when started 9 times out of 10 the engine will rev up to about 2000 rpms and back down over and over(about 2 seconds between each fluctuation) for about 5 minutes until it finally smooths itself out and then runs fine. it only does this in park and when its started (i usually let it run its course before i drive but have put it into gear and it stops instantly). I have been told that it could have something to do with the vaccume lines but have also read that it could be anything from an o2 sensor to gummy valves and so on. just trying to narrow it down a bit. thank you.
My car has 190000 miles.
My car has an automatic transmission.
Hi There, It sounds like you may have a dirty or failing idle air control valve causing your engine idle to fluctuate. The idle air control valve is a small valve on the engine’s intake system that reads the air intake as it comes into the motor. This is controlled by the car’s ECM which uses this information to make adjustments to the air/fuel ratio depending on various inputs such as outside air temperature, intake air temperature, load and various other things. As you accelerate, your car’s engine is receiving a much higher dose of fuel than when at idle and conversely, when you let off the gas pedal, there is a sudden change in this fuel supply as a result of your foot letting off the pedal. When this happens, the job of the idle air control valve is to bring this deceleration down to a slow and smooth idle rather than suddenly cutting off the fuel supply causing the motor to die. When the idle air control valve is not working properly, this cause a disruption in this process resulting in the engine not being able to idle properly.
The other possibility may be a faulty MAP sensor as it seems the motor is responsive to a change in load (i.e. putting it in gear). The Manifold Absolute Pressure (MAP) sensor senses engine load and generates a signal that is proportional to the amount of vacuum in the intake manifold. The engine computer then uses this information to adjust ignition timing and fuel enrichment. When the engine is working hard, intake vacuum drops as the throttle opens wide. The engine takes in more air, which requires more fuel to keep the air/fuel ratio in balance. When the computer reads a heavy load signal from the [MAP sensor]((https://www.yourmechanic.com/services/manifold-absolute-pressure-sensor-map-sensor-replacement), it adjusts the fuel mixture to slightly more rich than normal so the engine can produce more power. The computer will then retard (back off) ignition timing slightly to prevent detonation that can damage the engine and decrease performance. I would recommend having a professional come to your location to diagnose and inspect your vehicle.
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