MJ Comanche Tech. Stock & Modified. MJ (86-92)
All discussion regarding the Jeep Comanche, the pickup truck related to the Cherokee platform.

At my wit's end

Old 06-09-2015, 01:12 PM
  #1  
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Location: Brownsville, Ohio
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Year: 1991
Model: Comanche
Engine: 4.0L HO
Default At my wit's end

I purchased a 4wd comanche a month ago, and I knew it was going to need repairs done, I had no problem with it. But since then, I've run into a problem that I have done everything within my mechanical knowledge boundaries to fix, yet I can't correct the issue. The truck will idle all day just fine, and if in park or neutral, you can bring it all the way to rev limiter, put it in gear and it will idle fine as well, but give it gas and it will hesitate and stumble so bad it can't even move forward (reverse is O.K., but just ok). I have replaced EVERY. SINGLE. SENSOR on this thing and cannot seem to fix it. I replaced my fuel pressure regulator, distributor, coil, cap, plugs, wires, and stator pickup wire as well. The only thing I can think of left is the ECU itself went bad, but before I spend 125+ dollars on that i would like some confirmation from fellow jeepers.

Last edited by co-MAN-che; 06-09-2015 at 01:30 PM.
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Old 06-10-2015, 11:03 AM
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Had a similar problem, but, it was my fuel pump that was failing. Swapped it out, and it runs just fine. Apparently, it would pump enough fuel for idle and revving with no load, but, under load, it would starve for fuel. Check your fuel pressure.

Jerry
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Old 06-10-2015, 07:31 PM
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Can do boss. I happen to have a new pump but figured I didn't need it because I measured 39 psi while idling. I'll go ahead and install the pump
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Old 06-16-2015, 01:42 PM
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installed the pump, and had a noticeable improvement, but the improvement was that it will now wind up high with little power to the wheels rather than nearly stalling out this time
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Old 06-16-2015, 10:53 PM
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Originally Posted by co-MAN-che View Post
I purchased a 4wd comanche a month ago, and I knew it was going to need repairs done, I had no problem with it. But since then, I've run into a problem that I have done everything within my mechanical knowledge boundaries to fix, yet I can't correct the issue. The truck will idle all day just fine, and if in park or neutral, you can bring it all the way to rev limiter, put it in gear and it will idle fine as well, but give it gas and it will hesitate and stumble so bad it can't even move forward (reverse is O.K., but just ok). I have replaced EVERY. SINGLE. SENSOR on this thing and cannot seem to fix it. I replaced my fuel pressure regulator, distributor, coil, cap, plugs, wires, and stator pickup wire as well. The only thing I can think of left is the ECU itself went bad, but before I spend 125+ dollars on that i would like some confirmation from fellow jeepers.
Of my Tips in the Pink Link below, which have you completed?
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Old 06-17-2015, 04:36 PM
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i did 4 and 5, still need to do 1 and 3. I do not have a C101 connector as per the 1991 model year
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Old 06-17-2015, 08:12 PM
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Originally Posted by co-MAN-che View Post
i did 4 and 5, still need to do 1 and 3. I do not have a C101 connector as per the 1991 model year
C101 went away in 89.
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Old 06-17-2015, 08:57 PM
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I have repaired my grounds and checked for excessive resistance in my wiring between my TPS. I am still feeling it is my ECU because of no change in vehicles performance.
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Old 06-19-2015, 05:57 PM
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I have replaced my PCM/ECU (whichever it correctly is) and i now have a check engine light on. I'm getting the code for an open or shorted alternator field control circuit. Now while idling the jeep is acting like it did with the old computer in a drive gear, it's hesitating and stumbling really bad just sitting in park. Will acquire new alternator for that is what research is pointing out to me and see if that corrects my issue.
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Old 06-21-2015, 07:51 AM
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Bleepin Jeep has a good alternator test tutorial on video, could try that on the alt prior to purchase, the proceedure will also detect any battery problems that could be developing.
So with the original PCM/ECU there was good in park/neutral but bad in drive.
Could there be a trans control unit problem causing the torque convertor to lock up at a wrong time.
Did you replace or have you checked the setting of the Throttle Position Sensor or is that a self adjusting on the 91"s? A new TPS and good adjustment resolved some erratic issues for me. (I was surprised by how much it affected my overall engine management) If it's the HO self adjusting type (by the ECU?) could the TPS be sending some bad info to the control?
Sorry if I'm out to lunch here, is there a good vacuum line to the Manifold Absolute Pressure sensor? or a manifold vacuum leak that would provide incorrect data readings when under load off idle?
Guess I'd also be suspect of ignition timing, but you really seem to have that covered, perhaps others understand best how ignition advance is controlled on a 91?
Another area I've seen come up, could the cat convertor be blocked, seen so much junk come out of the honeycombs design of some systems.
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Old 06-22-2015, 11:53 AM
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New Development. This just in. The truck now runs halfway decent and the check engine light is off after an installation of a new computer and alternator, but seems to now possess a shifting problem. Now that I have really good power in R, 1, 2, and 3 is better, I put it in drive and paid close attention to the sound and feel. I noticed that the vehicle jumps (that regular little jump we are normally accustomed to when we first put it in gear) in the 1-2 location, very positive in R, the jump is reduced at 3, but in D it is nonexistent, and when I went down the little straightaway of low traffic road in front of my homestead, I heard a bogged-down sound that as I gained speed to about 40, turned into a sound of abnormally high RPM's with no significant gain in speed, as though the truck can't shift up or down accordingly with the load. Taking off in D it seems that it doesn't start in 1st then work its way up. Transmission fluid and filter changed within 30 miles

Last edited by co-MAN-che; 06-22-2015 at 11:55 AM.
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Old 06-25-2015, 08:11 PM
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Year: 1991
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I am thinking my problem could be a transmission governor pressure sensor/solenoid or a transmission control unit. Would I have both of these or only the governor pressure sensor/solenoid, and could these be my issue?
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