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1989 Jeep YJ Wrangler Sahara 4.2L Engine Missing Parts

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Hi, my name is Alberto, this is my first post here. Recently bought a Sahara Edition YJ 89. It has a 3 speed automatic transmission. I did a trip of about 700 miles from the city where I bought it. At the beginning I was at 1700 meter over the sea level, went almost 3,000 meter over the sea level where I felt a total loss of power, even at full throttle it didn't raise more than 2,500 rpm.


I'm at sea level right now... when going lower the engine was feeling too much powerful and now it seems to run like a dragster. The bad thing is that the fuel consumption is too high. During the trip the floor was getting really hot, under the rear seat on the right side the paint looks burned and the carpet a little melted, the RR wheel well  is melted...


So after the trip I've been reading a lot about this engine and found that there are a lot of things that have been replaced and removed. I will like to have this engine working as it should and as stock as possible. 


To start with, there are two hose connections under the air cleaner base, found those openings without hose or plugs, completely open to the atmosphere. Some wires are hanging out, some vacuum hoses disconnected, don't know if the PCV valves are the right ones, the heater hoses are not in place, and at the thermostat housing it seems to be a plug sealing where one of those hoses should be connected. There is a vacuum reservoir under the battery that only has one hose connected, the other one under the water reservoir seems to be with all the hoses connected.


Where the exhaust pipe is leaving the engine to the cat, there is an opening at the side that is sealed.  The fuel filter is not the stock one, it only has one input and one output instead of 3 total, in addition is not on the right side close to the carb, but is down on the left side next to the fuel pump.  I have a plastic fuel tank, I think is 20 gal. maybe that worried me due to the overheating that the exhaust is experiencing.


Is the rear axle upside down? or is the plug right at that height? I've checked oil level on both axles and seems to be a little low, the rear one is dark brown. I'm feeling a strange noise when I drive very slow, like human walking speed, there is a knock noise from the rear and I feel the car moving side way when it sound, suspecting that there is something wrong in the diff gears or the bearings.


Look at the pictures and please help me find out that are the things that I'm missing, if you think that more detailed pictures are needed to identify something else I can take some more...Thanks in advance.


Regards from Cartagena, Colombia.


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Welcome to the forums, Alberto! Your photos are very helpful, and so is your description of the problems...Here are some observations:


1) The floor getting hot and the high altitude lack of performance are related to the catalytic converter and exhaust system.  This engine uses a Pulse Air catalytic converter, and the air tube from the exhaust head pipe to the catalytic converter has been eliminated.  This prevents complete burn in the catalytic converter and also eliminates the pulse air system's cooling effect at the converter.  (Lack of a Pulse Air hookup would fail a U.S. emissions test.)  In your case, the catalytic converter, if still intact, is either not functioning properly or is likely getting extremely hot from a rich fuel mixture and the missing Pulse Air function.  (Tip: The Pulse Air catalytic converter should have an air inlet pipe that works with the pulse tube system.)


2) You have valid concern about the exhaust heat and the plastic fuel tank.  In many cases, the heat you describe melts the fuel tank.  In a worst case scenario, this could cause fuel to leak onto a super hot exhaust pipe and either set the vehicle on fire or cause the tank to explode.  From the tailpipe's "rich" burn signs (black and sooty at the exit), plus your poor fuel mileage, it's quite possible that the carburetor is running way too rich.  This could be either a carburetor issue or the wide open throttle/limp mode may be operating all the time due to a wiring or vacuum circuit defect or tampering. 


3) Click on this link for the detailed 4.2L BBD carburetor rebuild articles at the magazine.  The BBD carburetor requires careful attention to detail during a rebuild, Alberto.  You will also find a number of articles related to 4.2L engine tune and troubleshooting at the Search box keyword "BBD".  To save you time, here is a link to the lengthy list at the magazine:  http://www.4wdmechanix.com/4WDmechanix-search-results.php


4) The previous owner of your Jeep Wrangler apparently disconnected and eliminated a number of vacuum hose and coolant features.  Here are the vacuum circuit diagrams for your engine, you can compare what you have with these PDF diagrams: 1) for the 1989 4.2L Jeep Wrangler with a manual transmission YJ Wrangler 4.2L Vacuum Diagram.pdf and 2) 1989 Wrangler 4.2L Engine Vacuum Auto Trans.pdf for your automatic transmission equipped model.


Note: The emission and carburetor circuits are closely linked together on this 4.2L engine.  The ignition is somewhat independent but also works through vacuum, coolant temp and ECU timing controls.  This is a complex system, the reason many owners convert to EFI.  Howell, Mopar and MSD have EFI systems as you will see in the article on the MSD Atomic EFI conversion for a 4.2L. (Click on the link—there are five detailed pages to this article!)  Howell is the easiest system to install and cost effective if you want to do a conversion to EFI.  You can restore the BBD carburetor, vacuum circuits, ECU circuits and such; however, it will take time and patience to sort out all of the modifications and tampering that the previous owner has done to this engine's vacuum, emissions and coolant circuits.


