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Hello all. Newby on this forum.

I'm dealing with a '89 Wrangler with the 4.2 and an auto trans. The rig has about 165,000 miles and I don't believe there has been any work done to the 4.2. The original BBD stepper carburetor has been replaced with a non-stepper BBD, but all else appears factory.

The owner has not been able to get the rig through emissions testing and has asked me to help. I am not overly familiar with the 4.2 and much of the information I have seen seems contradictory, so...Here is where I ask for your help.

The base timing will change by merely revving the engine and letting it return to it's normal idle. In one instance the base timing was set at 6*BTDC, vacuum disconnected, and after reconnecting the vacuum and revving the engine the base timing had dropped to 2*ATDC, vacuum disconnected. The base timing has also advanced 8*-10* in a similar test. A static check shows the distributor and vacuum advance assemblies working as they should and the distributor is firmly clamped. The idle speed remains reasonably constant and the carb linkage is resting on the curb idle screw.

I have not checked for timing chain wear and suspect this may be part of the problem. I also plan to remove the distributor and check the internals for wear etc.

This rig has manifold vacuum to the distributor at all times. I have read where it should have ported vacuum.  I have studied vacuum diagrams for this rig and haven't been able to positively identify the vacuum source.

Any help or comments would be most appreciated. Thanks in advance.

 

Ford Guy

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Ford Guy...I understand your frustration, the BBD carburetor and feedback system is complex, and the ECU/MCU and vacuum circuit interface with the distributor can be confusing as well.

There are two concerns here:  A/F ratio of the fuel mixture burn at the emissions testing rpm and the ignition performance at these same speeds.  Also, the engine has several emissions related components, including the Pulse-Air system, the cat, an EGR valve and vacuum circuit plus an O2 sensor.

The ignition distributor is itself a Motorcraft conventional type with centrifugal and vacuum advance mechanisms.  This is overridden, however, by the ECU/MCU and a vacuum circuit that restricts vacuum advance and the total degrees of advance under certain engine operating modes and conditions.

Hooking the ignition distributor vacuum to ported source would be "normal" if there was not an emissions requirement to eliminate vacuum advance during the engine warm-up phase (consider the CTO vacuum switch as a trouble spot) and given engine operating conditions.  You do need to troubleshoot the vacuum circuits and the function of devices like CTO.  I would also concentrate on the EGR valve; if unseated or operating erratically, this could spoil the emissions testing results. The EGR valve functions from ported vacuum that has gone through the CTO and other vacuum circuits.

I'm attaching a CEC System wiring and a vacuum diagram for this year model 4.2L and application.  This will help clarify how the systems function and interact:

img138.jpg

Trace the vacuum circuits and sensor functions.  Also consider the Pulse-Air function and whether the EGR valve and Pulse-Air Valve check valves work properly.  Pulse-Air is part of the catalytic converter's efficiency at cleaning up the emissions.

img140.jpg

If the non-stepper motor BBD is new to the engine and its emissions testing, this could be a trouble spot.  Did this engine pass smog before with the non-stepper BBD carburetor?  If the engine passed smog in the past with the existing equipment, it should pass now.  If the non-stepper BBD is the only change, and if vacuum circuits and wiring remain intact and functioning, the carburetor could be involved in these failing tailpipe readings.

Moses 

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Mr. Ludel,

Thanks for the prompt reply and the schematics. I will check out the systems mentioned. Being unfamiliar with and not having solid information about this rig has been a large part of the problems I have encountered. The information you supplied will help me immensely.

According to the owner the non stepper BBD has been on the Jeep for 4-5 years and has passed the state emission testing twice before, but not without difficulty. It was installed by a local shop because the stepper BBD was not "working properly". The owner is a long time friend and I have been aware of his problems with this Jeep, but not involved until now. I have often thought there was more going on than just a timing or carb issue.

Did the original stepper BBD send inputs to the MCU or did the BBD just receive outputs from the MCU? With the non-stepper BBD in the system will all other functions work as designed?

Again, thank you for you help.

 

Ford Guy

 

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Ford Guy...The carburetor signals to the ECU would be throttle position for the "WOT" (wide open throttle) functions.  Make sure the WOT switch is intact with correct voltage signals.  WOT would not impact the idle and cruise speed tests of typical tailpipe emissions unless the system is "stuck" in WOT mode.

For a better look at the carburetor and its relationship to emissions, review my article on rebuilding a YJ Wrangler BBD:  

http://www.4wdmechanix.com/Rebuilding-the-YJ-Wrangler-4.2L-BBD-Carburetor?r=1

Let us know what you find with the vacuum circuits, vacuum devices, EGR and other components in the two diagrams.  We can go from there.

