Odd PCM issue.. I could really use your thoughts!!

I have a 1998 XJ Classic, 4.0, AW4, federal emissions, 108K miles that I just recently purchased. It was idling very high at about 2K RPM or more in park and 1.5K or so in gear when I purchased it. My thoughts were that it might have a bad TPS sensor or IAC valve. I replaced both of these with factory Mopar parts and still had the same trouble. I borrowed a PCM/ECU from a friend's 1999 XJ Sport, 4.0, AW4, federal emissions, 145K miles and my XJ ran perfectly. The only trouble I saw was that the oil pressure gauge was not working with the loaner PCM/ECU. It read "0", but reads around 40PSI with the original unit. Figuring that I had a bad PCM/ECU I purchased a used one off of the internet that was reprogrammed with my vehicle information. It has the exact same part number (56041534AF) and gives me the EXACT same trouble. This unit is supposed to be the correct one for my vehicle, so far as my research shows. I do not have the part number from my friends unit at the moment, but I will post it tomorrow if it makes a difference. I'm beginning to wonder if the wiring harness was replaced at some point or something like that.

Just guesses &thoughts-

-try the 'new' PCM in the friends XJ to verify it is really ok.

-Inspect all pins on connectors very carefully.

-is the fast idle changing with warmup?
-is it going to closed loop?
-any other clues?
-clean and check ALL grounds + engine and PCM.
-HVAC/cruise vaccuum controls working normal?

-Gently move harnesses and connectors at PCM and around throttle area watching for intermittent problem.

Good Luck,
Orange

orange said:

Just guesses &thoughts-

-try the 'new' PCM in the friends XJ to verify it is really ok.

-Inspect all pins on connectors very carefully.

-is the fast idle changing with warmup?
-is it going to closed loop?
-any other clues?
-clean and check ALL grounds + engine and PCM.
-HVAC/cruise vaccuum controls working normal?

-Gently move harnesses and connectors at PCM and around throttle area watching for intermittent problem.

Good Luck,
Orange

Click to expand...

Good thoughts... thank you. The fast idle is indeed changing with warm-up... but in the wrong direction. It actually gets faster! I will check the "new" PCM in my friends XJ as soon as I can. I actually sold the XJ to him last week because he needed a reliable DD and this "new" XJ is an extra vehicle at the moment. It's supposed to be my new DD, but I have something else to drive for now. With his PCM working so well in my vehicle I feel like I have two bad PCMs... which at the same time seems very, very unlikely. I will clean and check all grounds, etc. The PN on my friends PCM is 56041 492AE by the way.

Sounds like an intake leak around the manifold.

Get a spray bottle, fill it with water, set the nozzle to "stream" and then spray along the edge of the manifold where it mounts to the head, listen for changes in the idle.

I was going to say vacuum or intake leak. THey usually get worse as it heats up because the tolerances increase.
Have a good time getting to the underside bolts of te intake if that's the case. You'll need some extensions and someone with contortionist hands.

??? but its OK with friend's PCM ???

Sounds more like electrical.

Weak grounds causing weak signals from sensors?

-What is Battery volts while this is going on? (Jeeps do NOT like low voltage in the electrics.)

-check volts engine block to battery negative (-) post?

----OP- try all THREE (3) PCMs in the GOOD Jeep!!!----

-Check for scan codes.

Very strange problem, gotta be more clues hopefully.

orange said:

??? but its OK with friend's PCM ???

Sounds more like electrical.

Weak grounds causing weak signals from sensors?

-What is Battery volts while this is going on? (Jeeps do NOT like low voltage in the electrics.)

-check volts engine block to battery negative (-) post?

----OP- try all THREE (3) PCMs in the GOOD Jeep!!!----

-Check for scan codes.

Very strange problem, gotta be more clues hopefully.