5)  The rear diff cover looks normal, it does seem like a low fill plug, but that's the Dana 35 design.  The noise and hop sound like a factory limited slip differential that's either defective or in need of friction modifier and an oil change.  Remove the diff cover to drain, do a thorough inspection, and if there is a factory multi-plate Spicer limited slip, there should be a tag specifying use of special lube or lube plus friction modifier.  Friction modifier breaks down, and fresh oil and modifier often eliminates clutch plate chatter and wheel hopping.


6)  Your engine's fuel filter should have three attachment points and fuel lines.  The third line returns excess fuel pump volume to the fuel tank.  This serves two vital functions: 1) reducing load on the carburetor needle and seat and 2) lowering the risk of fuel vapor lock by keeping fuel moving and not stagnating in hot lines at the engine.  Vapor lock is a common phenomenon on mechanical fuel pump systems without a third pipe (fuel return) to the tank.  High altitude driving and hot under hood or exhaust temperatures increase the risk of fuel vapor lock.  You need to restore the 3-line system, the engine could have been experiencing vapor lock at higher altitudes under extreme load.


Worth noting, poor engine performance at 3000 meters is also a function of altitude.  A naturally aspirated (no supercharger or turbocharger) engine loses approximately 3-5% of its horsepower per 1000 feet of elevation.  You're talking about 9,000-plus feet above sea level, so that could be as much horsepower loss as 45%!  This would create a very rich burn condition on a carbureted engine, too, since the carburetor has fixed jet sizing.  There is a primitive "wide open throttle" (WOT) fuel enrichment mode, which is not intended to compensate for altitude. 


You have an oxygen sensor on the 4.2L engine, designed to help regulate fuel enrichment (to a very limited degree) and spark timing, primarily for tailpipe emissions purposes.  It does not change the air/fuel ratio constantly like an oxygen sensor helps to do on an EFI or MPI system with electronic fuel injection.  In the U.S., improved high altitude operation is another reason why 4.2L Jeep owners convert from the BBD carburetor to an EFI system.  EFI will instantly compensate for altitude changes and air/fuel ratio demands.


This should get you started on troubleshooting and evaluating your Jeep's exhaust system, fuel circuit, vacuum circuits and carburetor.  Ask questions as needed, we can take this further.


Trust this helps, Alberto...



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Moses, Thank you very much for the input...I'll start removing the diff covers and replacing the fluid if there is nothing broken, actually there is a leak on the front diff housing, I thing that resealing could stop the leak. Do you recommend use of silicone or gasket for diffs? or both?


Another issue is that the shift selector lever is not locking in Neutral, Drive, 3...etc. It only locks in Park position. I'm missing the indicator needle, it came with a bended wire with red tape as indicator, it does work but I would like to see a picture of the factory one to see if I can make one similar...


About the heat from the catalytic converter and fuel consumption it seems to be a lot of work to get the whole system working as it should, as it involves the exhaust system, carb and vacuum.


I need to start writing down a list of parts that I need and I can not find in my country. So I can improve the engine's performance.


Thanks for sharing your knowledge.



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You're welcome, Alberto...The fuel/carburetor, emissions system and ECU is so interconnected that the whole system must be restored in order to function properly.  This will take time and careful evaluation. 


It's unfortunate that previous owners and some mechanics believe that it is useful to "disconnect" and discard various components.  In tampering like this, the function of the entire system suffers.  "Restoration" seems the only real fix with this complex stock system—or an EFI conversion like the Howell Engineering TBI package if you can afford it.  (Click on the link for details.)


As for the diff cover sealing, silicone RTV works well.  Use a type recommended for exposure to gear lube.  Do not overcoat, apply a uniform bead that will smooth out evenly and seal properly.  (3/32" to 1/8" bead size is plenty.)  Make sure the diff cover is flat around the bolt holes, and torque the bolts snugly to specification and evenly.  Do not over torque the bolts, or you will indent the cover at the bolt holes and risk a leak.  Snug all bolts to just seat the cover, then let the RTV set up slightly before tightening bolts to final torque.


There is no need for a gasket if you do this properly; in fact, average quality cut paper gaskets are more prone to leaking than silicone RTV alone.  The later factory use of RTV instead of a cut gasket suggests that RTV works better.  (An exception would be rubber gaskets like those found on the Ram truck's AAM axles.  These work very well.)  I use RTV for Jeep diff covers.


Keep us posted, Alberto, enjoy your Wrangler and be safe in your travels!



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Actually I can afford the conversion kit, but I really like to keep the vehicles as stock as possible, I like to make small modifications, eg. some fog lights that match the YJ style, some lift to give it a better off-road look, I really like to keep the 80's style things that you can add, a modern stereo will be necessary as the stock one is a cassette player hehe...


