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I have battled rough idle for years have replaced: both O2 sensors on the exhaust system b/c engine light was on, Crank Pos Sensor b/c it was stalling at red lights, fuel filter, PCV valve and collapsed hose, sparks, and thermostat b/c it was not coming up to full run temp. Thermostat made a major improvement, but it still idles rough especially on cold start. No engine light at this time. Air & oil filter and oil are clean, Mobil/Exxon/Sunoco/Citgo fuel only. Any guidance you have will be greatly appreciated. </font>


I'm also wondering what oil is best for it? Mobil 1 makes idle worse. </font>


Thank you!</font></font>
Edited by: sw2cam

Sounds like you replaced everything but the most common thing to go bad. Replace the engine coolant temperature sensor and its connector if it has any corrosion on it.

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I agree. Back in October, my car was having some of the same issues and I replaced the engine coolant temperature sensor and it fixed the idling problem. I also ended up replacing the connector. Not too bad of a fix, anyway!

Its pretty nice when its a simple easy fix. People spend hundereds of dollars diagnosing the same problem and it all comes down to a $14 part.

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On this same subject, I just purchase a '99 SL2 and it seems to have a similar problem after driving it only a week. Of course ran fine when I test drove it and first purchased. Has 114,000 miles on it and I've got all of the service records. Everything mentioned above has been replaced except this same engine temp sensor. Will go ahead and replace it and see if it fixes it, but can someone please explain how this causes the problem.
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The ECTS plays a key role in engine operation.

From the FSM:

The ECT sensor is located in a coolant passage in the side of the cylinder head near the upper radiator hose. The ECT sensor is a two wire thermistor type sensor that varies resistance according to changes in engine coolant temperature. The PCM supplies a five volt reference through a resistor to the sensor, which is connected to ground. When the sensor is cold it has high resistance, high signal voltage at PCM. As the sensor temperature increases, its resistance decreases, lower signal voltage at PCM. The PCM uses the signal voltage to determine engine coolant temperature.

The ECT sensor is used by the PCM for fuel control, calculation of temperature of air/fuel mixture at the intake valve in conjunction with the IAT sensor temperature, Coolant Temp telltale illumination, coolant temperature gage position and cooling fan relay operation. The PCM also uses the ECT sensor for ignition timing, exhaust gas recirculation (EGR) operation, evaporative emission (EVAP) purge solenoid control, idle speed control and oil life monitoring.