Click to expand...

vacuum leaks can come and go. especially if it's a rubber hose, they change with temperature.
you are correct that a better test would be to take the suspect PCM over to the other jeep. it would at least rule out the PCM as the issue if it works in the other XJ.

orange said:

??? but its OK with friend's PCM ???

Sounds more like electrical.

Weak grounds causing weak signals from sensors?

-What is Battery volts while this is going on? (Jeeps do NOT like low voltage in the electrics.)

-check volts engine block to battery negative (-) post?

----OP- try all THREE (3) PCMs in the GOOD Jeep!!!----

-Check for scan codes.

Very strange problem, gotta be more clues hopefully.

Click to expand...

I agree that it sounds more like electrical as well. I am going to try to check your suggestions today. I know where some of the obvious grounds are, but are there any elusive ones that you know of?

the most important one relating to the PCM and it's sensors is the one on the back of the head.
adding one from the battery to the body is a good idea.

87manche said:

the most important one relating to the PCM and it's sensors is the one on the back of the head.
adding one from the battery to the body is a good idea.

Click to expand...

Thanks for the information on the grounds. It does indeed have a ground from the battery to the body, and I am going to run a new ground from the back of the head to the body. It's a little oily (not much), but I don't care for the mesh (aluminum maybe??) ground that is there.

All of the PCMs worked just fine in my friends XJ last night. My XJ with the "new" PCM will run just fine sometimes and sometimes not, but it trows CEL codes 122, 123 and 1694. I am starting to suspect the TCM is possibly having itermitent trouble due to a bad ground somewhere, or the TCM itself is struggling. I will see how it goes. I can use my friends TCM some tomorrow so I can rule it out if nothing else.

That ground strap is braided copper, tin coated, for easy flexing as the engine rocks around a lot starting, etc.
The grounds need to be disconnected, cleaned, coated with a anti-corrosion, and reconnected. It just takes a tiny layer of corrosion to mess with 12 vdc. Just looks ok means little, clean them ever 2-3 years, battery terms yearly or so.

Went to google 'obd2 codes xxxx' and you have found the problem!
1694- turbo charge relief circuit malfunction. That's it, its missing that signal!

Run your codes thru, as above, about 4th down is a wikianswers with interesting info on the buss testing and how the CPS can mess up things. Also looking at the TPS.
The part about gauges going bonkers fits too.

Kinda-Sorta indicates CPS (or?) is messing up the buss.
Might try to get it failing then disconnect/reconnect CPS to see if that changes it, also test the CPS ohms.
TPS is very suspect too. (two problems? / or one causing both?)

Good Luck,
Orange

Did a master reset on the ECM and then borrowed my friends TCM. CEL codes 122 (TPS low voltage) and 1694 are no longer coming up... as of yet. The vehicle is running much better at the moment. The only symptom I have right now is high idle on start up. Revs to about 2K and then drops down to about 700 RPMs. No more bouncing up and down like it was. Code 123 (TPS high voltage) is still coming up. This is a brand new OEM TPS, so I will start checking with my meter. Willl check CPS and grounds too.

Turned out to be a bad clock spring... disconnected the horn relay and all problems are completely gone. Can you believe it?? Thanks for all of the input guys!

How about sharing with the rest of the kiddies how you isolated the problem?

Could be a great help to someone else down the road (or alongside the road?).

joe_peters said:

How about sharing with the rest of the kiddies how you isolated the problem?

Could be a great help to someone else down the road (or alongside the road?).

Click to expand...

Crazy thing... I went to honk my horn at a dog in the road and my cruise control light came on. I figured voltage had to be bleeding to the cruise control indicator somehow. If it could bleed to there, who knows where else it might go (like to my TPS or at least to that circuit???!!!:confused1). I sat at a stop sign in (in the country) and honked my horn like 50 times. I could make the idle go up or down from time to time, so I pulled the horn relay and the problem hasn't come back yet and no CEL either! One clock spring coming right up.

Way cool, and thanks for sharing.