Mechanically I know that the engine can be more efficient with a conversion but I really like the carb and the idea that is "easier" to service. It's really cool to see a machine from '89 that can be close in mileage to my past '09 Toyota LC90, even with all the electronics improvements and sensors that should increase the efficiency.... It has a 2.7L 4 Cyl. that was running around 18 miles per gal at maximum. You can not compare the power of that 2.7L with the 4.2L that the YJ has and almost the same mileage rate...


I think some mechanics suggest to disconnect things and they argue that there is no difference doing that, and in addition the owner will save money avoiding spare parts purchase... anyway, my task now is to restore it to a good efficiency level, environmentally and economically talking.


I Will try to do the diffs inspection and fluid replacement and will post some pictures.


Thanks again.

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Alberto, you'll find the BBD carburetor step-by-step article at the magazine helpful.  The vacuum diagrams, too.  As you proceed with the restoration, we can fill in other "factory" details.  Since you're willing to do this work, I know that you will be thorough.  Each of these systems overlap, and the only "right" way is a restoration to original equipment performance as you suggest...You always have the option of an EFI conversion if you discover that it is cost effective or if you simply cannot find the many parts you need for the restoration.


I am pleased with your enthusiasm and believe you will draw the correct conclusions about whether to restore or not, Alberto.  Looking forward to pictures of the Jeep.  The forums' Garage Photo Gallery would be a good place to share.



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Good morning Moses, yesterday I did another inspection to draw a sketch as the Vacuum System is now... Here are the results. Some pictures to clarify what the drawing is showing. Could you please let me know a good website where I can buy new Vacuum hoses, check and delay valves? which ones will meet the factory specs? which internal diameter hoses I should look for? which filter to install in the PCV rear valve?


There is a picture where you can see the Air cleaner cover, It has a crushed connector, What is this one for? 

There is a connection under the steper motor that is not connected, is that the one that goes to the EVAP SYS VAC MOTOR?


What I see is that the last mechanic that worked on this engine just plugged any hose where it reached...


Thank you and have a good day...Regards.


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In the U.S., NAPA retail auto parts stores have always been helpful.  Car Quest also has extensive catalogs of individual parts.  NAPA's Echlin line and catalogs are a good source.  Do you have retail chain stores like these in Colombia?  Is there an AC/Delco, Motorcraft or Bosch warehouse?  These would all be sources for the more generic parts like check valves and vacuum delay valves.  Otherwise, your best source, and many do this in the U.S. with older engines and emission control parts, would be recycling yards ("wrecking yards") that have older Jeep vehicles for dismantling and used parts sales.


The vacuum hoses should be the easiest items to find, as these hoses are "universal" and come in specific sizes for the diameter of the vacuum nipples and pipes.  A vernier caliper (even an inexpensive plastic one) is helpful for determining the correct diameter needed.  You want the hose to fit snugly on the pipe or vacuum nipple, so some experimenting with samples of vacuum line would be helpful.  In the PDF I have provided below, the Mopar listings show actual vacuum and fuel hose sizes for the various locations on your engine and vehicle.  These measurements are for bulk hose that you can buy locally.


To save you a great deal of time on your Jeep 4.2L Wrangler project, I combed through the factory Mopar parts catalogs for your model era.  I gleaned out pages that provide both detailed illustrations and also the part numbers.  You can see the relationship of the parts. 


Some (very few) of these parts are still available from Chrysler/Mopar.  The Mopar part numbers will help you cross-reference to other manufacturers that may still offer the parts.  Hose sizes are noted in "I.D." (inside diameter) and shown as "bulk" source.  This means they are generic and can be purchased as bulk hose for vacuum or fuel hose in these sizes. 


Make sure you use hose that is specified as vacuum or fuel grade.  Always use fuel-grade hose for fuel lines or for lines exposed to fuel vapors.  Your 4.2L Wrangler with carburetion does not require fuel injection rated pressure hose.  (If you convert to EFI, you must upgrade the fuel supply line hose for the added fuel pressure.)


Note:  The fuel tank for your vehicle is either 14.5 or 20 gallon.  The 2.5L TBI/EFI engine uses an electric pump in the tank, your 4.2L carbureted engine does not.  At the tank, your Jeep has only the fuel gauge module without the electric pump unit. 


This will speed your search along.  Both the PDF text and the images can be "zoomed-in" for great detail.  I trust this will make your project much easier.  You can determine what you have, what you need and the part numbers for each item. 


In the page footnotes and model references, Mopar labels your Jeep YJ Wrangler as a type "81".  I am unclear whether your vehicle was for U.S. (North America), Canada or the export market.  In some cases I provided sets of information for both U.S. and export market vehicles.  You will need to compare the illustrations with your vehicle's actual equipment:


4.2L YJ Wrangler Fuel & Emissions Systems.pdf


From experience, I know that if you do not have this "big picture" information, the project can become very challenging.  With the extensive information provided in the PDF, you will be able to identify the parts you have and those you need.  At that point, you can try to source the parts by part numbers or from generic sources.  You will discover what is still available as new replacement parts.  You will also know the relationship of the parts and how they fit together.


