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Q: drop into gear engine revs out of control

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start engine, when gear selector is positioned into drive engine revs full throttle. the car is in pristine condition otherwise. have owned for 20 yrs, started with this issue 5-6 years ago. has happened only 4 times. most recently was this past spring. engine revs , tires screech, and is very ,very scary. when ignition is turned off then restarted all is fine until the next time. mb dealer cant explain, nor can the local import service garage. can you explain any reasons for this???

My car has 29000 miles.

A: This is often times a sign of a vacuum leak...

This is often times a sign of a vacuum leak. There is a plate that is supposed to be closed when the engine is cold to preheat the air going to the throttle body. As the engine warms up this plate gradually opens to maintain a certain temperature of the air coming into the engine. This accelerates warm up time, controls hydrocarbon emissions, and prevents icing of the throttle plates in cool damp weather.

A "controlled" vacuum leak when the engine is cold and plate is closed, creates a normal fast idle condition to prevent stalling until the engine is warmed up, although it sounds like your issue is not quite a controlled fast idle.

I would recommend having an expert from YourMechanic come to your home to diagnose your vehicle and help you make the necessary repairs.

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