faulty PCM?

[1swift1]

I have a 93 GC that stalls after about 15 minutes of run time. After sitting for about a half an hour it will start up fine but die again. Fuel pump checked out ok and there are no trouble codes. After reading some threads on similar problems I think it may be the PCM. Is there any way to check it out before buying a new one? Dealer wants $600, I found one at a parts dealer for $350. Does anyone know where to get one cheaper in Canada? (Ontario)
Thanks for any help out there.

[bringselpup]

I just did my computer maybe 3 weeks ago. I had no luck finding a used one and I scoured the net and called every parts yard near me. I bought one via local auto parts store for $210 US.

There are a LOT of different computers for 93. You have to match up what you have. It's located in the engine compartment on the pass side firewall kind of hidden by the radiator overflow tank. My sticker was outside on the top covered with a LOT of dirt. You're looking for a number like 56029-009 which was my PCM number for an early 93 with AW4 trans and no factory installed anti-theft. The transmission was changed mid year 1993 to the 42re so there are computers for both as well as with or without anti-theft, calif emissions or not, or maybe you have a manual trans etc. If you do in fact need one you need to get the one that matches up to the number on your PCM.

www.rockauto.com had the cheapest price I could find. I just didn't want to wait for it to ship, and then have to deal with shipping the core and waiting for that refund. I brought my old one in to the parts store and handed it over as I got the new one and never had to front the core $$.

With all that said I would certainly want to be sure it wasn't just the crank sensor or the camshaft sensor first before blowing that kind of dough on a new PCM. If either of those stops sending a signal to the PCM it's going to think the motor isn't turning, which will cause it to kill the power to the fuel pump.

How do you know the fuel pump is good? Once the vehicle stalls out can you hear it do the 1 sec burst as you key on to try to start the vehicle again?

I knew my computer was bad, my vehicle would take me anywhere, 1 mile or 50 miles, once I got it started.

[1swift1]

I had a shop run diagnostics and test the fuel pump and they found no problems with the pressure and no engine problem codes. I can hear the pump activate when I put the key on the ON position. At that point the engine will fire up fine. After it stalls out on me, (now after about 5 minutes of run time) I can't hear the fuel pump until I let it sit for a good half hour. Then the pump will go and it will start for me. Maybe that is an indication that a sensor is bad. I did take off the computer connector to see if anything looked funny but all looked fine. I'm already about $300 in the hole (tow, diagnostics, new oil pressure sensor) and I don't want to play roulette installing new parts. Chrysler dealer says they'll perform a number of tests for me, big $$$ but that may be what I have to do.

[1swift1]

Just went out to tinker again and now it won't start at all. So here is the history:
About 3 weeks ago it would lose power occasionally, but not stall, while driving .
3 days ago it stalled while highway driving, started up again about a half hour later.
Today it would only run for maybe 5 minutes before stalling and again needed about a half hour of rest before it would start. When I could hear the fuel pump activate, it would fire right up.
Now it won't start at all and I don't hear the fuel pump while the key is in the ON position anymore.
PCM, CPS, fuel pump???? Anyone?

[oljeeptek]

Likely a cps. When it won't run, try something..

Turn the key off for 30 seconds. Listen carefully when you turn it back on. The pump will only run for 1-2 seconds and shut off. It won't come on until you crank the engine (the pcm is waiting for the cps signal to turn it back on) or until you cycle the key back off for another 30 seconds.

You can also connect a voltmeter or a test light to the power feed wire at an injector and see the power at key up for 1-2 secs, and again during cranking.

If you're not sure how it responds normally, check it when it runs to know what it should be doing when it doesn't.

I wouldn't dump money into a pcm until you have run a few more checks.

[bringselpup]

With the fuel pump not doing that initial 1 sec burst when you are turning the key on makes it sound very much like the computer now.

You can do several things. Go turn it on and just sit there and wait to hear the pump. It seems that as the problem begins it takes 15-20 sec but as it gets worse it can take 15-20 min. Shut everything else off so you won't miss hearing the pump. Eventually mine just wouldn't spin the pump at all anymore.

If you don't hear the pump when you key on open the PDC and find the fuel pump relay. Get it out and carefully take the cover off it. A small jewelers screwdriver can be used to pry the cap off the tabs. Put it back in without the cover and have someone in back at the tank to listen. Manually close the relay just for a moment. They should hear the pump. If it's quiet you might be able to hear the fuel rail to the injectors pressurize. (I could) The ignition key does not have to be on to do this.

That's how I proofed my fuel pump and knew for sure then that it was good and did the computer.

Also for whatever this may be worth, when I did pull my computer I heard an odd sound. I thought at first the blower motor for the heater was running and I was hearing it through the firewall. And then I realized the key to the Jeep was in my pocket. The Jeep was NOT on. That sound was my computer, it stopped the moment I got the connector off it. I don't know how to describe it any better than a lower frequency squeal.

