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Hi all,

I am new to this forum, and to Jeeps in general. I recently got a 1990 Comanche 2wd, with the 2.5 TBI engine, and over the past few months have been trying to get it going. It was parked for about 10 years due to a bad transmission, and possibly the issue that I have now encountered. I am going to try to be as thorough as possible, but I am stuck and need some outside eyes. My dad is a professional mechanic with his own shop, so I have tools etc. at my disposal, but this thing has both of us stumped.

Issue:

I replaced the fuel pump and tank, flushed the system, along with did some wiring repairs that rats had eaten. I believe I have everything on the fuel system and electrical system correct. I can turn the key on, the pump will prime, and the engine will start, but immediately dies. I can start it, and begin "pumping" the accelerator, and it will remain running as long as I do so, but if I stop, or try to hold the accelerator in one position, it will die. It appears to have sufficient fuel pressure while running, it seems as though the injector pulse is going away before it is able to idle. I can manually ground one side of the injector, and force it to fire, and keep the engine running this way, by pulsing the injector myself, but it will not do this on it's own. I know the ISA is bad, and it is not working, but it doesn't seem like it would cause this issue?

Parts I have replaced with new ones, no change in symptoms: Computer, fuel pump, fuel tank, CPS (Crank sensor),

Parts I have replaced with other used parts, no change in symptoms: TPS, Ignition module, Fuel Injector, Complete throttle body, fuel pressure regulator.

Any help would be greatly appreciated. I am attaching a youtube video of everything it is doing, along with a shot of data from the scanner while it running and attempting to run. For some reason my scanner doesn't pull any codes, it will only show data.

Thanks,

crunknastyvdubs@hotmail.com

 

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Crunknastyvdubs...From the video, it sounds like your engine is starving for fuel.  Before second guessing and replacing more parts, I would address the ISA issue and adjustment it properly.  Also, to provide some level of insight, use my recommended induction timing light test described in red text at this current 2.5L TBI topic:

My suggestions will narrow down the ignition versus fuel supply trouble, though this does sound like a fuel issue.  Also check out the laundry list of 2.5L TBI topics and replies at the forums' Jeep YJ Wrangler section.  Here is one series of exchanges where I include PDFs of ISA adjustment and troubles, including forum member's comments about a stripped ISA motor gear:  

Glad to discuss this further once you narrow down the issues.  ISA has been covered, use the forums' search box to find additional troubleshooting and ISA comments.  You've replaced the core of parts, there is something left undone.  From your description and graphic video (helpful!), the injector pulse opening does not occur.  Also check all grounds for continuity and ohms resistance back to the battery ground post.  Check the fuel pump ground wire for resistance...Ground gremlins are common.

Moses

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Thanks Moses,

I am going to get a new ISA ordered, and go back over the grounds, and let you know what I come up with, and we can go from there. Thanks again for the help. All of my searches and research on this topic seem to have led back to you being the leading authority on these, so I figure if anyone can help me, it'd be you. This forum seems to have the most info on these by far. I have been looking for a complete running 2.5 truck locally that I can at least look over to make sure I've got all my connectors and everything right, and nothing is missing, but no luck on that yet.

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Thanks for the kudos, crunknastyvdubs....I look forward to your findings.

I strongly encourage finding a factory workshop manual for your Comanche.  1987-90 would work equally well when looking for a used FSM.  As a professional mechanic across my entire adult life, a work history approaching half a century now, I have been academic in my troubleshooting and work.  As the author of seven technical books with Bentley Publishers, including "best sellers", I must emphasize the importance that first generation factory service manuals have played.  My books earned Chrysler/Mopar Performance, G.M. Motorsports and Ford SVO official part numbers due to their accuracy and reliability.  That accuracy was a combination of in-depth experience plus use of the right OEM resource manuals and data.

My office has four bookcases, each 72" high, packed with factory workshop manuals and OEM reference materials.  When I volunteer information on the 2.5L TBI engine, many facts come from a 1989 YJ Wrangler FSM on my reference shelf.  Every vehicle that has stayed in our family or work "fleet" gets a new or used FSM.  The '89 manual was provided by Mopar staff for my Jeep book projects.  It has also served well when working on my son-in-law's '87 YJ Wrangler, which began its life as a 2.5L TBI model.

Quality parts manuals are helpful for discovering parts interchangeability and usage.  I have plenty of catalogs and access to online catalogs like we all do. 

So, when you talk about viewing a facsimile vehicle, by all means, do so if you can find one.  Also look for an appropriate MJ Comanche FSM.

Glad to assist with both my experience and research...I am also planning more how-to videos on relevant subjects, videos that can stream as Vimeo On Demand rentals.  (I have experimented with a few that serve well.)  These days, many prefer audio-visual training materials!

Moses

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SOLVED!

Thanks for your help. It ended up being a bad MAP sensor. I checked all of the grounds and everything as you said, and everything checked out okay. Installed a new ISA and got it working right with no change. We weren't completely satisfied with the replacement fuel pump we put in, so we pulled it and come to find out it was the one for the 6 cyl engine, and apparently there is decent difference in output. I don't think this would have made much difference, but while we were doing that, I just thought to myself, since I've already thrown so many parts at it, the only thing I haven't replaced pretty much was the MAP, so I got one, installed it, and she fired right up and idled great.

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crunknastyvdubs...This is great!  The MAP would show up on a higher end scan tool for OBD-I.  You nailed it anyway...

Yes, pump pressure is much higher for the 1990 4.0L six-cylinder engines, they have multi-point injection, which operates at higher pressures than TBI...This would be too high for TBI.  Even with the pressure regulator returning excess fuel to the fuel tank, the 6-cylinder fuel pump has higher fuel volume than the TBI can handle.  

Did you install the correct (2.5L application) TBI fuel pump?

Moses

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