philwarner

My 96 Limited refused to start a long time ago and I had left it parked where it was because it appeared to be a bad fuel pump and it was in a bad place to try to drop the tank, but recently I jumped the fuel pump relay socket and the pump was good and made 50 PSI so I bought a replacement PCM on eBay with the same part number as my old unit. It came yesterday and after first plugging it in and verifying that my scanner found it (ISO9141) and that the engine would start, I reassembled everything and was able to start and drive the Jeep up to my garage for a cleanup. However it ran very rough and it now "idles" at 1200 to 1500 RPM (higher when it is warm) and surges 100 to 150 RPM up and down every half second and the tachometer needle dips every 1 to 5 seconds as if an invisible hand slaps it down 500 RPM and back up. This is with my foot off the accelerator and the scanner showing throttle position as 10.6% (which is as low as it goes and it will go up to 69.5% if I press it to the floor with the engine off). I had sprayed the TPS and CPS connectors with contact cleaner and then WD40 in the process of cleaning up the engine when I reassembled everything. Also, while running, the low fuel light will sometimes come on and the gas gage drops to empty and then later the light will go out and the gage will read anywhere from 1/4 to 1/2 tank and when first started, it may take quite awhile for the low fuel light to go out and the gage to register. Very strange.

So, it does run with the replacement PCM now, but badly, and I have not tried to drive it any distance yet, because this morning it did idle down at one point while warming up and then suddenly died and it took several tries and a very extended cranking to finally get it to fire again and then it "idled" at 1500 RPM while surging up and down with the tach needle dipping the whole time it ran. I understand the PCM may need to relearn, but I don't want to get stranded on the road with a dead Jeep with expired tags.

Does anyone know if the fluctuating high idle, the tach needle dipping, or the gas gage level & low fuel light is controlled by the PCM and if this one might be malfunctioning? It was missing 3 of the 4 screws on the top cover (and has a screw broken off and wedged in one of the mounting lugs), so it has likely been opened before. The last time I ran the engine this morning, shut it down, and turned the key on again, after the bulb check lights went out the check gauges light stayed on, but the scanner says there are no DTCs set.

Noah911

"I had sprayed the TPS connector and CPS connectors with contact cleaner and then WD40 in the process of cleaning up the engine"

You said this above in your other last post. May I ask what type of contact cleaner was used? If it was something like BCR Brakes and Contact Cleaner? Then, I believe it is very possible this may have messed up the TPS. Those sensors on the throttle body get ruined very easily by this type of stuff, which could explain some of those symptoms you are experiencing now with the new PCM in there.

philwarner

It was "CRC Quick Drying Electronic Cleaner" with the picture of automotive type connectors on the label and I actually sprayed the TPS connector this morning after experiencing these same symptoms yesterday. I had previously cleaned the CkPS but not the TPS and I had not used WD40 on the CkPS, just the Electronic Cleaner. it was just an after thought to use the WD40 on the TPS; Any for or against recommendations about WD40?

Any way to know if these functions are controlled by the PCM?

Noah911

It is my understanding the PCM really only has an indirect control of those things. They are controlled by it but, it is also only based off of the signals it receives from all of the various sensors. However, a faulty PCM can display any variety of symptoms.

I would focus my efforts on investigating at the TPS to do every last test in the book on it for those types of poor running conditions you have described.

I would also focus effort on investigating at the CkPS to do all of the tests on it again.. and unplugging this sensor whenever the fuel gauge is acting up to see if doing so brings the gas gauge back to normal too. It sounds like there were previous abnormal readings on this sensor. The readings need to be verified as being within normal parameters now that the new PCM is in there. You are still describing signs and symptoms of having a faltering CkPS with having the abnormal readings at the fuel gauge, and with the extended crank/no-start.

philwarner

I checked you tube for TPS testing and found to my dismay that the connector I sprayed was NOT the TPS connector, but the connector for the other device on the front of the throttle body. For the record I did once have a high idle problem before the PCM crapped out, but it was a mechanical problem of a sticky throttle shaft spring not fully returning and I made sure that was not the case this time. I am not aware of any previous TPS problems, but then I am ashamed to admit that the Jeep sat for more than a year without starting or moving after it failed to start so anything is possible.

I'll check the TPS tomorrow if the rain stays away, but right now it is wine time. Thanks for the advice

Noah911

I am going to think about it a little more. I will let you know whatever else my brain comes up with..

I think I know why you are asking regarding the PCM controlling all of those things. I would be interested to hear what other people may think about it too. It does not seem a PCM would be the culprit to cause those specific types of issues how you are seeing them happen on your Jeep to me.

Noah911

The TPS is real easy to check too. It is a lot easier to do than the CkPS anyway. Whenever I get off the road I will provide this thread with the way in which I would go about testing the TPS.

Noah911

Here is a diagram. It comes courtesy of another member who was CCKen, to mention.. who seems he was exceptional.


