MJ runs poorly when warmed up

[phokur]

Hi all,

When my Comanche cold starts it runs fine. It will run well until I shut it off. When I try to restart it runs really rough, almost like it is missing. The truck lurches at low rpms and almost stalls unless I give it some throttle. Keeping the truck at higher rpms is the only way I am able to get it home. Once it cools off it starts right up and runs fine again. I also noticed if I am WOT and the engine is straining, I can get a miss and the truck lurches. Not consistently enough to be a symptom though.

My thought is fuel filter but I want to confirm this before I go tearing into the lines.

Any ideas? Thanks!

[kro10000]

O2 sensor is my guess, when your engine is cold it is in "closed loop" meaning it ignores the O2 sensor and runs on a preset fuel map. Once it warms up it gets the bad reading.

 

Should've asked this first. What motor/year? If its a v6 then disregard above

[phokur]

Thanks for the reply! That's something I haven't considered. I have an 89 4.0 I6. Is there any way to test the O2 theory without having to buy one first?

Thanks!

[kro10000]

http://www.lunghd.com/Tech_Articles/Eng ... ostics.htm

 

This site is full of useful info, if you own a renix jeep a multimeter is your best friend.

[kro10000]

Also just a thought, but I'd unplug your temp sensor and try driving it that way. That should force it into closed loop and will disregard the O2 sensor entirely. If you're still having issues then it rules out an open loop issue.

[phokur]

I'll check it out tomorrow, thanks again!

[phokur]

Finally got the O2 sensor in it, it solved it! Thanks a million! :clapping:

[phokur]

Sorry to be grave digging but since this is the same problem as before I'd like to continue this thread.

 

The O2 sensor seemed to solve the issue for awhile and it ran great for around four months when I drove it. (Not my DD) But on an unfortunate trip to the dump it started its bad habits again.

 

I pulled into the dump and it started lurching and backfiring. When I backed up it finally quit completely. It wouldn't start back up right away but after sitting for around 15 minutes it fired back up. To get home I had to keep the RPMs high to keep it from stalling. It seemed to run fine when cruising at a relatively high rpm but to accelerate it spit and backfired like crazy.

 

My first thought was bad gas to I drained the tank and filled it with 91 octane and dumped in some stabil just to be sure. Same issue.

 

To recap, truck runs great when it first starts. But when it warms up it starts to run horribly and almost sounds like it is missing. It backfires and sputters and when it accelerates it lurches wildly. I have to park it for hours, then it will run fine again for another 15 minutes.

 

Help :hmm:

[ComancheKid45]

Not much help, but i had a similar issue a few weeks back.

 

Here is my topic.

viewtopic.php?f=2&t=32312

 

Just go through and check all your electrical connections and sensors, when was the last time you did a tune up? Fuel pump and Filter in good shape?.....just little crap like that can make these trucks throw a fit, just be patinent and youll get it figured out :thumbsup:

[phokur]

Thanks for the encouragement. It has been awhile since a tune up. Time to start replacing things!

 

Quick question though, for whoever knows. Are there vacuum lines that would cause it to run like this? Does anybody have a nice diagram of what should go where? A bunch of stuff is dried out under there and seems like it could be my gremlin.

[Dave Stark]

I had a pin hole leak on the rubber line attached to the fuel sending unit inside the tank which caused very similar issues until the hole got big enough to kill it altogether...

 

Chasing gremlins sucks.

[Jeep Driver]

Also what about temp sending units going bad? Could this be a cause?