My new computer did not sound like that at all! It made very smooth sounding soft clicks after I hooked the battery terminals back up. Sounding very much like a boot up process in there.

100% fix for me and after 3 weeks or so driving I am noticing better gas mileage than before too.

[1swift1]

Ok, I tried closing the relay manually as you suggested. At first there was nothing from the fuel pump so I let it sit for 10 minutes and tried again. Then I could hear the fuel pump prime so I tried to start the engine and it fired up. I let it run until it died again (about 10 minutes) and watched the relay at the same time. The relay remained closed until just after the engine died. I then closed the relay again but again there was no sound from the fuel pump. Would this suggest that the fuel pump itself is faulty and is shutting down after several minutes of use?

[bringselpup]

If this were me at this point I think I would try to proof the system by taking the tube to the air filter off the throttle body and being ready to put some fuel down the throat after you stall.

The Jeep runs when you get to start it, and you saw the relay stay closed until it stalled which is normal because the PCM will kill the power to the pump if the CKS or CPS sensor aren't telling it the motor is turning. That and the fact you can start and run the engine for periods of time suggest strongly those sensors are ok. You're getting no codes. That indicates nothing else being controlled/watched by the PCM is bad.

So I'm thinking if the whole system is good except for the pump, and the PCM is in fact trying to get it to prime at key on after you have stalled you might be able to get her to fire with some manual fuel delivery. And if you do get the motor to fire after it stalls it would really seem like the pump.

Logic says do everything you can to proof that pump. The fuel filter is less than $10. It's underneath in front of the tank. Get a multimeter and trace the line from the relay down to the connector by the fuel tank. See if closing the relay makes voltage appear down in that connector. Clean that connector. Anything you can do cause I know sure as hell right now I wouldn't want to be dropping my tank this time of year.

[1swift1]

I really appreciate all the tips I've been getting. I've tried a couple more things but am still at a loss.
If I manually feed gas into the throttle body the engine will start and run unitl that fuel is burnt.
I am getting a positive reading from the connector by the gas tank, but only if the key is in the ON position. I get a reading both when the relay is in the open or closed position.
Now I can't get the pump to prime at all anymore, even after sitting for at least an hour.
So the fuel pump does not appear to be working. Could there be any other reason for this or has it finally died on me? And does that even sound normal? Would a fuel pump work sporadically for a while before eventually quitting or would it not just die all at once?
I'm only a backyard mechanic but to me it still sounds like an electrical problem. Any more suggestions would be greatly appreciated. Thanks.

[bringselpup]

Hey man it's a Jeep. Could be any number of reasons that pump is doing what it's doing.


But I really think:

If you can run it till it stalls, (at which point you can't get the fuel pump to spin) and then make it run by putting fuel down the throttle body then the whole system save for the fuel pump has been proven to be functional. By being able to do this it tells me your PCM is good. In my case with a bad one (PCM) even though I could manually make my pump go by closing the fuel pump relay my vehicle would not start. I assume there are other connected events from the PCM that must occur at key on, not just the 1 second pump prime. But with you, they are happening, because you can make the vehicle run by dumping gas down the throttle body.

That pump isn't giving you fuel all the time. I would imagine that once it stalls you won't be able to detect pressure at the injector rail. Before you drop the tank change your fuel filter. I can't really see it being the problem but it would definately suck if it was because it's a $10 part and less than 15 min to do.

But looks to me like you're in the market for a fuel pump.

[1swift1]

Thanks bringselpup. I changed the filter but that didn't seem to help although the old filter looks pretty rough. I then read another thread that said if you tap on the tank that could loosen up whatever is jamming the pump. That worked like a charm. Right after I tapped a few times on the tank, the pump started priming. I was able to start it up and it ran for a couple of minutes before stalling. Then again, no priming noise. Then I tried tapping again and the engine has been running now for half an hour. Is it possible that replacing the filter and banging on the tank a couple of times was all it needed?? Or should I still look at replacing the pump?
I owe you big time if this is the case! I'll update what comes next.
Thanks again to everyone for talking me out of buying an unneeded computer.

[bringselpup]

I think you just proved it's the pump without a doubt.

But alas no I think unless you are prepared to get out every so often and bang on the tank you are gonna be doing a pump soon.

I noticed here http://www.autopart.com/specials.htm that they have a 93/94 assembly on sale. I was going to get my computer through them because they are local to me but turned out they didn't have one in stock. So make sure this is if you try for it.

[1swift1]

finally had my new fuel pump delivered last week, dropped the tank & put the new pump in. Runs perfect now. Thanks again for the help.

[bringselpup]

Way to go!

How much was the pump?

[1swift1]

$140 on ebay. (brand new pump)
$15 for new fuel tank strap bolts as I had to take the bolt cutters to the old ones they were so corroded.
$100 for the orignal tow back in late December.
$200 to the gaarge to repair an unrelated problem they thought would fix it.
1 month with no 4 x 4 in the middle of winter.
40 hours of my time for diagnostics & repair.
satisfaction of doing it yourself - priceless!