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4.0L GC stalling after starting, backfiring?

22K views 35 replies 14 participants last post by  Ovedz 
#1 ·
Hey everyone. I have a 1998 Jeep Grand Cherokee with 150K with the 4.0L. I just developed a problem in the past two weeks. When you go to start the engine it will start fine but will idle for a second or two, stumble and then die. This can go on 3-5 times before you can actually get the thing to run and drive. Rarely it will happen while waiting at a stop light. It seems to do it more after a warm start (like after you shut it off and go into the store and then come back out 30 minutes later).

While driving and accelerating (this will occur right after it was first started) the car will sometimes also stumble and lose power (rpms will drop to idle even though your foot is on the gas and it will stumble). In as few cases while trying to accelerate, there has actually been a knocking/backfiring noise that sounds like it is coming from directly under the driver's side floorboards. These symptoms occur sporadically and the car usually runs fine once it gets driving.

Check engine light is not on. I had a somewhat similar problem last year and replaced the 02 sensor and I tried replacing both again a few days ago (along with a whole new exhaust system) but it did not get any better. Within 10K the engine has had new plugs, cap and rotor, wires, air filter. I'm kinda stumped cause the check engine light is not coming on. I would imagine it is something fuel related. Anything special I can look for or test? Let me know!! Id rather not take it to the dealer and spend alot of money.

Nick
 
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#3 ·
I had the same thing happen to my '98 4.0 a while back. Acted just like yours for a few weeks then it got worse. After a couple weeks it would die once warmed up and not run again until it cooled off.

Take your Jeep to a parts store with a code reader and see if they can pull any codes. I didn't do that and ended up changing a few unessesary items before I finally got a code reader. Turns out I had a bad magnetic pickup inside the distributor. Not saying this is what is wrong with yours but it is best to see if there are any codes before you start swapping things out.

The horn-to-fuelpump relay swap is free so that's a good place to start.
 
#6 ·
Well i had a day to work on it: I

1. Cleaned the IAC/Throttle Body
2. Replace the Fuel Pump Relay
3. Replaced the Camshaft Position Sensor
4. Replaced the Crankshaft Position Sensor
5. Checked fuel pressure when it did run (45psi at idle)
6. Cleaned the battery contacts
7. Replaced Ignition Coil

So I did all that (on top of the two o2 sensors, new cat, cap and rotor, and wires) and I STILL HAVE THE PROBLEM. The car starts and drives okay from a cold start (very rarely I get a slight "bucking" while cruising) but if you drive it and warm it up, park it, and then come back to start it 30-60minutes later while it is still warm it will fire up, immediately die, and will just crank and not start from there on out or take at least 10 times to actually get to start and run (and then have the "bucking" or "jerking" in the beginning of the drive cycle).

Brought it to the Jeep Dealership yesterday and the Tech Notes said that they found a vacuum leak at the intake manifold which will cause weird voltage readings from the TPS/MAP sensors, and said they reset the computer (they said one the parts I replaced probably fixed it, but the computer needed a "baseline" to relearn from). Of course I drove it home from the dealership (it bucked a few times), parked it and attempted to start it 30 minutes later and it took 10 times to get it to run and drive.

Going back to the dealership tomorrow. I've never been at more of my wit's end then with this car....never should have sold the Chevy Blazer!!!

Nick
 
#8 ·
Back off the pcm bolts like 2 or 3 turns mine had a stalling problem in cold weather and I did that and it stopped it. Have done it with 2 others it wont hurt to try it it only takes 10 min if you know what your doing.
 
#9 ·
Ill clean the pcm connections and untighten the bolts (how does this effect anything?!). I figure Ill change the plugs too cause that is the only other thing I havent changed (they are about 20k old).

Dealership wants $200-400 just to diagnose the problems so I will try those things before returning it.
 
#11 ·
Yea I am leaning towards the ECU at this point too. Where did you get a replacement? I looked at Advance Auto and for $400 I can get the computer and it looks like it will already be flashed with all my vehicle information.

Oddly enough tho, the problem does seem to be getting slightly better (now it only stalls 2-3 times on a warm start before being fine).

Nick
 
#25 ·
Yea I am leaning towards the ECU at this point too. Where did you get a replacement? I looked at Advance Auto and for $400 I can get the computer and it looks like it will already be flashed with all my vehicle information.

Oddly enough tho, the problem does seem to be getting slightly better (now it only stalls 2-3 times on a warm start before being fine).

Nick
Any luck on the fix. I know it's been a while and I hate to reopen a thread but having the same issues.
 
#13 ·
Upstream 02 sensor . Little doubt . Unplug it , expect a CEL to appear . Drive it for a few days , if symtoms are gone , replace sensor . CEL goes away when the sensor is plugged back in .
 
#15 ·
Ill try to unplug it the next time I have problems starting it....but the mopar sensor in there was less than a year old when I started having these problems a couple weeks back...and then I got it replaced at the shop (not mopar) a couple weeks ago and same problem.
 
#18 ·
Just an update but I brought the jeep to get tires and the mechanic there took a look at it, and said there was a stored code in the computer (I think po351 which indicates some type of coil circuit malfunction).....he drove it and the car acted up for him.

He traced the coil wire back and found that the connector which has this coil wire in it that goes to the ECM on the firewall was loose and wasn't on all the way. He said he could get it to stall just by wiggling the connector. He put a big ziptie around it and the ECM to make a better connection. The Jeep has gotten better and we thought it was fixed, but it stalled while idling (after a warm start while driving in the parking lot) and took a couple times to start back up yesterday. The trick helped, and it stalls more rarely, but it is still stalling. I may take it back to him to see what he can find out.

Nick
 
#19 ·
FIXED!!!

Took all the connectors at the pcm off, cleaned them with electrical cleaner and a toothbrush and then put zip ties around all three of them. Then I changed the spark plugs (they were only 20K old but I figured why not because I changed everything else).....turned out I gapped them a little big at .045.

Then I found the new converter I had put on it had been blown because of the backfiring I would assume so I got it warrantied out.

Hasn't stalled/hesitated or anything in about a week and is running better then it ever has. And to think I prolly spent over 1500 bucks on all those things when all it was was a bad connection at the ecm.
 
#20 ·
I have the same problems with my jeep now, right down to the same code and everything you described. My question is how long did the zip tie thing last. I just rigged my pcm like that and it only lasted two days. Did you end up getting a new PCM? Let me know what happened.
 
#29 ·
#32 ·
If you PM hunter he can change it for you, if you're on his good side, which I can't imagine you not being.
 
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