When I am at a red light with the lever on Drive position, there is a vibration in the car body. I did some experimenting and found that if I slightly increase the engine speed while holding the brakes, the vibration disappears. So I asked the auto repair shop to do it - to increase idle speed of the engine. They told me that the car is pre-programmed and that they couldn't do anything there. Can you explain this to me please and tell me how I can get my car fixed? Thank you.
A few different items can cause the vehicle to have a vibration. The most likely would be a misfire, engine mount, or transmission mount.
Most vibrations have a RPM range at which they are most noticeable. Increasing the idle if it was possible would not fix this issue long term. The manufacturer sets the idle at the factory and unlike order cars, is not something that can be changed.
I'd suggest having the vehicle looked at by a qualified technician, such as one from YourMechanic. They will be able to diagnose the vibration you're experiencing firsthand and perform any repairs that are required.
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