The only thing is the wiring color and general shape of the TPS in your Jeep may be different than what you see in the diagram. This is okay though. The orientation of terminals on the TPS in your Jeep should still be the same, as well as the expected results.

For the sweep test it is easiest to see it happen when using an analogue style multimeter (DVOM) but, not at all necessary. A digital DVOM works to get an idea and see it too. All you need to do is ground one DVOM lead to the negative battery terminal, and use the other DVOM lead to backprobe the middle terminal (signal wire) of the TPS for doing the sweep test. Keep the sensor connected as it normally should be during normal operation. You can use something like a paperclip to backprobe at the TPS. Turn the ignition key to run and keep the engine off for the test. Watch the voltage from idle to full throttle as you smoothly fully depress the throttle pedal. There should be a smooth rise of the voltage from about 0.20 VOLTS - 4.5 VOLTS.

An extra tip or two:

Do the tests twice. Once on a cold sensor.. and another time after you let the sensor get warmed by running the Jeep up to normal hot operating temperature prior to test the second time. This is actually good advice for when testing anything like these electrical vehicle sensors, as temperature has a definite influence on how electricity behaves.

At some point while getting the readings as you are testing the TPS.. I recommend to use a screwdriver, or something like it, to gently tap on the sensor during the test procedures. If gently tapping on the sensor skews any of the results from their normal, then the sensor should be replaced.

I think I have some other thoughts to put your way if the TPS is tests okay.

philwarner

Thanks, Noah, for that info. It is pouring down rain right now, but I'll definitely check the TPS signal in the morning if it is dry. would actuating the throttle at the throttle body be the same as pushing the accelerator for this test?

I am still puzzled by the tachometer dipping every few seconds. Is there an RPM sensor that could cause that? Or is that a function of the crank sensor or the cam sensor?

Noah911

It definitely sounds like there could be more than one type of problem present contributing to the issue.. possibly?

I was thinking about the same thing with what the RPMs are doing too. The Jeep is hunting for the proper RPMs within a range. A lot of times something like this might just simply be the Idle Air Control Valve (IAC). This is actually where I would want to go looking first, if I was not seeing the more extreme symptoms you are describing.

I was thinking to suggest pulling the IAC, and thoroughly cleaning all of the carbon from out of the inside of it. Also, cleaning all of the carbon from out of the throttle body assembly with it also pulled from off the Jeep while you are at it too. Cleaning out the throttle body and the IAC really is what should be routine maintenance in my opinion. This seems it may be more important for in your case, since the Jeep has sat for over a year being at its' current age. The sliding portion inside of the IAC may be sticking to make whatever the bigger problem is worse. If you are not very familiar with all about the IAC, then it would probably be well worth it to look some information up on this particular piece of the puzzle.

You certainly may use the throttle body linkage thing to take it up to full throttle when you do the sweep-test checking on the throttle position sensor.

philwarner

I found a good video on cleaning the IAC and throttle body and will first test the TPS signal voltage and then proceed with cleaning.

In the mean time I reconnected the battery and made a short video of the gages starting with foot off the accelerator from cold. The gas gage behaved this time and the cold idle surged around 1000 RPM with the tach needle dipping periodically. I think the check gages light remained off this time. Sorry for the shakes of my hand held camera.

Hmmm. perhaps the 2 minute mov file was too large to upload?

UODATE: I back probed the TPS connector with a paper clip and measured the signal voltage with my digital volt meter with the key on. The center orange/black wire (#2) signal voltage was 0.552 volts at throttle closed and 3.42 volts at wide open and appeared to change smoothly as the throttle was opened. The rearward most white sensor supply wire (#1) and had 5.16 volts. The forward most black/blue wire (#3) had 8.6 mV at throttle closed or open. That makes it .0086V VS the spec of .015 to .020V; does that indicate a problem?

Next to the IAC cleaning and then decide if I need to pull the throttle body itself. It was not overly dirty, at least above the butterflies.

Noah911

That sounds like a good plan to me.

The CEL comes on at first though, right? If so, this is a good sign for the new PCM.

philwarner

Yes, the check engine light came on for the bulb check and then went out.

OK, the IAC is out and ready to be cleaned

Noah911

The IAC removed along with its housing.. which I also removed and recommend cleaning now at this time while it is all apart. It gets real dirty in there...

The inside of mine when I did this service on my ZJ last year. It looked pretty nasty inside there.

Clean

If doing the throttle body too, be sure to remove the TPS sensor prior to spraying and scrubbing, to not cause it any damage. Be careful of this small connection of the TPS when prying it off the throttle body.

Everything looks much better all together when it is all clean and shiny.


I tried to link you a thread I made when I did mine last year but, could not get the URL right for some reason. Be careful of that little o-ring for the IAC! If you wanted to search it to look it is titled:

ZJ Throttle Body and IAC, a cleaning guide c pictures

Noah911

I just looked at your pictures and it appears yours definitely needed it!

Hopefully it cleans up well for you? Sometimes the little stepper pintle motor (the sensor) itself may need replacing as it can become 'weak' and go bad.