Topic: 2 weeks chasing a running problem NEED HELP!

[lax22]

I am reaching out to you guys for help here. I have been chasing down this problem for two weeks and cannot figure it out. I am going to try and explain it as best i can. What happens is as the jeep is warming up it runs fine and idles smooth. At about 160 degrees, weather driving or idling, it starts to stumble and the rpms begin to jump. Then they get really low like its going to stall out but never does and after it get to operating temp 210 degrees it runs fine. When driving and it reaches 160 degrees the engine bogs down and loses acceleration, then it pops and backfires but sounds like its doing it in the intake and begins to stumble like its going to stall. then when it gets up in temp it runs fine like nothing happened. Main problem is its throwing no check engine codes at all and we have ran it with a scan tool and diagnositic tool and it cant really pick up any negative readings. it has great oil pressure and fuel pressure. we cleaned teh injectors and even replaced the magnetic pickup in the distributor. Before i buy anymore parts i figired id check in and see what your take was on this situation. What dose the engine or computer try to do between 160 and 210 degrees that could cause this issue. The help would be greatly apprechiated. 

[VA_YJ]

Let's think about this.  What happens when the motor heats up?

1. Coolant temperature sensor sends updated signals to ECU - Coolant temp sensor could be bad
2. Intake manifold temperature sensor sends updated signals to ECU - IM temp sensor could be bad
3. Once the engine reaches a certain temperature (based on the signals) it goes from open loop to closed loop (part of the time).  This means that certain sensors (like the O2 sensor) don't affect the fuel to air mixture until the engine warms up.  Focus on the sensors that are used only in closed loop modes.
4. As the block heats up, metals expand.  This can cause vacuum leaks and coolant leaks if things aren't tight.  Check for combustion gas in the radiator, make sure intake manifold fasteners are properly torqued and gaskets look good.  Look at your spark plugs are see if one looks different than the others.

Use the appropriate Factory Service Manual (FSM) to check out 1, 2, and 3.  I suspect your coolant temp sensor might be reading incorrectly- start there.  #4 is less likely, but possible.

[lax22]

Thanks alot! I will check those things out and report back if anything changes.

[VA_YJ]

Hope you find your problem.  What year is your Jeep?  I may be able provide more info from FSM.

[lax22]

it is a 97 with a 5-speed tranny and 116,000 miles. Its been sitting for almost three weeks now and is covered in snow. I was going to give it to my sister to use for awhile but now that all this has happened who knows. Hopefully the white stuff melts off and warms up outside so i can get to work on it. Thanks again for your help.

[VA_YJ]

I have a '99 FSM that might be useful.  The fuel system section covers the computer functions and goes through all the open and closed loop modes, indicating which sensor inputs the computer uses in each mode.

[lax22]

Yeah i am going to try and do something today after work since we have some more light in th evenings after work now.

[lax22]

Well it finially threw a code today so i had it ready it showed several soft codes and the one hard code for a low voltage on the upstream o2 sensor. So I replaced the upstream o2 sensor since the downstream sensor was replaced not too long ago. Seems to be doing better but will post results after its driven a bit more. Hope this works.

[VA_YJ]

The O2 sensor is a real possibility, as I mentioned before.  Keep your fingers crossed...

[lax22]

Well after replacing the updtream o2 sensor and clearing the code it seems like everything is working right again. I have been driving it two days strait in varying conditions, Rain and regular with deforst and heat on and everything is smooth again. Thinking the o2 fixed the problem just weird that we couldnt find that out with the scanner sooner. figured it would have picked up on something like that. Thanks for the help and glad i am able to drive the jeep again.