I trust this helps, Alberto...



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Of course it helps much more than what you can imagine, I want to thank you for sharing all this information that is not easy to get in short time... I hope I can take good pictures when performing all this restoration to share them and retrieve some of your help.


That last PDF has really detailed and accurate diagrams, very helpful. I'm sure I can find some hoses and parts here, but I will need to order some more, and it will be easy to order almost all from a single vendor.


Will keep you posted.


Thanks a lot.

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You're very welcome, Alberto.  These kinds of issues come up frequently for restorers of the 1981-90 Jeep CJ and YJ Wrangler models with the 4.2L inline six-cylinder engine.  In the U.S., it is often a state-required emission inspection that turns up the "troubles" with the emissions and fuel system...I trust that the information provided in the PDF will help you and many other Jeep members and guests at these forums. 


Colombia must be a wonderful country for four-wheel drive access.  You mention "3000 meters" elevation, and that is much like the Sierra Nevada Mountains near our home.  Snow, permanent snow packs on the north slopes, this can be great country.  At Colombia, you have access to the Pacific Ocean, the Caribbean Sea, Ecuador, Panama, Venezuela, Peru and Brazil!  Lots of rivers and mountains, too.  I imagine that the magazine's Honda XR650R dirt motorcycle would be a great way to explore Colombia. 


Photos would be great with your Jeep at these kinds of backdrops...Please share photos when you can.  Enjoy your 1989 Wrangler Sahara, Alberto.



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  • 5 months later...

Moses, long time away from the forum but still working on this restoration.


I still have the rear axle noise as I hav had no time to pull the axle, did pour one modifier to the fluid with no positive results, it seems that is not a trac-lock diff.


Regarding the AT transmission, is a TF999 which has a grinding noise, the noise is more noticeable in 3rd gear, in 2nd you can hear it, and in 1st shift is not hearable at all.


I've been working much more in the engine, I've pulled the carb out the car, disassembled and noticed that it has two different metering jets (100 and 85 numbers stamped.) In addition, there are no metering rods. the choke linkage was missing and there was a lot of particles all over those parts.


I've read in your article about rebuilding the carb that you should not try to pull the idle pickup tubes out of the ventury assy, mine were not loose but came out with no effort.

Another thing is that the Idle mixture screws seem to be adapted from other carb, the tip is not a perfect cone, it seems like it has been done with a file.

I see no movement of the stepper motor. Could be a bad oxygen sensor or a bad coolant temp. switch.


Last Saturday I removed all the wiring from the MCU to each one of the sensors, stepper motor, A/C components, distributor, Ignition module, etc. My intention was to check continuity on all the cables and compare the factory wiring diagram with what was actually connected. This because I've read about the Nutter Bypass and I was afraid that it was done on my YJ. Fortunatelly it was not done and all was in the stock shape. I've put some soldering on the splices found.


I've noticed duct tape in the stock splices/taps done. Is this made from factory?


Now, I need a little help from you on some things.


I've been trying to get the right idle mixture screws, metering jets and rods. Found some online but there is no specification on the model that they will fit, there is just the overall length, thread pitch, etc. Which one will fit on my BBD Carb? --> 


Idle Screws:

Idle Mixture Screw- 10-32 x .900", OEM # 30A-51

Idle Mixture Screw- 10-32 x 1.206" OEM # 30A-58

Idle Mixture Screw- 10-32 x 1.311"

Idle Mixture Screw- 10-32 x 1.391"

Idle Mixture Screw- 10-32 x 1.502" OEM # 30A-198

Idle Mixture Screw- Carter BBS, BBD, RBS 10-32 x 1.442"

Idle Mixture Screws Pair of replacement idle mixture screws. 10-32 pitch thread 1.675" overall length


Metering Jets:

Carter Metering Jet .070"

Carter Metering Jet .071"

Carter Metering Jet .074"

Carter Metering Jet .077"

Carter Metering Jet .080"

Carter Metering Jet .083"

Carter Metering Jet .086"

Carter Metering Jet .089"

Carter Metering Jet .092"

Carter Metering Jet .098"

Carter Metering Jet .101"

Carter Metering Jet .104"

Carter Metering Jet .107"

Carter Metering Jet .110"

Carter Metering Jet .113"

Carter Metering Jet .116"

Carter Metering Jet .119"


If I reduce the Metering Jet size, Will I have less fuel consumption, sacrificing power?  I cannot find the stock metering rods listed, do you have any retailer suggestion?



Thanks in advance. Regards!

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Hi, Alberto, good to get your update.  Glad you're doing well and finding time to restore the YJ Wrangler 4.2L.


