91 Cherokee start then die

[alroosa]

I am learning a lot, thanks for the help guys!!

Vacuum is there. Tested the pressure again and @ prime it jumps to 40 for a split second; then down to 32, @ start it jumps to 40 but back to 32 when it runs for the few seconds it does and dies. I did a leak down test and it was fine, the pressure only dropped 16 PSI, the failure range is above 20.

When the pump primes it only runs for a second and there is voltage @ the pump for that time, after it is started there is voltage there after the engine dies.

I did a volume test and I get about 1 cup for 10 seconds. Is that the normal amount, the haynes manual doesn't have specifics for a volume test.

Could this be caused by a flaky sensor such as CPS? or maybe the computer since the power antennae pulled the battery down?

Thanks!!

 

[alroosa]

Or could it be a bad ground or a short?

 

[joe_peters]

I'm getting suspicious of the PCM/ASD relay/harness, not so much the CPS/CKP crank sensor although it is part of the equation, possibly a security system interrupt for the ignition.

When you turn the key to ON the PCM provides grounds to the fuel pump and ASD relays. The fuel pump runs for about 1 second, the ASD is supplied power for about 3 seconds. If the PCM doesn't get a signal from the crank sensor it cuts the grounds.

So. Security system?

There is something in the FSM 93 about not be able to use a PCM that has been used with a security system on a vehicle without the security system, but you haven't changed the PCM.

 

[alroosa]

Yea no security system that I know of. Wouldn't the same issue have happened when the battery died the first time and i replaced it?

When I unplug the CPS it doesn't start. I tried to Ohm it out with help of the haynes manual but it says the reading should be on hot engine (200 +- 75 Ohms). I cant get a hot engine since it'll only run for 2 secs..:bawl:I can only get a reading when i first touch the pins with the multimeter for a split second and it seems to be in range...Could the CPS have grease on the wires causing it to get a funky reading? Its pretty messy where the CPS sits.

I pulled the PCM to take a look and the harness looks good. Also cleaned a couple grounds (@ PCM & from Neg Battery Terminal). If the PCM is bad will it still send codes? b/c I am getting a 12(BAt or PCM disconnected) on the key trick, even when the bat/pcm hasnt been disconnected in the last 15 or so starts. Had a friend with a scanner come over and he could only read bad cooling fan relay.

Also tried swapping oil pressure sensor b/c I was told that if it didn't show correct pressure it would shut things down...that wasn't the issue.

If the PCM has failed is there a way to test it?? Also would a re flash be just as good as a new/used PCM?

Thanks for all your help!

 

[joe_peters]

And another guy HOSED by the Haynes/Chilton cabal of incompetence!

Those IDIOTS are telling you how to test a RENIX crank sensor, 87-90.

See Tim_Mn's post here: http://www.naxja.org/forum/showthread.php?t=1025391

He covers BOTH the Renix and 91+ tests.

Do yourself a favor and throw away the Haynes.

You can purchase a LICENSED copy of the FSM for your year for $37 from http://www.autobooksbishko.com/index.html or you can haunt eBay and Craigslist for original FSMs--don't waste your money on junk or pirated copies.

Good luck.

 

[alroosa]

Yea...I was doubting lots of stuff in the Haynes manual...Seems a little broad (84-01) to get good info.

It says that test is good for 92 or earlier...what a joke.

Joe, per that post the cps is testing fine then. Are you leaning more towards a computer issue?

Also is a reflash as good as getting a new PCM?

 

[joe_peters]

Well, I'm wondering if the PCM is cutting the ASD relay ground and killing the engine by removing power to the coil.

Relay diagram: http://autorepair.about.com/library/illustrations/bl184a-lib.htm

Ok, locate ASD relay in the PDC. Use a short piece of thin gauge wire stripped on both ends, pull the relay and shove one end into the "30" connection, then replace the relay. Now, hook the negative of the voltmeter to the other end of the wire, the positive to the battery terminal positive. Have someone turn the key to ON--should get a ground for about 3 seconds from the PCM, and then it should be interrupted. Ok, then have them start the engine and you observe the voltmeter to see if it is the chicken or the egg--does the ground interrupt and the engine die or does the engine die and the ground interrupt. Any of this making sense?

 

[alroosa]

Yep makes sense. I'll give it a check tomorrow morning.

Thanks!

 

[alroosa]

I get a little voltage (.2 or so) for a couple secs when the key is turned to on

Then the same for about 3 seconds, the engine stops before the voltage is gone...what's this mean?

Thanks!

 

[joe_peters]

Ok, not 100 percent, but it sounds to me like the ASD relay is still energized by the PCM when the engine dies. This means that 12 volts is still going to the primary side of the coil when the engine stops running, so the problem MAY not be the PCM cutting the ground to the relay.

So, if that is what is happening we need to figure out what could be killing the engine while the coil primary side is still getting power.

Things that come to mind--the PCM is not TRIGGERING the spark. That could be a faulty PCM, crank sensor, or the harness--something that will allow the engine to start then heats up and creates an open in the circuit.

The crank sensor tested fine, so how about you start at the PCM and using electrical contact cleaner and dielectric grease clean all the connectors. Once you have that done, if the problem persists, see if you can score a used PCM at a salvage yard.

 

[alroosa]

IT WAS THE PCM!!!!

THANKS SO MUCH FOR ALL YOUR HELP! LEARNED ALOT!

 

[bchulett]

Hi Everyone,

My 91 Cherokee Briarwood, 4WD, 6 Cyl. starts fine and then dies after 1 -3 seconds.

It started with an electrical problem. Got a brand new battery and it died within a week, got a jump and the jeep made it home (about 5 miles) but i noticed when i slowed down at stop lights it would almost stall, i figured this was related to the battery problem. When i parked it in the driveway the radio and door alarms were working, when i went out the next day to start it the battery was dead! Traced it down to the power antennae (after testing alternator, replacing coil, checking dist), it was stuck and would pull 5 Amps every 5 secs or so, pulled the fuse for that and the battery held a charge!

Now it will start perfectly and run normally(no sputtering) for 1 to 3 seconds then die. After starting it usually idles up to about 2kRPM then stabalizes at 1.5 but it cant even make it to 2. If I push the gas in before i start it will make it to about 3KRPM then it goes back down. I did notice the antifreeze was pretty low so i added some. Its not storing a code, the check engine comes on for a second when it stalls though, which i think is normal if the vehicle fails..? If there's no idiot light does that eliminate the possibility of a sensor getting a bad reading (TPM, Oil Pressure, CPS, etc?). I can hear the fuel pump kick on when i put the key to the on position and i have swapped relays on auto shut down and fuel pump to no avail. I have also tried bypassing the balast resistor and resetting the ECM by touching the pos cable to neg for 30 secs. Anyone have any ideas?

Thanks!!

It it hasn't been said already, run some Chevron Techron fuel injector cleaner through your tank... this same problem on mine was the injectors and fuel system. As a result, I only run Chevron gas now and I've never had a problem since... 172k miles and I still have the original injectors.

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