Q: Low idle when stopped unless the a/c is on or i shift into neutral

asked by on December 23, 2016

Everytime i come to a complete stop without the a/c on it drops to a low idle but never stalls, if i turn on the ac or shift into neutral it returns to a normal idle? Also if i shift back into drive or turn the a/c back off while still stopped the idle will remain normal... that is until of course the next stop light or sign!

My car has 91000 miles.
My car has an automatic transmission.

Due to the change in load put on the motor when shifting into gear vs neutral and the change in load when the a/c is on vs not on, this may suggest you have a bad MAP sensor. Both of these situations represent a change in the load put on the motor, which changes the pressure inside of the motor as well. 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 (spark knock) that can damage the engine and decrease performance. This may also be caused by other closely related components such as a dirty or failing mass air-flow sensor, a faulty throttle position sensor or potentially a vacuum leak. I would recommend having an expert from YourMechanic come to your location to diagnose and inspect your vehicle.

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