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Hi 4WD Mechanix,


I love the site and am so happy you are fielding questions from common folk like me!  Your troubleshooting guide for the 2.5L TBI helped me turn my limping MJ into a reliable daily driver!


Current set-up:  1987 MJ, 2.5L with AX-4 manual transmission, 2WD.


Future:  I want to convert to the multi-port FI.  I have all the equipment, ECU, harness, manifold, etc., from a donor engine.


Question:  Do I need to change the RENIX flywheel to the MPFI flywheel?  I have the MPFI flywheel on my work bench (the RENIX is in the truck), but whether the flywheel needs to be change will affect when I do the various upgrades.  A flywheel change requires me pulling the engine.


Thanks again!

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CasualMechanic...Your compliment is what makes the magazine and forums gratifying...Thanks!  Response to the 2.5L TBI coverage has been significant and meaningful...


The flywheel on the 2.5L TBI and MPI engines, as you know, provides the crankshaft position pulse.  This is accomplished by a set of carefully positioned "teeth" that set up a field as the flywheel rotates.  The odd "tooth out" helps the ECU or PCM determine the top-dead-center (TDC) position of #1 piston and the corresponding crankshaft position.  The sensor at the top of the clutch housing is a pickup that interacts with this clocked set of cogs on the flywheel.


Every other rotation of the crankshaft, #1 piston is up to fire.  As for ignition, though, the ECU or PCM is simply looking for a reference to TDC:  The rest of the spark timing and fuel management is controlled by the computer itself.  On your TBI engine and the MPI engine, an ignition distributor directs the controlled spark to each of the distributor cap spark leads, using a typical rotor.  Like a conventional distributor, the distributor shaft gets driven by the camshaft.


You actually have two concerns: 1) whether the TDC signal is the same (pickup and flywheel wise), and 2) will the TBI distributor interface with the PCM and other components.  The second point simply clarifies whether you need to change out the distributor as well.


Here's the rundown on the parts...Beginning with the distributor, the TBI distributor is not the same as the MPI distributor.  You need the MPI distributor for this conversion...As for the flywheel, again you find a different part number for 1991-up MPI engines: Mopar 53005526.  Your OE flywheel for the TBI engine is a Mopar 33002408.  The TBI flywheel is no longer available from Mopar, the 1991 up flywheel is still available.  Here are the Jeep models using the TBI type flywheel, none is newer than 1990:




The 53005526 MPI 2.5L flywheel has two counterpart flywheels: 53020578 and 53010555AA.  Jeep coverage is wide, from 1991-2002 in 2.5L MPI applications.  (The late TJ 2.4L four uses another flywheel type.)   There is a difference between the TBI and MPI flywheels, and it's good that you have the donor flywheel. 


If I were doing this job on a 2WD Comanche MJ, I would simply remove the transmission and clutch housing to access the clutch assembly and flywheel—unless you prefer removing the engine to install the MPI parts.  The engine is relatively short and easy to support while removing the transmission, there's enough room to tilt the engine slightly for access to the clutch housing bolts, and the flywheel is readily accessible at that point.  The factory method for changing out the flywheel is just a few steps beyond a clutch cover and disk replacement, and this might be a smart time to renew the clutch assembly. 


Without a transfer case, you only have the rear driveline to remove, the Aisin AX4 is relatively easy to remove, and you're there. Make sure the used flywheel has a flat, crack free and usable clutch surface.  If the flywheel needs machining, find a capable shop that understands the Jeep/AMC flywheel designs and has proper tooling for the surfacing.


Let us know what the actual difference is between these two flywheels, it should be apparent.  A couple of photos would be very helpful, CasualMechanic...Thanks!


Everyone is here for more questions on your project...You will also need the higher output fuel pump that I discuss in the recent/ongoing YJ Wrangler Jeep 2.5L TBI troubleshooting forum exchange: 





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Thanks Moses, and darn! 


I will be converting to 4X4 in the future but I like to do small bits at a time rather than all at once.  Since the flywheel needs to be changed the best time is when I change the transmission.  So it seems like I will be doing all at once: change transmission, change flywheel, add MPFI, all required components, transfer case, and drive shaft.  I have the other distributor so that will be part of the swap.  and oh, the fuel pump too.  yikes!


