Q: Car shakes at idle, worse when compressor/accessories are on

asked by on September 15, 2017

When in R,D,or L the car vibrates quite a bit at idle but not when driving normally. In park or neutral the shaking isn't there. When you run things like rear window defrost, AC/Defrost (compressor) or many accessories at once it gets much much worse almost to the point of a vibrating massage chair. I've noticed when the compressor is on if you give it too much gas at once and make the RPM's jump say 700-2000 the drive belt will squeal for a split second. I've been told the motor mounts but there is hardly any movement when shifting through gears and the stationary torque test didn't show excessive movement either. Wondering what to do next.

My car has 60500 miles.
My car has an automatic transmission.

This may be a sign of a faulty MAP sensor as the vehicle responds adversely to a load put on the motor (i.e. putting it in gear, making turns (which makes use of the power steering pressure switch) turning on AC or engaging the brakes). 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, 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. 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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