Rough Idle, Engine dies, but only when warm?

[S1L1SC]

I have looked through some of the older threads, but can't find anything conclusive.

Here is the problem:
'91 Roadmaster Wagon.

Car runs and idles fine when it is cold. Once the car has been running for a while and the engine is up to operating temperature I run into the following issue:

When I stop at a traffic light the car goes into idle ok. But after a little while the idle starts surging. The needle starts jumping up and down and the variation starts getting worse the longer the car sits. After a while the engine will "stumble" and cut off.

From my research it seems that it could be the Ignition coil, Idle sensor of Mass Air Flow Sensor. Since the car only does this when the car is warm, I am wondering if I can eliminate the sensors?

Anyone have any idea what else it could be? Vacuum leak somewhere?

Thanks for any help guys!

 

[bdblly]

First off, with your car being a 91 (91-93 are TBI), you do not have a Mass Air Flow (MAF) sensor, so you can rule out that possibility. When you mention Idle valve, do you mean Throttle Position Sensor (TPS)? This sensor gives a reading to the computer as to where the throttle is, and if you're sitting a long time at idle, and the sensor isn't working right, it could be giving a bad reading. Or do you mean the Idle Air Control (IAC) valve? Is your Service Engine Soon (SES) light coming on? Even if it isn't, you can easily check for stored trouble codes. The diagnostic plug in is under the dash just to the pass. side of the steering column. There are 12 terminals on it, 2 rows of 6. Take a wire or paper clip bent into a U shape, and plug the ends into the last 2 on the upper right (pass.). Turn the key to the on position, but do not start the car! The system will flash codes through the S.E.S. light, first a code 12 to show the system is working. This is 1 flash, short pause, then 2 more flashes, there is a slightly longer pause between each code flashes, but each, including 12 will flash 3 times before moving on to any others. If there are any other codes besides 12, they will flash, 3 times each, then 12 will flash again to show any other stored codes have been shown. Hopes this helps, and good luck.

 

[buickwagon]

Unplug the O2 sensor and see if the problem goes away.

It sounds like the ECM is chasing the mixture -- a lazy O2 sensor can cause this, as can a sticking IAC (or restricted IAC passages).

 

[dcraig1000]

i second the oxygen sensor. sounds like when it goes into closed loop, your problems seem to start

 

[S1L1SC]

Sorry, yes - IAC & TPS...
Terminology still gets the better of me at times.

No Check engine light at this time, although the Low oil light is on permanently... Most likely I need to clean the sensor again.

 

[pbrktrt]

Could be the temp sensor dumping more fuel than needed.

 

[buickwagon]

You mean the engine temp sensor? I think the O2 sensor overrides the engine temp sensor when it's operating in closed loop mode.

The intake air temp sensor coupled with the MAP sensor estimates the air mass, and contributes to block selection but again, the block learn value is ultimately set by the O2 sensor (via the integrator).

My thought is that if the O2 sensor is slow to respond then the integrator may be drifting between way low and way high, causing the oscillation the OP describes.

 

[S1L1SC]

Turns out it was the distributor. The shaft was worn out, so the idle air control kept chasing the timing and over/under adjusting eventually causing the engine to stall...

Go figure. Wish it had only been a bad sensor.

 

[buickwagon]

Interesting. I'll have to remember that one.

 

[RandNotAyn]

2000 LeSabre, 118k, starts clean, stalls when warm.

Tracked it down to EGR (only place where hot exhaust can get back into the intake manifold).

 

[RSS94RM-Concord]

I have looked through some of the older threads, but can't find anything conclusive.

Here is the problem:
'91 Roadmaster Wagon.

Car runs and idles fine when it is cold. Once the car has been running for a while and the engine is up to operating temperature I run into the following issue:

When I stop at a traffic light the car goes into idle ok. But after a little while the idle starts surging. The needle starts jumping up and down and the variation starts getting worse the longer the car sits. After a while the engine will "stumble" and cut off.

From my research it seems that it could be the Ignition coil, Idle sensor of Mass Air Flow Sensor. Since the car only does this when the car is warm, I am wondering if I can eliminate the sensors?

Anyone have any idea what else it could be? Vacuum leak somewhere?

Thanks for any help guys!

Had same problem. ...the fix for me was changing the two oxygen sensors. And MAF sensor.