I was able to scrounge a picture of the Renix flywheel before I swapped the engine last time.  Upon inspection, there is definitely a difference: the RENIX has a bunch of rounded teeth (lack of better word) whereas the MPFI FW has four sharp notches for the CPS to work from.





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CasualMechanic...Thanks for confirming the parts difference with these very useful photos. This is dramatic, the Renix flywheel and the Chrysler MPI flywheel each use a different "pulse" approach. The parts books were correct this time!


From your quality photos, the MPI flywheel looks to be in good condition. True?



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  • 5 months later...

Hi Moses,


To confirm; the fuel pump needs to be changed when converting from TBI to MPI.  Per your link, you listed the Chrysler #'s 4637192 & 4637193.  On the linked site, they indicate the kit is for a wrangler.  Do you think the same kit will work in my 2.5L Comanche fuel tank?




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CasualMechanic...Here are two PDFs with illustrations and part numbers for your 1987 MJ and a 1991 MJ.  Yours is lower pressure TBI, the '91 is higher pressure MPFI:


1987 Comanche Fuel Tank and Pump.pdf 1991 Comanche Fuel Pump.pdf


Zoom in for details and part numbers.  This should clarify...





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Yep, I'm still thinking of your 2.5L TBI conversion to 2.5L MPI.  The two fuel pumps listed in the parts PDFs I provided represent your current pump and the pump you need for MPI...See the content and references to the MJ model pump for 1987 and 1991, respectively.  Zoom-in on the illustrations, the electric pump is lower left (item #25) in the 1987-88 illustration.


I do have a major concern around the location of the pump module opening in the fuel tanks.  From the illustrations, it looks like your '87 has a side mount; the generic drawing for the 1991-up pump shows a topside fit (like the Wrangler) if the drawing is accurate for all applications.  I would compare the two MJ Comanche fuel tanks to confirm.  If you keep the side mount type, maybe a later pump (MPI type) can be fitted to work with your pump module stand if the fuel tubing size is adequate.


Note: There are aftermarket sources for these pumps, and you can change just the pump without replacing the whole module.  Hasbro and others make replacement pumps, you need the '91-up version for MPI pressures.


Another wild card here is the PCM for 1991 and triggering the fuel pump.  You do need the later engine and chassis wiring for the PCM and also verification of which wires run to the fuel tank/pump/gauge sender on a Renix versus Chrysler (1991-up) system.  Compare the two wiring systems (1987-90 versus 1991-up), including color coding, wire gauge, etc.



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  • 7 months later...

Hi Moses and CasualMechanic...Excuse me for "reviving" this old topic, I'm thinking of doing this conversion myself and this forum proved itself one of the most reliable sources of information.


My first question is: was the conversion worth it? I understand the distributor has to be swapped in the process, but does changing the flywheel require to change ignition control module and coil too (maybe also crankshaft position sensor)?

I didn't find anywhere a good explanation of the amount of components and work required for the entire conversion job, it would be really awesome if you could share your experience about it. :)

Thank you for your time, guys!

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Maybe an update available from CasualMechanic here?  It would be great to get an overview of how this conversion turned out...


An MPI swap, if practical and manageable from a parts and labor standpoint, would provide some performance gains.  In the late 'eighties, when GM stayed with TBI on truck engines, I attended a press launch of the '89 models.  I recall a journalist colleague asking a GM engineer why TBI was in place instead of port injection like the GM performance cars.  Ford at the time had MPI on its truck engines.  The response was interesting, as this engineer was a motorhead and not just a corporate spokesman.  His belief was that the use of an intake plenum with a central TBI unit actually produced more torque at a usable "truck" engine speed. 


I would argue that the uneven flow of air/fuel in any intake manifold with a central carburetor or TBI is obviously inefficient.  When we played with the original "Mopar" EFI conversion* for the 4.2L Jeep inline six, the Electromotive fuel-and-spark system used a Holley TBI unit from an OEM Dodge V-6 application.  That system was a big improvement over the BBD carburetor, though the best was clearly yet to come.  By the mid-'nineties, Mopar latched onto factory off-the-shelf MPI from the 4.0L inline six and adapted that as the second generation Mopar Performance EFI Conversion for 1981-90 4.2L/258 retrofitting.  It was a 50-State legal system and added nearly 60 horsepower to the 4.2L engine with no other changes.  This was way beyond the gains of TBI.


