My Chevy P.U. idles around 2500rpm then gradually falls to 1100rpm each time I restart it from previous running. Normal idle should be 800rpm. There are no DTCs; I checked all vacuum lines for cracks - all seem ok. The car recently passed our state’s emission test so I’m assuming that O2 sensor is ok. It’s a 5 spd; 4.3L TBI V6; 106,000 miles…any suggestions on what to check next?
One thing to check is the signal from the engine coolant temp sensor to the computer when the engine is hot. If this sensor malfunctions while the engine is hot, it can cause the computer to believe the engine coolant is cold. The computer then sets the idle speed high to try to get the engine up to operating temperature faster.
Disconnect the electrical plug from the iac (idle air control) valve on the intake tract. Start, and run, the engine. If the idle rpm is more nearly normal, then, the iac valve is, indeed driving the idle rpm too high. (The iac valve responds to the engine computer’s commands.) It would, then, be a matter of finding out why the engine computer is ordering the iac valve to do this.
Thanks for the suggestion…the problem is solved! While warming the engine so I could check the “hot” resistance of the temp sensor, I noticed that the distributor cap was vibrating. It was very loose so I tightened the screws and the idle is back to 800 rpm…