1993 Xj 4.0, low idle, stutters, and check engine light.

So I bought a 93 xj with a 4.0, AW4 automatic. the engine had a blown head gasket and ran like it had a dead hole. a compression test confirmed (2) cylinders @ 90psi.

So i installed a low mile excellent running engine out of my rolled 94, and it still runs the same way.

It will start for a second, then die. eventually, it does start, but it idles very low, and hesitates when i give it fuel. Also the check engine light is on.


To date, all of the engine sensors were replaced with the '94 stuff, as well as a new 02 sensor. I also replaced the CPS with a god used one from the 94.

I have tested fuel pressure out of the fuel rail, and with the fuel pressure reg vacuum line unplugged, I am getting a solid 40psi.

I also rigged up a factory fuel pump from a 5 gallon can to the fuel rail intake, no change. In my estimation, that eliminates the fuel delivery system from the equation...agreed?


I checked the codes for the CEL, and it showed up as a PCM hard failure.
Ive replaced the PCM with a functioning unit, still no change. I have several jeeps at my disposal and plugged the pcm into one of those, it ran fine with it.


I have another wiring harness, but I hope I dont have to go that route. It sure seems to me that there is a short somewhere.

Id love to hear the experts opinions....

So, what is the missing element?

You have fuel pressure up to specs, but are all of the injectors working?

Lets A**ume--yeah, dangerous--that spark is Ok as the engine runs. A**ume also that the compression is good in your transplant 4.0 engine.

You can check for 12 volts to each fuel injector with a multimeter. With the engine running you can listen for the injector to "click" as it functions. You can test resistance across the injectors. You can check for the PCM firing impulse with a NOID light or a top of the line digital meter like a Fluke.

Did you have the distributor out of the engine at anytime? Might be off a tooth or two.

Thanks for the response, Joe.

Actually, the new engine is flawless. Ive owned it for several years, and have done religious maintenance. there are no bad injectors, no bad sensors, compression is 150 in every hole, and it was running perfectly with its components 10 minutes before I pulled it.

I haven't had the dist out at all. I pulled the complete running engine out, and set it in the 93.


when I checked on the Check engine light, the code was "PCM internal failure". now, I know the PCM is good because it runs fine in another jeep, so i really feel like this is the area I should be focusing on.

moneypit_k5 said:

Thanks for the response, Joe.

Actually, the new engine is flawless. Ive owned it for several years, and have done religious maintenance. there are no bad injectors, no bad sensors, compression is 150 in every hole, and it was running perfectly with its components 10 minutes before I pulled it.

I haven't had the dist out at all. I pulled the complete running engine out, and set it in the 93.


when I checked on the Check engine light, the code was "PCM internal failure". now, I know the PCM is good because it runs fine in another jeep, so i really feel like this is the area I should be focusing on.

Click to expand...

Well, the PCM code, which you didn't give, is telling you the PCM is bad, but when you hook it up on the other Jeep it runs fine without a code.

So, where is the problem? Sounds like it might be in the harness, but where? To the synch sensor? To the injectors? Engine with sensors and injectors from the 94 and everything was fine. Now something isn't right--Ok, gotta ask again--are the injectors firing, i.e., are they receiving the ground impulse from the PCM through the harness?

Good luck.

ok, got a chance to screw around with it tonight.

I checked power at the injectors, im not getting anywhere near 12v. More like 1-3v. I checked it with the key on, and with it attempting to run.

The origional check engine code must have been a fluke, because now im not getting the same one.
Now Im getting a code 12 and a code 14.

I understand the code 14 to say that the map sensor signal input is too high or too low? I checked the map sensor with an ohmmeter according to the haynes manual, and it failed their test. I have an idea their test is bunk, but i went ahead and threw a new map sensor on it. It changed nothing.

aside from checking for 12v at the injectors, what other tests should i do on them?

Ohms test on the injector itself, 14.2 ohms + - 1 to 2 ohms.

Sent you a PM.

Well, the injection system checks out according to joe's diagnostic sheet:

injectors all at 14.2 ohms give or take.
test light @ injector plug is lit while cranking, and upon startup.
fuel pressure is present, injectors are opening and working as they are supposed to.

Ok.

I would start and test each sensor, measure reference voltages and resistances, that sort of thing.

Next, run the engine management harness with an ohm meter back to the PCM.

That will probably take you longer than it would to just swap out the engine management part of the harness, but if you just swap the harness and its still not fixed you will have to go back through it all anyway.

Good luck.

I actually spent the day running resistance and voltage tests on all hte sensors. everything checks out as far as i can tell, which brings to mind the map sensor. ECM code 13, code 14 are still coming up, but the map checks ok.

My extra harness is from a 94. I started looking at it today, im thinking they are not interchangeable. Any thoughts as to what I would do with the ballast resister if I did swap to the 94 harness?

One thing im thinking, is that I would be swapping out the 93 ECM for the 1994. If I swapped both the harness and the ECM, wouldnt that circumvent the need to ohm check the existing harness?

Ok, you can just eliminate the ballast resistor, the factory did 94/95.

I don't see any reason you couldn't do both, heck, they are both OBDI.

Well Joe, I wanted to give you an update. After spending a dolid 8hr day ohm checking the harness, I im done. I sold it yesterday, and frankly Im glad to see it go. I told the guy to let me know what the problem was if he found it.

Wish we could have been more help to you.

Getting another?

joe_peters said:

Wish we could have been more help to you.

Getting another?

Click to expand...

Hey I appreciate the help youve given me already!

this XJ is just one of many in my garage. I bought that one to drive over the winter and resell in the spring.