*The Electromotive system came into existence in the early 'nineties and is now virtually unknown by most Jeep inline six buffs.  It was a bold attempt to improve the 4.2L engine without changing the intake manifold.  The TBI had an adapter to the OE BBD two-barrel carburetor intake manifold.  Unfortunately, the inline Jeep 4.2L six's stock intake manifold is very inefficient.  Flow to the end cylinders is impaired, cylinders were inherently out of balance, and this engine begged for MPI. 


On a 2.5L TBI change to MPI, the gains would be proportionate, yet MPI with injection at each port and minimal intake runner length issues is clearly a major gain.  As a retrofit, however, MPI from a 1991-up engine, swapped onto a 1987-90 model TBI engine, would have to offer a major incentive, something as serious as TBI parts no longer being available.  This conversion is not for the faint of heart.  It can take considerable time and thought to sort out each of the distinct differences between these two systems.


CasualMechanic is enthused and objective.  I'd really like his input here, as this task, in my view, is anything but simple.  The wiring alone would keep one busy for some time.



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Thank you Moses,

that's very interesting information.
I'm afraid I have no major incentive other than power gain, please consider that I cannot legally change displacement (so no 4.0l swap) here in Italy, hence real driving impressions from CasualMechanic would be very valuable to me. Even if the gain was small, the conversion makes other modifications possible (4.0l throttle body, injectors upgrade, ecc), so I'm still uncertain on the way to go.

Moses, I hope you excuse me for an off-topic question: around here TBI is 76KW on documents and MPI is 89KW: do you know of any 76kw engine?

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Hi, evercloud!  Here is a Wikipedia account of the horsepower and torque ratings on the 2.5L TBI versus MPI engine.  The carbureted version was used in 1983-86 Jeep CJ models:


AMC 150/2.5 Compression Horsepower Torque:


One-barrel carburetor 9.2:1 105 hp (78 kW; 106 PS) at 5,000 132 lb·ft (179 N·m) at 2,800


Throttle body injection (TBI) 9.2:1 117 hp (87 kW; 119 PS) at 5,000 135 lb·ft (183 N·m) at 3,500


Multi-point fuel injection (MPFI) 9.1:1 120 hp (89 kW; 122 PS) at 5,250 139 lb·ft (188 N·m) at 3,250


There is no account of a 76 kW variant.  Perhaps you found a reference to the late 'seventies to 1983 carbureted G.M. 2.5L four (Iron Duke) used in Jeep CJ models.  Here is some information on that engine:




What I find very interesting is the relatively insignificant horsepower gains one could expect from an MPI conversion on the 2.5L AMC four.  In fact, other changes beyond just the MPI improvement could account for the difference between 1984-90 (TBI) and 1991-up (MPI).  Worth noting, the MPI 2.5L develops maximum torque at a lower rpm, and the TBI develops maximum horsepower at a lower rpm.  Not dramatically different speeds in either case, though. 


Both engines need to spin quite fast to develop peak horsepower and torque.  I recall this from testing the new TJ Wrangler in 1997, comparing the 4.0L six's performance to the 2.5L four.  I wondered why Jeep did not strive for more low-end power in the 2.5L pushrod engine.  These higher speed power peaks were the trend for import engines, and Jeep was in step with the competition at least.  I found that holding the 2.5L MPI four in lower gears on long grades, just to keep up with traffic, did little for fuel efficiency.


There are a number of upgrade possibilities for the 2.5L TBI engine that would even the horsepower and torque without doing an MPI conversion.  Air intake and exhaust improvements, in particular, could make up the horsepower and torque.


So, rather than plunge into an involved MPI conversion or a 2.5L MPI engine swap, I would consider some moderate engine upgrades to the 2.5L TBI setup.  One example would be a cold air intake.  My son-in-law made the system depicted here from a Honda AEM kit:




For exhaust, I could not turn up an aftermarket header for the 1987-90 2.5L TBI engine.  Perhaps another member knows a source.  For the '91-up MPI 2.5L, there are several header sources. 


For the TBI engine, I would concentrate on a performance catalytic converter and open up the exhaust system with a custom head pipe and smoother flowing pipe to the cat and muffler.  Use a performance muffler and a less restrictive, smoother tailpipe.


Here is a Random Technologies lower restriction cat installation on the magazine's 4.0L exhaust system.  This provides some ideas:




These basic modifications could be easily "reversed" if necessary.  The gains would be measurable and certainly equal to or exceed the stock output of a 2.5L MPI engine.


