Miss when warm

[clearman]

I have a 93 trans am that misses only after warming up. When its cold, it runs great, but as the engine temp goes up, it begins to miss under acceleration as well as have a slight burble in the idle that can barely be heard. Anyone got an idea???

[Joe 1320]

How many miles on it? How old are the plugs and wires?

[clearman]

Plugs, wires, and optispark are all brand new. It has 160k miles on it.

[Craig 94 TA GT]

I would put my money on the ICM. Could also be the coil, but when I had a bad ICM, it had similar symptoms.

[Joe 1320]

I would put my money on the ICM. Could also be the coil, but when I had a bad ICM, it had similar symptoms.

if the plugs wires and opti are known to be good, then what the guy above me said^^^^^^

[TraceZ]

When you install the new ICM stick a couple washers behind it to space it away from the cylinder head a little. This will create a little air gap that will help it stay cooler. Electronics and heat do not get along well. I have no idea why GM would mount something electronic directly to a hot cylinder head, unless they thought that the cylinder head would be cooler than the ICM and help displace its heat.

[Craig 94 TA GT]

Yup, I've done it and have had no ill effects. It seems to be a bit more responsive when warm, but it could very well be the "placebo effect."

[zer0signal667]

I have a 93 trans am that misses only after warming up. When its cold, it runs great, but as the engine temp goes up, it begins to miss under acceleration as well as have a slight burble in the idle that can barely be heard. Anyone got an idea???

I had this exact same thing happen shortly after I changed plugs and wires. It turns out one plug had backed itself out of the head and was loose. I guess when the head expanded, it was loose enough to loose some compression through there and cause a miss under load.

[clearman]

Ill give the ICM a shot, thanks for the help.

[clearman]

Well, tried a new ICM, and no change. I have found though that there is a change in the way the car runs with temperature changes outside. When its very cold, it runs almost normal even as the engine warms up. As the air gets warm, it begins to miss and even cut completely out when I hit the gas quickly. It dies when I stop, and will not hardly start back up. Even if the air is warmer, and the engine is cold, it runs fine until it warms up. I did get a code...the ECT sensor, but that checks out good, and it is brand new as well. The coil is also brand new. I unhooked the egr valve thinking it might be that, and no difference. It is closed because I can manually open it and the engine will stumble. Anyone got any ideas???

[TraceZ]

It sounds like it needs to be scanned. I'd be interested in seeing what the PCM is getting from the ECT sensor, and making sure all other sensors are reading within tolerance.

[fastTa]

Well, tried a new ICM, and no change. I have found though that there is a change in the way the car runs with temperature changes outside. When its very cold, it runs almost normal even as the engine warms up. As the air gets warm, it begins to miss and even cut completely out when I hit the gas quickly. It dies when I stop, and will not hardly start back up. Even if the air is warmer, and the engine is cold, it runs fine until it warms up. I did get a code...the ECT sensor, but that checks out good, and it is brand new as well. The coil is also brand new. I unhooked the egr valve thinking it might be that, and no difference. It is closed because I can manually open it and the engine will stumble. Anyone got any ideas???

There are many codes involving the ECT sensor and circuit. Which exact code was it?

Did you place the washers behind the Coil/ICM bracket as suggested?

[clearman]

Yes, I placed 2 washers behind each bolt. It was code 15 for the ECT. I just took it for another drive and it seemed to drive better, very little miss and no stalling when coming to a stop. While idling, it has a thump in the exhaust that can only be heard behing the car every once in a while. It is not a regular timed noise, but obviously something is not right. It does not do this either when cold.

[fastTa]

If the ECT circuit is truly malfunctioning and not reporting the proper coolant temp at the waterpump, it is likely that the PCM is adding way to much fuel to compensate for the extremely low coolant temp it falsely perceives. If the ECT circuit is intermittently or completely inoperable, it is possible that the PCM is using -40 degrees F as the reference coolant temp. This can make cold starts a PITA because of the extra fuel being added and "flooding" the engine.

In your case it sounds like the ECT is actually just reporting a very low intermittent signal and making the car run overly rich. The "thump" noise you hear is likely a misfire.

Was there no misfire history? Do you still get the miss at WOT? You might want to put a few more than 2 washers...more like 5.

[97LT1Z28]

Given 160K miles could a worn timing chain cause the problem?

My car (when it had a timing chain problem) would run great when cold - then after it heated up, it would miss - especially at low RPM acceleration.

When the engine was cold, it would idle at a higher RPM which would help hide the engine miss - then after it warmed up and the RPM's went back down, the miss was plain as day. It was worn cam/crank gears and stretched timing chain.