To any automotive technician who will offer some advice to an aircraft technician::

I have a 1997 ZJ (86K) that doesn't like to run when it's cold. It cranks and runs good except when it's idling during warm up, it quits twice without stumbling. No MIL or codes. When it dies the MIL flashes and the lights on the inst panel stay off. It cranks right back up.

If allowed to idle all the way through the warm up cycle, when it reaches operating temp (after two times dieing), it stumbles, then catches up and runs. After this, there are no drivability problems.

If driven while cold, it may stumble or die at any time until 10 minutes after operating temp is reached. If cruising, the instruments die for about a second, makes a big jolt, and keeps running. Still no code.

When the engine is hot, it always runs fine. Every now and then, it runs good cold.

I've replaced the coil and crank sensor, plugs, wires, cap, rotor.
Taken the PMC off and cleaned the connectors and shortened the two screws that are believed to cause problems. Checked voltage drop across PMC grounds, cleaned all grounding studs and rings. Took throttle body off and cleaned it and everything attached to it. Took the power box apart and cleaned all the relay and fuse terminals, checked the inside for corrosion. Moved all relays around. Changed fuel pump module. Checked O2 sensor heaters and signals.

A couple of things I found are what I think may be abnormal:
The front and back O2 sensors are both cycling at the same rate. The front from .2 to .8 and the back from .1 to .5. If I understand the concept, the back one should be steady or cycle at a slower rate? A blown CAT maybe?
Would this make it stall or cause the PCM to fail?

Another thing that may be abnormal is the coolant and intake air sensors are reading lower ohms than my Haynes manual says for the ambient temp. They both read 8K at 80 degrees. Not sure if Haynes is accurate...

If I turn the switch on while it's cold, sometimes I can get it to cycle the relays by touching the PCM and the truck at the same time. This just happens every now and then. Sometimes I can get them to cycle by flexing the PCM case. I tried gently moving the wires at the pcm and that doesn't affect it, unless i flex them hard enough to flex the pcm too.

I took all the plugs off the PCM checked the integrity of the pins with an ohmmeter. If back probed so many things, I cant remember them all. I have a full wiring diagram, so I have been busy.

On thing I did catch it doing with a meter: I had the switch on and was checking voltage drop on ground, black lead on neg batt terminal and hot lead back probed to PCM ground. The relays cycled and the voltage drop went to .2 volts for a split second. This is out of range according to what I've read. This cued me to check the ground wires again, but didn't find anything.

I don't know a lot about automotive test equipment, but I tested for ac ripple with a lab scope. It was nasty looking, coming from ignition, 004-024 V pp. Turned on the 10KHz filter and got rid of it. Don't know if this a prob or not. I read that the PCM is supposed to be shielded from this, but it looked like this HF AC was riding in on all the wires. If I check it with a meter, it reads about the same as the scope.

I think the PCM is toast. The weird thing is, if it's rolling with less than 1/4 tank of gas and it does it's trick, the gas hand drops to empty and the low fuel light comes on. Today, unless it was a coincidence, I made the gas gauge start to work again by cycling the A/C switch.

If any of you techie guys or gals have any tricks up your sleeves to figure this one out or confirm a bad PCM, I'm open to suggestions. I am getting a scan tool soon, maybe it can tell me something...

Thanks in advance!

David

 

This is going to be slightly out of left field ( but not completely , i have an XJ and a WJ) But how is the fuel pressure?

I assume this is a 4L? I had issues with my XJ acting very similar to that, turned out i was having fuel pressure issues.


Other than that , it sounds like you either A. Have an intermittent short , or more likely B, your PCM is going uniform tango. Check for a local offroad club, there is usually someone within that realm of people who has a spare everything, you might find someone with a PCM you could try.

 

What engine 4.0, 5.2 or 5.9??

 

It's a 4.0 2WD with factory alarm. Stock.
The fuel pressure is good at the rail. Holds 49 psi.
The intermittent short has been in the back of my mind, but i haven't figured out a way to find it.

 

First thing to check on the jeeps is the battery. Even just cleaning the connections. When the battery starts to go, it makes them do all kinds of crazy things because the PCM uses so much juice, they say even more than the starter. It won't show codes ususally either. Not sure on the ZJ, but the WJ has the regulator built into the PCM, so it throws the alternator charging off too. After it warms up, it may have been running long enuff to have a decent charge and curb the problem temporarily. Not a tech, just ran into this problem with my 4.0 myself. If the battery tests ok, then have the alternator checked.

 

I dont know much about the jeep motors but I have seen this on a 4 cyl dodge. It was a bad Idle Air adjustment Solinoid. It is located on the throttle body and idles the motor up when it is cold or the AC is on.

 

Two things come to mind.

1- With the engine running tap the pcm with a soft mallet and see if the problem duplicates, if it does put a new pcm in it.

2- Check for a vacuum leak at the intake gasket, they LOVE to fail there causing your exact symptoms. Also, at 1/4 tank they are empty, it is a known problem for jeeps.


Tim

 

I had a Jeep truck with the 4.0 and the crank position sensoer went south. it would do all sorts of crazy things, including be diffrent as it ran from cold to hot. It was at the back of the motor on the bell housing, looking at a magnet on the flexplate.

 

I tried the tapping with the mallet. BINGO!

I had been tapping on it with a screwdriver handle. In the Air Guard, we call it the "7-level tap". It's not a well looked upon thing to be beating on airplane parts

Who knows anything about Cardone reman? A Auto parts store has one already programmed for $225. One year warranty. I think i want a lifetime warranty...

Anything else I should know about reman PCMs? The thought crossed my mind about trying to fix this one by resoldering. I have an ESD bench and soldering/desoldering equipment... I imagine the board is going to be coated?

Thanks for the replys!

 

That's one thing I haven't had off the Jeep. Looks like a bear to mess with. I did check it's resistance though the harness and looked at the output with a meter while turning the motor over. Doesn't mean it's not breaking down as it operates.

I'm hoping something hasn't caused the PCM to fry.

 

Not sure on the board or if I would even attempt to repair it. Unless something on the board is obviously fried (like a burnt resistor) it would most likely be a shot in the dark to try and fix it.

 

I sprayed carburetor cleaner around the intake gasket and the idle didn't change. How else can you check?

 

Just a 2 cent bit of advice.
When it is cold, the O2 sensers are not in play, forget about that avenue.
It will run in open loop when cold, and switch over to closed loop as remp rises. This makes me think the computer is ok.

 

Thanks for the reply,
The computer shuts off when tapped lightly and rapidly (vibrated)with a rubber mallet. It only does this when the engine compartment is cold.
Going to order a reman from Cardone. At this point I have nothing to lose but $225. I am going to do some more checking of the the IAC and VSS to be sure there isn't a short that fried the PCM.

I think it's just a heat related problem in the PCM. That engine compartment gets extremely hot and the heat stress would cause anything to fail eventually.

 

If it were a substantial leak the carb cleaner would have found it.

As far as Cardone parts goes, I have had good and bad luck with them. So that's up to you.

You will be hard pressed to find a lifetime warranty on any electronic component.

Good luck,


Tim