If the engine is in top condition, turbocharging might be a consideration.  There is also some potential gain from a custom grind camshaft; however, a higher rpm torque or horsepower gain would be of less value than the bottom end and mid-range power boost of turbocharging. 


Turbocharger boost would have to be very mild at the current 9.2:1 compression.  A lower compression ratio would be desirable for turbocharging, and this could be accomplished with either a thicker head gasket height or piston changes.



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Thanks for your help, Moses!
It's inspiring to see somebody so generous with his time and willful to provide so many informations. :)

Before anything else, I have to say that you were right. I found sources referencing to the Iron Duke, there is some confusion about european jeep engines.

Then I found the answer: horsepower was measured until 1990 with DIN system in Europe and "SAE net" in US. That's why my jeep documents (and most of 2.5 tbi wranglers in europe) say it's a 76kw engine.
This ultimately confirms that I have the same US engine (117hp SAE net), making the MPI conversion less and less appealing. 

Here's the reference:
http://www.automobile-catalog.com/make/jeep/wrangler_yj/wrangler_yj_hard/1990.html says

displacement: 2471 cm3 / 150.8 cui, advertised power: 76 kW / 102 hp ( DIN ), torque: 180 Nm 

As for my upgrades:

My car has no cat (I can legally do that), no "horn" before the air filter, a sport muffler and some other small improvements. As turbocharging here is illegal, I guess I will follow your suggestions and try a cold air intake and a sport exhaust manifold.
Thank you again, I hope you will find my information useful too :)

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Evercloud...If you find an exhaust header source for the TBI 2.5L engine, let us know.  I did some research on cylinder head and exhaust manifolds for the 2.5L EFI engines.  The 1987-1990 TBI cylinder head is part number 83502487.  The 1991-up MPI head is a #33007115.  (I can provide casting numbers, too.)  The exhaust manifolds also changed with the head redesign.  This may explain why I could not find '87-'90 header offerings for 2.5L TBI engines.  There may not be enough demand for an earlier style header.


Note: This may also point to a problem converting TBI to MPI.  If the MPI injector system requires the later cylinder head, it would be necessary to update the cylinder head as well.  The cylinder head gaskets are different by part number, and while this could indicate possible cylinder block differences, the factory replacement short block assembly (fitted block without the head) is the same part number for both 1987-90 and 1991-up 2.5L fours: 04637399.  We still need to confirm whether the later head is a "bolt-on" to the earlier block.  (As a point of interest, carbureted 2.5L engines use a different cylinder head and exhaust manifold than either of the EFI engines.)  Curious where CasualMechanic got with this issue and his findings.


The cold air intake and conical air filter is a gain of sorts with one caution:  An open face air cleaner is vulnerable to water exposure.  For a Jeep that fords streams, a snorkel system would be advisable with the filter near the vehicle's roof height, typically alongside the windshield.  4x4 engines have been known to "hydro-lock" when a large dose of water gets drawn through the air cleaner.  Water entering a running engine can cause piston and connecting rod breakage.  The moving rod and piston cannot compress the water!


There were several OEM air filtration systems with grille opening intakes that could suffer from hydro-lock.  When the vehicle plunges into a stream and sucks water through the intake air stream and into the cylinders, the result is catastrophic.  In the 1980s, Toyota 4x4s and the Ford Bronco II and Ranger pickups were known for this problem. 


Jeep developed more isolated air boxes for the Wranglers and XJ Cherokee, upward in the engine compartment with intake ducts pointed away from the water source.  The stock YJ and TJ air boxes are good examples of an OEM concern for keeping water from entering the induction system.



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  • 2 weeks later...

Hi Moses!  Engine upgrades slowed down because of Christmas and New Year holidays (best wishes, BTW!), but that gave me time to do some research. 

Talking about intake manifold, I believe TBI and MPFI  headers to be the same as I read people swapped engines between tbi and mpi (and vice versa) easily by leaving manifolds (together with harness, tb, ecc) from the recipient car.  I also found websites selling the same intake/exhaust gasket for all 2.5 engines.

As for my choice, I found specific Borla headers on the bay, but I'm starting to think it could not be worth the money (more than 500€ for a few hp from what I read).

Thank you for the warning about water, I'll consider that although I'm not brave enough (and my car is not ready) to cross streams or rivers at the moment! ;)

Have a nice day and a happy new year!



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