Moses

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Moses,

Thanks for the link. I'm going to have to read it over more than once, that's for sure!

The owner has been talking about the Nutter Bypass. I tend to try to keep things as originally designed and am not sold on this modification, though from the positive comments it seems to be something worth looking at. I would appreciate your comments on the Nutter Bypass.

I won't have the Jeep back in the shop until mid next week. I will do the checks and verifications at that time and report the results.

 

Ford Guy

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Ford Guy...I'm like you and lean toward the academic.  My approach is restoration, not setting the vehicle up for failure with more modifications.

Is the Nutter Bypass likely to "fail" the visual inspection?  How will the smog test station view this and the non-stepper carburetor?

Moses

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Moses,

I appreciate you thoughts.

From what I have found regarding the Nutter Bypass, it will most likely pass the visual inspection part of the smog test, providing everything appears original. Put the bypass wiring in the loom and make the vacuum lines appear untouched. Typically the visual part is nothing more than a quick glance under the hood to see if the major components are in place and appear to be functioning. As long as there is still a maze of hoses and wiring, the vehicle will usually pass the visual. The inspectors typically don't spend the time to check for every component, wire and hose. The non stepper carb would most likely go unnoticed, as long as the disconnected stepper wiring wasn't visible.

The tail pipe sniffer on these older rigs is the big item the local inspection stations will fail a rig for. The rig is first checked at idle, then revved to a fast idle for a few seconds and allowed to return to a normal idle and checked again. This second reading is the "official" test reading. The allowable emission limits are a bit more lax than original limits, but there is no grey area. over the limit, even by .01% and you fail.

After reading more on the Nutter Bypass this past weekend, I am less opposed to it than I was originally. Please correct me if I don't have a good understanding of the change. Simplified, it appears the Bypass removes the ECM from the ignition system and changes the advance vacuum from manifold to ported.This allows the ignition to work as a typical early Duraspark system with vacuum advance. When coupled with the non-stepper carb conversion already on the Jeep, this bypass returns the Jeep's ignition and carb systems to a mid 1970 norm.

Again, any input would be appreciated.

 

Ford Guy

 

 

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Ford Guy...With the Nutter Bypass in place, and assuming that the engine and emission system pass the visual inspection, there's only one hurdle left:  Will the engine pass the tailpipe test?  This needs to be tested.

If the engine will meet required emissions standards, that should be enough.  Whether it will comply with the factory ECU off line is the key question.  You could try running a simulation test; run a tailpipe test at the two rpm points, and see if the tailpipe readings meet requirements.

This begs the question:  If a non-stepper carburetor and Nutter Bypass do meet emissions requirements, why would Mopar bother with an ECU and other devices plus a maze of vacuum lines?  There are likely other parameters that Mopar had to meet beyond these simpler rpm tests conducted in state smog tests.  Emissions labs and the EPA test with the vehicle on a dynamometer and subjected to a variety of loads and operating conditions during the test.  The state test you describe is much simpler with far fewer parameters, and the Nutter Bypass or a non-stepper carburetor may not impact tailpipe readings under the two rpm/zero load tests.  Like you share, the key would then be to pass the visual inspection.

If these modifications do turn back the engineering to mid-'70s Duraspark equipment, they also turn back the clock on meeting emission requirements.  The 1989 emissions standards were far more stringent than 1975.  Mopar/Jeep's legacy engine, the BBD carbureted 4.2L engine, struggled to meet the 1980s standards.  This resulted in one of the busiest emissions equipment packages of that era.  (EFI was the typical method for meeting these emission standards.)  From 1981-90, the BBD carburetor and a mechanical advance HEI ignition distributor demanded a plethora of add-on emissions devices and unique solutions to meet EPA and California emissions compliance.

Without a doubt, many CJ and YJ Jeep owners have swapped later EFI/MPI engines into their vehicles just to avoid restoring and troubleshooting the OEM emissions system.  Often, the original 4.2L pieces have been stripped off and tossed, and the visual inspection fails.  Sometimes, a defective device does not have a service replacement part.  (There is a petition/exemption process in most states when repair equipment is unavailable, but this is just one more hurdle.)  A complete and emissions legal engine swap is often easier, with a referee station (the California procedure) inspecting the swap and its equipment to see whether all parts are in compliance.  With the cat and other required pieces in place, the tailpipe readings for a later EFI/MPI swap engine should always meet compliance in a 1981-90 passenger car or light truck.  

