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Exact symptoms of a bad Coolant Temperature Sensor


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After a couple months of throwing parts at my 92 CSS, I finally fixed the problem. Just for the sake of troubleshooting, here are the symptoms of driving with a bad CTS for a month:



- 1992 Olds Cutlass Supreme S, 3.1L

- New IAC, TPS, ICM, Coils, Plugs, Wires, MAP, known good ECM, Vacuum leak check passed, Injectors tested OK, FPR not leaking, not losing coolant...



- Car will not start on its own. Needs throttle in order to start.

- Car will die at any speed if throttle is not maintained.

- Idle can be achieved by "feathering" throttle, but only lasts for a few seconds before stalling.

- CES light intermittently turns on and off, but this does not happen very often.



- High idle (1250-1500 in drive). Car will accelerate from 25-40mph on its own, no throttle needed.

- While rolling, putting car into Neutral causes idle to raise to ~2500 RPM.

- When slowing down, car will maintain a high idle until speed reaches ~3 mph, then idle suddenly drops to ~500 RPM. This happens both in Neutral and in Drive.

- Car lopes at idle and stalls regularly, unless throttle is applied to compensate.

- In traffic, car gets pretty hot. This is caused both by having to keep throttle applied, as well as the fans not working because the ECM doesn't know the actual coolant temp.


I just thought I'd put this all in a thread in case someone ever has the same combination of problems. I hope this can help others in their troubleshooting.

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Well, I spoke too soon apparently. Car drove great for a day and a half, now it's doing the same thing again... :willynilly:


What are the characteristics of a bad ECM? It seems that the car runs great for 24 hours after I do an idle relearn, then it's back to being shitty. Sound about right?


I'm ready to drive this thing off a cliff...

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dude a lot of those symptoms you described are very simlar to what my car is doing right now. i haven't driven it in almost a month now and nobody can tell me what is wrong. i have a new coolant sensor, new iac, new tps.....

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That's classic symptoms for the 3.1 MPFI for almost everything. I still to this day have a random HIGH idle I can't figure out.


I don't really know what causes it to be so funky, b/c our cars seem to have the same symptoms for every broken part. If you have access to a scan tool that will show you read outs of what the ECM is seeing, that might help. It's expensive, but doesn't sound much more expensive when you think of parts thrown at the car!

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It has to be something that gets hot and starts to leak as the motor gets hot. The car runs perfectly every morning when I take it to work, which is about a 20-minute drive. Sometimes, though, right before I get to work it will start acting up.


At lunch time (~3 hours after parking it), when I get in the car to go out, it won't start or idle without additional throttle applied. Same story with driving home.


Car sits overnight, runs great the next morning (for 20 minutes).


I'm beginning to think it may be intake manifold gasket related, since the problems seem to coincide with the actual temperature of the engine block itself.

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I know you said new ICM, but those sound like the same exact symptoms I had when mine was bad on another car. I had a new one also, but didn't put the thermal paste on right I guess. The car would run great for 20 min then act up and die. I would wait and it would start with some throttle. When cold, started great. I never got an SES light.


Also check your engine to body grounds.

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Mine is doing about the same thing, when starting it up you have to feather the throttle unitl the CES light comes on then it will idle on its own but kind of erraticaly and will idle for a while then shut off. other times the CES light doesnt come on at all and will idle and run perfect but still randomely shuts off. also it will idle fine until the fan comes on and then idle down and sputter. sometimes the idle will come back up but other times it will just shut off. also it will heat up till about 240 then the fan comes on and it goes back down to around 210. any suggestions?

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