To avoid confusion, let's start with the carburetor tag numbers available in 1989 Jeep YJ Wrangler 4.2L engines.  These are official Mopar complete carburetor part numbers (a carburetor assembly new is by now obsolete) reflecting where the vehicle was originally built:



83320007  U.S. and Canada - Tag #BBD 8383, BBD 8394 Carter, Man. Trans.

83320007  YJ U.S. and Canada - Tag #BBD 8383, BBD 8394 Carter, Auto Trans.

33000486  15,26 Export NRM - Tag #8366

33000486  YJ Egypt and Venezuela - KDX - Tag #8366

When I look up the Mopar metering rods for a 1989 YJ Wrangler 4.2L BBD Carter carburetor, I come up with Mopar part number J8133949 for both metering rods (same rods and size).  This Mopar part fits a huge range of 4.2L BBD applications:
Tag #8383, 8384; carburetor tagged 8302S, 8303S; carburetor tagged 8306S, 8307S, 8341S; carburetor tagged 8338S, 8339S, 8383S, 8384S; carburetor tagged 8340S, 8383S, 8384S
Years and models of fit include 1984-90 4.2L engines in the Grand Wagoneer, J-10, YJ Wrangler (1987-90) and Jeep CJ models through 1986.  This would be the factory metering rod for your Jeep 4.2L application.

The metering jets, by Mopar part numbers are two types, dependent upon the carburetor tag number.  Each carburetor takes two of the same metering jets.  These are the listed carburetors for 1989 YJ Wrangler 4.2L:
J3207756 Tag #8383, 8384
J0948827 Tag #8366
If you're sourcing outside Mopar and from a direct Carter jet source, you need to cross over the Mopar number to Carter.
Idle Mixture Screws are each the same, there are two under Mopar part number J8133451.  Again, you would cross this number over to a Carter number if you cannot find the Mopar parts.


Trust this is helpful.  I can furnish more Mopar information if you need it, Alberto.


As for jet sizing, stay with factory type.  These engines were designed to run with the emissions package and stepper motor.  They deliver reasonable fuel efficiency and performance.  I would not get creative here. 



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Moses, Good morning.


Thank you for the information provided, actually I have been trying to find the Mopar parts with no luck, and I cannot get a cross reference to a vendor that has those parts in stock.


I will be traveling to the US next week, will spend 2 weeks over there from Fort Lauderdale/Miami zone to Virginia by car. So I will try to find a place where I can buy a lot of stuff. I would like to visit a salvage yard to see what can I find.


Another concern is that the Pulse Air system was removed from my YJ by a previous owner, don't know if the catalytic converter is the stock one. I know this is an emissions device, but I don't know what else this Pulse Air System does, does it reduce the temperature on the exhaust pipe?


I've tested the EGR valve pulling the actuator with my fingers and blocking the hose connection port and it did not hold. another $ to spend on that. I will check if the Coolant Temperature switch is closing when the engine is warmed up (how long it should take to close? I'm at sea level, about 34 Celcius).


I have Tons of questions for you to help me out, but I will go step by step.


I really appreciate all your time reading and replying to all us here in the forums.



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You're welcome, Alberto, glad to provide information. Your trip to the U.S. should be productive. A good core carburetor would be a smart option, a unit that is original.


Pulse Air sources fresh air at the air cleaner and feeds this air to the catalytic converter for a more complete burn at the cat. The system is unique and passive, there is no need for an air pump. Exhaust pulses create a low pressure suction that the Pulse Air valves regulate.  The suction draws clean air from the air cleaner Pulse Air ports to the catalytic converter (cat).  Check valves (one-way) prevent hot exhaust from creeping into the upper Pulse Air tubes and the air cleaner.


You cannot damage the engine with this system removed and blocked off.  That would not be true with an air injection (pump) system, as the pump flows air into the exhaust stream at the exhaust manifold or exhaust ports.  (This fresh air helps complete the burning of hydrocarbons and CO in the exhaust.)  Tampering with Air Injection can cause overheating of the exhaust valves, and in the worst case, the melting of injection tubes.


If you decide to restore the Pulse Air system and get an OEM catalytic converter, make sure you use a silicone high heat resistant hose at the catalytic converter's air inlet tube.  Regular rubber hose will burn up immediately and create an exhaust leak from the catalytic converter.


Here are the parts involved with the Pulse Air system.  Not shown is the OE catalytic converter with air inlet tube.  (You have an automatic transmission, so the deceleration valve and pieces are not needed.)  The factory cat has an air pipe facing toward the Pulse Air tubing.  The Pulse Air tube connects with the hose I described.  Using the Pulse Air system is optional unless you have vehicle smog inspection at Colombia.  As long as the cat is hot enough to burn emissions, there should be no issue:


1989 YJ Wrangler 4.2L Pulse Air.pdf


Note this example: With the Mopar 4.2L EFI conversion, Pulse Air is eliminated.  The OE cat's air inlet tube is pinched and folded over.  I seal the pinched end with a gas weld or MIG bead.


Your access to stock and unmodified YJ 4.2L Wranglers at the U.S. recycling yards would be beneficial.  Take lots of photos!



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  • 8 months later...

Has been a long time away from the forum. I've been working Night Shift and Monday to Sunday.


Anyway I've made some progress on my YJ restoration. Did a search with all the YJ and CJ7 owners to see if any of them had a dirty Carter BBD sitting in their garages and finally got one with the ECU as well.


