QuestionI have a 97 corolla, 5-speed with 135,000K that has a fluctuating idle problem. When I first start my car it idles at about 1500RPM (higher if colder) and will gradually fall to about 1100PRM if cruising along a stretch of road for say 10 minutes. However if my car is sitting idling in my driveway until it warms up fully (about 15 min) the rpms will drop to about 750 which is the correct operating temperature. Once I start driving for stretches of road and come to a stop the rpms will go back up to 1100RPM.
So what I've noticed is that my car will only go to normal operating temperature if i'm in stop and go traffic or its been sitting for a while. My temperature gauge will sit just about in the middle when correct idle, and a few degress below when around 1100RPM. My cooling fans go on when at 750RPM and once cooled, the rpms go back to 1100RPM.
I'm thinking I may have a vacuum leak but not sure. Things I have replaced recently were air filter and cleaned throttle body, Replaced PCV value, spark plugs/wires, distributor, fuel filter, and new rad.
Thanks for your help.
AnswerHas the thermostat also been replaced? If not it could be the problem.
An intake air leak could also be the cause but would not be temperature related, there is also a coolant temp sensor for the ecu that can directly affect idle speed, it's located near the thermostat housing, it should have a green connector with two wires, in most cases a failure of the sensor will make the check engine light come on but it is possible to fail without turning the engine light on, you can also have it checked to see if there may be some trouble codes stored in the computer.
