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'95 Buick Century Idles Rough

My mechanic brother and I am racking our brains over this one. I have a 1995 Buick Century with 149,000 miles that will occasional and randomly idle rough when started warm. Also, on several occasions it will idle so rough it kills the engine and I’m unable to start the engine until the thermostat reads completely cold. I’ve taken it to 2 separate mechanics who both are unable to pull any error codes (even after the engine has died) and are unable to duplicate the problem. When it does idle rough most of the time I can rev the engine, put it in gear and drive it for a few minutes and the problem disappears. Other times I try to put it into gear and it kills, I put it into park and try to restart the engine and the engine will start for a brief second and then die and will die faster each time I restart until the engine doesn’t turn over at all. Then once the thermostat reaches ‘C’ it starts up like there was no problem at all.

 Here is a list of recent pertinent repairs: 
  Replaced Battery 12/2/11
  Replaced plugs and wires 4/11/12
  Throttle Body Gasket and Fuel Filter Replaced 3/18/13
  PCV Hose replaced 2/19/14  (last mechanic's guess on the problem)

I don’t notice any signs of a rich fuel mixture or abnormal exhaust (there was a discussion with a Buick with no start that had this). Does anyone have any ideas?

Thanks for the help!

The first thing I’d check is the engine temp sensor.

The engine temp sensor is a critical input that the ECU uses for fuel metering. If also tells the ECU to ignore the oxygen sensor and go ahead and run rich. If it’s bad, and continues to tell the ECU that the engine is cold even when it warms up, the ECU will continue to run the engine rich. The engine will run fine when cold, but run too rich when hot.

I’d also want to put the engine on a scope while it goes through its various stages of warming up. I’d guess that if the ignition system had a problem the engine would not be running well even when cold, but one never knows what’ll show up unless one looks.

Yeah sounds like heat is the enemy. Ive never experienced it myself, but have learned on the forum that sensors like a bad crank or cam position sensor can cause your exact scenario.

Good comments above. Could indeed be the ECM’s coolant temp sensor that is causing this. That part is usually fairly easy for a mechanic to test. When you say eventually it won’t turn over at all, does it still crank ok? That rrrr --rrr --rrr sound when you turn the key to Start? Do you mean by “not turn over” that it cranks, but won’t start and run? If so, heat related, if the coolant temp sensor checks out, I’d lean toward an ignition system problem. Crank or cam sensors, coils, etc. Usually these will throw a code though. No check engine light along with this?

A '95 GM car is horribly difficult to pull codes from. GM put ODB-II style test connectors in many models, but they are still running ODB-I electronics. ODB-II readers and scanners cannot connect to them. And, jumping terminals A & B doesn’t bring codes up on the CEL like avlotvof other ODB-I systems. These cars needed a now obsolete reader to pull codes. Personally, I’d go old school and check for spark at the plugs and a noid light on the injectors when it is acting up to see if the problem is fuel or ignition related. Lots of things can fail with heat, like sensors, coils, and ignition modules.

Somebody posted a link the other day to a site that showed how to easily check and read OBD-1 codes. Wish I saved it.

Here it is. A quick google search turned it up.

Looks like according to busted knuckles this wont work?

Yeah I’d be looking at the engine temp sensor first too. They are fairly cheap. Then maybe the air sensor. The other thing would be to check the fuel pressure to make sure the injectors aren’t sticking open and leaking fuel in the cylinders. They aren’t going to throw any codes but when the computer is hooked up to it, you check the actual temp and the temp the sensor sees.

Thanks all for all the quick and helpful comments!

To GeorgeSanJose: To answer your questions, when the engine won’t turn over there is NO cranking of the engine (no 'rrrr" sounds). In fact the car is just silent making me think briefly I had depleted the battery with trying to restart it but all the other electronics in the car work (windows, lights, etc). Also there was NO check engine light and no battery light on which boggles my mind. The car won’t start but there are no codes and no check engine light?

To Fender and Busted Knuckles: The first mechanic I brought it too was used to working on older cars (had 80s-90s cars in the garage and some classic 60s cars too) so I assume he would have know about using the obsolete reader and he was able to pull codes off my Buick before when other problems arose, but next time I’ll ask. Good suggestion on checking electricity on the wires. I’ve got a wire tester in my car for when the next time this happens.

Thanks again all for the advice I’ll be running through your suggestions until the problem is solved and I’ll try to comment back when I find out what it is.

Thank you!

If there is no cranking of the engine"rrrr", then I would replace the starter. That will not fix the rough idle. Fuel pump and low fuel pressure would also be suspect.