I didn't knew that my ECU was not working good until I replaced the stepper motor on my carburetor and found that the misxture pins were not moving as the FSM says. So I swaped up the ECU module and all started working better. Is there any way to test the ECU module isolated from the vehicle?


I've taken some parts from the carburetor I received, as Metering Jets, Rods and Stepper motor (as stated above). So I have mine now "complete". I'm not sure if the Rods are the right size, as they are stamped with the number 2376 and the FSM calls for 75-2384.

The Jets are 120, so I think those are OK. But if the Rods should be bigger that could be a reason for being running a rich mixture.


The Idle running has improved, I still have a hard time to get the engine started when it is siting for more than 3 days, so I have to pump the gas pedal too much times to get it started, but now it holds running once started (before I have had to play with the gas pedal for a few minutes while it was warmer). I'm at sea level and an average weather temp around 29 C (84 F), so I can't tell how bad could be a cold starting of my YJ after one week sitting in the parking lot in a really cold weather.


Took it for an emissions test and it has failed. Readings are as you can see in attached image.


I will have to follow up the carb setting procedure again, step by step and for the ignition timing as well, and see how it goes, anyway, from your experience and knowdledge Moses, What do you think can be the big problem to get those readings? Oxygen sensor? Catalytic converter? let me know what you think.

I'm noticing some misfires and can't get the source, they are random at alla range of Low (600) to Mid (2000) RPM


Do you think it is possible to pass this test without the Pulse Air System?


Thanks in advance.




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Alberto...Very useful details about your tuning with the Carter BBD.  Yes, the cruise enrichment could be the rod diameter.  You're not very far off here, so let's first concentrate on the causes for failing the emissions test.


Some basics.  The choke must open completely after the warm-up cycle, it must also work properly when the engine cold starts.  You have a high CO reading, and if the HC is not way off, high CO is most often an overly rich idle mixture condition.  Spark cable shorts or weak spark can apply here, too.


The idle mixture screws could be set too rich.  Use the factory adjustment approach for a "lean drop" setting.  If you need clarification, I will suggest how to set lean drop.


Make sure you do not have vacuum leak(s).  Check for vacuum leaks at the intake manifold-to-cylinder head gasket, the manifold fittings, hose connections and the carburetor base plate and gasket.  A quick check with the engine idling would be use of a lower-volatility solvent spray mist like WD-40 or even spray carburetor cleaner.  Be aware that carburetor cleaner is more volatile.  Avoid spraying on hot surfaces like the exhaust manifold!  Listen for changes in engine rpm as the mist finds vacuum leaks.


Resolve the over-rich idle condition, and the CO should drop.  The HC reading may increase slightly with these richer rods in place.  Cruise rpm is tricky, as the engine is moving off the idle circuit and onto the main jets (rods and jets).  Try to find a balance here.


With a quality timing light (built in advance), verify ignition base timing (vacuum advance hose disconnected from distributor and taped, engine idling), and check spark timing as the mechanical advance moves.  Timing errors can affect the emissions readings.  Also eliminate ignition misfire possibilities.


Pulse Air is a passive system that simply pumps air into the catalytic converter to increase the burn effect.  Unlike air injection with a pump, the Pulse Air takes advantage of engine exhaust cycle pulses to move air.  This system does reduce tailpipe readings, helping the catalytic converter dispose of additional CO and HC.  You do need a CAT system to get acceptable tailpipe readings.  Pulse Air helps, too.


Eliminate any vacuum leaks, and try the idle mixture adjustment.  Correct spark misfire or timing errors...Get another emissions tailpipe reading.  We'll compare and go from there.



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Hi Everybody, I've set the carburetor as specified in the service manual, actually the only setting off was the vacum piston lifter setting, that was too high I think.

Timing was too advanced as I was following the timing marks in the wrong way, I was taking each slot as one degree, actually the scale goes from 0 to 24 degrees, resulting in something like 4 degrees per mark with a small one in between, corresponding to 2 degrees.


I read that retarding the base timing to zero and adding some alcohol based octane booster would help to reduce emissions, Did that and took the YJ to be inspected again. See results in the attached drawing.


It is noticeable that retarding the spark reduces the emissions. But at the same time it does make your engine to run worst, you feel you get less power... So some questions arise:


1. How is the fuel mileage affected by retarding the spark?

2. How is the emissions/power-delivered relationship?

3. Is the factory timing spec the optimal fuel consumption/power ratio?

4. How can I obtain the highest fuel mileage? (sacrificing power?)


Now, I've cleaned the spark plugs before the test, #2 plug was the worst one. After running the engine for timing purposes I've pulled #1 again and it was black as you can see in the picture.

I'm tempted to pull the valve cover just to see how bad are the head components looking but as I have no gasket kit I will wait to avoid the vehicle to be "out of service". I've read that replacing one quart of engine's oil with diesel fuel can help cleaning all carbon deposits, read also that adding diesel fuel to the gas tank will make the same, and pouring water trough the carburetor will make some celaning as well. So, there are too many old-school techiniques but not sure if they are all right.