An LS Chevrolet V-8 engine is just one popular swap example, and here is my interview with Steve Roberts at Advance Adapters on performing such a swap to meet California's emissions requirements:  http://www.4wdmechanix.com/HD-Video-Advance-Adapters-Jeep-TJ-Wrangler-LS-V-8-Conversion?r=1.  Many would find this 50-State legal conversion easier than restoring a 1989 4.2L emissions equipped Jeep YJ to pass a smog test.  Just sayin'...

Moses

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Moses,

Thanks for the comments and the additional thoughts. You touch on a number of points, that I while agree with most, I feel are best left for another thread. The LS /TJ swap was interesting, but it looks to be a bit more work than my friend is interested in.

I was supposed to see the YJ back in the shop this week, but that was pushed to next week, so...The YJ's owner is concerned with getting his rig to pass the sniffer test, though his tags don't expire until November. Given that and based on your last post, I plan to give the Nutter Bypass a try, as a test. Then we can run the Jeep through the sniffer test and check the results. If it passes, great, and if not we are back at square one. Either way I will begin digging into and confirming the operation of the emission system components. I will get back in contact once we have done the test bypass and run the rig through the test station.

Question. Do you think that with all other emission related components functioning properly and the ECU not receiving errant throttle position signals, the use of a non stepper carb  should allow the rig to pass the basic tailpipe sniffer tests?

Moses, your thoughts, comments and insight are valued. Thank you.

 

Ford Guy

 

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Ford Guy...The loaded question:  Will a non-stepper carburetor pass a tailpipe test?  This remains to be seen.  If the cat, Pulse-Air, in/hg vacuum switches, CTO, EGR and other components are, with certainty, working properly or repaired, you could narrow down the tests to the Nutter Bypass and non-stepper carburetor.  I'm trusting your thoroughness to get credible results and findings!

I've always taken the OEM equipment restoration route, mainly because this eliminates second guessing aftermarket wild cards like we're discussing.  In this case, the non-stepper carburetor has passed in the past, so you should be able to achieve that result again—unless engine wear is a factor now.  Some component or device is creating the problem.  I would thoroughly test and assess the stock emissions equipment before trying the Nutter Bypass.  You'll be able to separate issues that way.

In this assessment, there are the on-engine devices, the chassis devices (cat, EVAP and Pulse-Air) and the sensors, vacuum switches and electronics.  Try to separate segments during your tests.  Don't overlook straightforward fixes like a new cat or fixing the Pulse-Air.  Downstream cleanup of the exhaust affects tailpipe readings.

Moses

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Good day Moses,

Thanks for the comments and suggestions. I too like to work from a known baseline, like the factory settings. I learned a long time ago those factory engineers are paid the big money for a reason. I rarely try to second guess them.

I didn't get back to the Jeep until early this week. I started by identifying what I could, checking routing of wiring, vacuum and vent lines and checking the operation of the components.

Starting with the original problem of the timing changing, I took a close look at the distributor and found what I thought was excessive lateral movement in the distributor shaft. Closer inspection showed there to be slight contact between the reluctor and the magnetic pickup. So, in went a new (not rebuilt) distributor, rotor and cap.

The timing had been a bit erratic, but with the new distributor is now steady. The Jeep sounds better, Hard to quantify, but I am sure you know what I mean. The base timing (vacuum disconnected) still will vary about 8* up or down when the engine is dry revved to about 2,500 rpm and let return to idle. That is, with base timing set at 6* BTDC (vac disconnected) and the engine dry revved and returned to idle, a follow up timing check may show the timing at 12-16* BTDC or it might be 2-6* ATDC. This timing change can be duplicated at will in the shop. Idle speed will vary 200-300 rpm with these changes, from a curb idle of about 750 rpm. Returning the idle to  750 rpm in Park doesn't affect the timing. I set the base timing at 10* and let the owner have his rig back. It still has the non stepper carb but everything else appears factory.

The owner took the Jeep through an State emissions testing station the following day. The Jeep passed on all accounts, visual and tailpipe. The HC and CO readings were both about 25% of what was allowed. These were the only measurements the test station took. It appears the erratic timing was causing the test failures.

So...The jeep is licensed for another two years and the owner is happy. But...I still want to identify the reason for and correct the changes in base timing, if that is a problem. I am supposed to get the Jeep back next week and I will continue with the identify and confirm part of the task. Pretty sure I will have more questions.

Ford Guy

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Ford Guy...Sounds like you avoided use of the Nutter Bypass.  Since the engine and emission system passed the visual inspection and tailpipe test, your mission seems accomplished.

