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Corolla Idle Issue

91 Corolla,automatic, 138k miles. When car is idling in gear, it vibrates the whole car. Not really too rough but enough to be annoying. I usually shift up to neutral when stooped at lights/heavy traffic(stop & go). It runs smooth in park and neutral. So smooth, it feels like the car is not even running. All motor mounts have been replaced and are good. I turned up the idle just a little and it helped somewhat to the point where it doesn’t feel like it’s going to stall. When I come to a stop, It starts out smooth then starts the vibration like if the air kicked on and the idle isn’t set correctly.

How did you turn up the idle on your car isnt it fuel injected? Your car still has idle speed adjustment possibility? Does sound like a low idle speed concern but why the low idle. Does your car need to be emission tested? does it pass easily. Post back with your type of induction system (carb. or fuel injection)

It’s a 1.6L fuel inj 4A-FE. Idle was turned up at throttle body where accelerator cable attaches. It does have a hex screw with 8mm nut. I just turned it up very little. No emmission testing required. Also there is no check engine light.

Note: If you turn this screw to open the throttle plate, the Idle Air Control servo will close down the bypass air the equivalent amount taking you back to where you were. Now that the throttle plate is being held open the Throttle Position Sensor will be off the idle indicator postion so the computer does not know you are at idle. The only time I have seen the throttle stop help anything is when the throttle plate is catching i.e. the car will by hard to get going when starting out and will tend to jump on leaving a stop. Even then it is adviseable to check that the TPS is still working correctly at the closed throttle position.

You probably just need to have the throttle body cleaned and the bypass passage and IAC cleaned.

Hope this helps.

I will clean the throttle body and IAC. Will post with results when done.