Regarding the misfires, I've noticed some improvement but still feel like the engine's rpm are not steady when idling. It feels like it is swinging around an rpm value but does not keep the same rpm all the time, seems like a delayed compensation from the closed loop system or an intermitent signal to the ECU, I'm just guessing.


By the moment I will set the timing again to the factory spec and replace the air filter. Looking forward to your coments.








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Alberto...I see the dramatic drop in some of your emissions levels.  The spark plugs are revealing:  #2 cylinder is not getting optimal combustion and likely misfires.  Spark plug cables, cap and rotor are new?  These plugs look like they could use replacing...What is the cranking compression on #2 cylinder?  Where did the single, fuel rich spark plug fit?  Was that enrichment fouling from before the latest tune?


Best fuel efficiency is typically at the maximum timing advance just short of detonation (ping).  This increases manifold vacuum.  Peak vacuum (again short of ping) is desirable for performance and fuel efficiency.


Timing advance closely relates to horsepower, and horsepower equals BTUs (approximately 45 BTUs per horsepower).  Carbureted engines in the emissions era are "detuned" to produce less fuel-enriched power.  Since the driver controls the throttle, the only built-in emissions remedies are spark timing to lower emissions and lean mixtures—but only to the point of detonation and way below the point of creating pre-ignition. 


In early attempts to lower emissions at California, it was mandatory on some smog retrofit kits to disconnect the vacuum advance on certain engines to comply with emissions standards.  This retarded timing (cut off vacuum advance) also caused engine overheating.  Engine overheat damaged engines and increased NOx, as NOx is a direct product of fuel combustion above 2,500-degrees F.


During the late carburetor era, factory timing was more concerned with emissions compliance than performance.  In the 1991-up 4.0L EFI era, both performance and compliance became possible, but for the more primitive 4.2L carbureted engines, the only remedy was to "detune" the engine.  Horsepower dropped with each new and stronger emissions control mandate.  The trend continued until the introduction of EFI and electronic spark management. 


Note: Carburetors and emission controls like your BBD model were popular from 1981-90 on the 4.2L engines; the TBI 2.5L four went from carburetion to TBI on the 1987-1990 Wrangler.  There was a 2.5L carbureted four prior to 1987 in the CJ Jeep.  TBI first appeared on the XJ Cherokee 2.5L four.


Experiment with gas mileage and spark timing.  Your carburetor and jetting are fixed and controlled by the ECU (enrichment) to some degree.  Fuel enrichment for WOT is controlled.  If you need the BBD Carter carburetor for emissions compliance, I would not play with it.  Spark advance (vacuum side) is controlled, too, and I would leave that alone.  The only incremental change you can make is base spark timing adjustment.   This cannot be done beyond the point that you hear the engine ping under load or when you run lower octane fuel.  There is not much gain here, and a few degrees added would be the limit.  I would not set the 4.2L Motorcraft distributor beyond 6-8 degrees base advance timing at idle with the vacuum hose disconnected and taped.  Retard slightly if you experience ping (detonation) under heavy throttle.


The real performance and mileage fix is a conversion to EFI.  The Mopar EFI Conversion Kit, for example, boosts power around 50 horsepower with no other significant changes.  Driven prudently, the EFI converted engine will also deliver improved fuel efficiency.  There are greater gains with MPI (Mopar) than TBI (Howell Engineering) due to the inherently poor fuel/air distribution when running the stock engine's long intake manifold with its carburetor (central) induction point. 


Emissions improve dramatically with EFI, and both systems are 50-State legal in the U.S.  The Pulse Air goes away, a cat is still required (considered a chassis emissions component by law), and the EVAP stays.  Howell TBI uses the stock distributor or an aftermarket distributor that has a California E.O. number for emissions requirements.  The Mopar kit is very pricy but patterned after a Wrangler MPI 4.0L system, including the intake manifold, throttle body, distributor and wires, fuel pump pressure levels, all sensors and a 60-Way PCM (basically an off-the-shelf '94-'95 4.0L YJ or XJ Jeep stock type).  Some owners convert from scratch with stock parts, but California will only recognize the official Mopar retrofit EFI/MPI kit, as it has an E.O. compliance number.  Others will often install a complete 4.0L engine with its MPI and emissions equipment.  They certify the 4.0L as a complete retrofit/conversion or swap.


You may be more constrained at Colombia.  Or perhaps you don't want to spend $1400-$2,600 to convert to EFI.  If the stock and original engine is all that you are allowed or prefer, regardless of emissions gains, then the stock engine in the tune you current have is the only way.  If Colombia recognizes California or U.S. 50-State compliance terms, then the (expensive) Mopar EFI Conversion Kit or the less costly Howell Engineering TBI kit is a solution.  A subtle improvement, if legal at Colombia, is the 50-State legal MSD ignition distributor.  It offers an improved spark advance curve, ample spark with the MSD box added, plus direct vacuum hook-up to ported vacuum.  This distributor and the MSD "box", which both have California E.O. numbers, may not be emissions legal at Colombia.