So, why would Mopar bother with an MCU (ECU) and other devices plus a maze of vacuum lines?  These were necessary to make a 4.2L carbureted engine with a conventional (traditional mechanical and vacuum advance mechanisms) electronic distributor meet the 1980s to 1990 emissions tests.  The simpler rpm tests conducted in state smog testing (vehicle not under load nor on a dynamometer) are not the same as an emissions lab EPA test with the vehicle on a dynamometer and subjected to a variety of loads and operating conditions during the test.  A Nutter Bypass or a non-stepper carburetor may not impact tailpipe readings under the state's zero load rpm tests.  States are typically concerned strictly with the visual inspection and actual tailpipe readings.  

As for doing a Nutter by-pass and installing a non-stepper carburetor, these modifications do turn back the engineering to mid-'70s Duraspark era equipment.  They also turn back the clock on meeting emission requirements.  The 1989 emissions standards were far more stringent than 1975.  Mopar/Jeep's legacy engine, the BBD carbureted 4.2L inline six, had a huge challenge meeting 1980s to 1990 standards.  This resulted in one of the busiest emissions equipment packages of that era.  By 1987, as you know from your Ford experience, EFI/MPI and electronic spark management were the typical methods for meeting emission standards.  

From 1981-90, the 4.2L Carter BBD carburetor and a mechanical advance/vacuum advance ignition distributor demanded a plethora of add-on emissions devices and unique solutions to meet EPA and California emissions compliance.  For 1987-90 in particular, it's actually amazing that AMC/Jeep engineers were willing to tackle the EPA and California standards with a carburetor and an independent distributor ignition.  Why they did not put the Renix 4.0L EFI/MPI engine into the new Jeep YJ Wrangler chassis is confounding.  The engine was available to the 1987-90 XJ Cherokee; the YJ Wrangler ultimately went this route with the Mopar MPI 4.0L in 1991.

As for the fluctuating base timing and changes you're seeing under deceleration on this 4.2L, consider how the vacuum advance system and MCU overrides work.  According to Jeep engineers, the CEC (Computerized Emission Control Fuel Feedback System) attempts to control "undesirable emission to the atmosphere and maintains an ideal air/fuel ratio to provide an optimum balance between emission control and engine performance."  Can "ideal" be met with a feedback carburetor and a mid-1970s design ignition distributor?  Wishful thinking? 

CEC controls spark timing at specific load conditions, manifold vacuum conditions, and engine situations like deceleration or unloaded idle/base timing.  The MCU and vacuum circuits are capable of modifying the spark timing to suit load and operating conditions.

To clear the air on why the ignition timing fluctuates on your friend's 1989 Jeep YJ 4.2L inline six, these excerpts from the 1989 Jeep FSM explain the role of the various fuel/carburetor and ignition devices, including vacuum switches, that interrupt and modify the spark timing:

Jeep 4.2L CEC Fuel-Spark Feedback System 1981-90 Era.pdf  [For clarification, pages 4-126 and 4-134 have been re-copied and attached at the end of the PDF.]

In the FSM, the factory flow charts on troubleshooting and testing devices goes on for the next 21 pages.  If you are keenly interested in delving deeper into your friend's 4.2L CEC system and spark timing, I heartily recommend finding a 1987-90 circa AMC/Jeep or Mopar FSM.  (If more readily accessible, even an '81-'86 AMC/Jeep CJ FSM could prove beneficial.)  

I have a 1983/84 CJ FSM and this 1989 Mopar/Jeep FSM, and the two manuals cover all of the needs for 1980-90 Jeep 4.2L engines.  The 1989 manual's information in the attached PDF is valuable for typical 1981-90 4.2L Jeep inline six CEC system emissions troubleshooting.

When you find a copy of the manual and read the additional 21 pages, you'll see why a Mopar EFI conversion has been so popular for the 1981-90 Jeep 4.2L inline sixes.  Not only does that conversion update to 1991-up era Mopar 4.0L MPI parts and tune, it provides an immediate 50 horsepower bolt-on boost over the carbureted, emissions constrained 4.2L in line sixes.  The Mopar EFI Kit for the 4.2L is also 50-State emissions compliant and will produce considerably lower, 1991-up tailpipe readings.

Interested in the 4.2L Mopar EFI emissions legal conversion?  Here's a link to my article: http://www.4wdmechanix.com/Carburetion-Versus-EFI-Conversion-for-the-4.2L-Jeep-Inline-Six?r=1.  You'll find additional info at the magazine as well.

Moses

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