Idle speed on my 1987 Mercury Grand Marquis is going up and down - what' s wrong ?
When my Mercury is running for a while at idle speed, engine speed suddenly starts going up and down. Sometimes the engine even turns off after a few seconds. I checked all vacuum hoses - they seem to be OK. Can it be any sensor or valve causing this problem ? While driving everything is OK. This only occurs when enging is idling and the selector lever is set to "P".
Sounds as if the air idle control is defective or needs to be cleaned..
http://www.partsgeek.com/catalog/1987/merc ury/grand_marquis/fuel_injection/idle_control_v alve.html
Could it also be the TPS? (Throttle position sensor). They do wear out sometimes.
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