As a footnote, the engine lasts much longer with the tune and efficiency of EFI.  I cover a Mopar EFI conversion at the magazine.  (Type "Mopar EFI" into the magazine site's search box at www.4WDmechanix.com, you'll get returns on both the Mopar EFI/MPI and MSD Atomic TBI with notes on the Howell system.)  The MSD Atomic EFI conversion that we pioneered on a 4.2L Wrangler engine is nearly identical to a Howell 4.2L TBI kit installation and tune.  Though reducing tailpipe emissions over a stock carbureted 4.2L, the MSD Atomic system is not emissions approved at this time for use on the Jeep 4.2L engine.



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Moses, Distributor cap and rotor are brand new, Cables installed are medium quality, I will look for a quality cable set as all I was getting was cheap cables that will work the same as the ones I got installed.

All spark plugs were replaced and I got like 10.000 miles on that set. Regarding the compression I do not know as I don't have the gage to do so, I will see if I can find a workshop with the equipment to make a complete test (Are you thinking I could have a gasket or valve issue?).


The single fuel rich spark plug came off of the #1 Cylinder and it was during the tunning process, I should clean them all after tunning the engine and run it some miles, then pull all spark plugs again to compare with the previous picture.


Timing label states that 9 degree is to be set when engine is at 1600 RPM, vacuum advance plugged and 10 and 4 Hg switches harness disconnected. So if I understand you, air/fuel mixture and timing when cruising are controlled by the ECU, so the only adjustment could be for the idling timing and mixture, What makes sense to me as the only fixed adjustments are the idle screws and distributor base. 


In the other hand, I understand by what you said, that fuel eficiency is obtained at the max timing advance without getting the engine to run really bad. So, a high HC level doesn't mean that the fuel is not burned completely and we are wasting fuel? are the CO and HC emissions low or high at the maximum fuel efficiency setting?


Now moving on the legal matter, here in Colombia we have to take our vehicles to a Mechanical-Technical inspection in a annual basis, this starting the 6th year after the vehicle was registered to obtain the licence plates. So new vehicles are not inspected.

Cops won't check beyond the certificate printed by the Inspection center unless your vehicle is running without muffler or throwing too much smoke through the tailpipe, or if you have more than one bulb off (tail lights, head lights).

I can even remove the catalytic converter and go up in the emissions values and I still have the certificate for a year, but I prefer to stay under the emissions levels and try to be less harmful to this planet.


Said that, we can install any conversion kit on our vehicles, if they will pass the emissions test, it's ok.


Some day I will try to go at least across all South America, then North America (hope all around the world). And I will be glad to do that with my stock engine and carburetor. I may add some Fog lights, maybe a 31" mud tires and some 2 or 2.5" lift, roof rack, etc. But always keeping the shape and look of this vehicle's era. That is just a dream, but it can come true some day.

That is the reason I'm not thinking in swapping the engine, or the head, or put a conversion kit, I know all the advantages and improvements that I can get with an EFI kit, but for now I will stick to the old times.


As always, Thank you very much for the information provided, I really appreciate the time you spend sharing all this valuable information.




Alberto H.

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Alberto...I'm not pleased with the rich plug and the #2 plug's buildup could be poor combustion.  This could be telltale signs of low compression.  A cylinder leakdown test would be conclusive.  A cranking compression test would be okay and a quick test with an inexpensive diagnostic tool.


Clean the plugs and get a current reading on the spark plugs.  The ECU controlled spark is primarily an emissions constraint.  The distributor actually has a conventional mechanical/centrifugal advance and a vacuum advance unit.  These devices get over-ridden, in a modest and primitive way, by the ECU.  This is not bona fide "spark management" like with EFI/MPI engines.  Spark is only modified by the ECU, your distributor is like older engines in most ways.  You can bypass the ECU spark controls but will find the engine will not meet emissions standards. 


Likewise, the ECU provides controls, to a limited degree, especially fuel enrichment at wide-open throttle.  Beyond the fuel enrichment system, the carburetor has fixed metering rods and jets.  The ECU plays a limited role in fuel control, mainly at WOT.  The BBD carburetor, by design, dates to the 1960s and the pre-emission controls era.  Solevac and electronically controlled fuel enrichment rods are the updates that help your engine meet emission standards.


You've done great tuning and have restored your YJ 4.2L's emissions system and carburetor.  It will be reliable now.  That and your plans for a mild lift and 31" tires makes excellent sense.  Good plans...Let's check compression and do a leakdown test if possible.  I have tips on an inexpensive approach to a leakdown test here at the forums:  http://forums.4wdmechanix.com/topic/164-how-to-make-an-inexpensive-engine-cylinder-leak-tester/?hl=leakdown.  Make sure the engine is in top shape before the N/S America tour!



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