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Re: vibrating @ stop lights
Check for vacuum leaks. Hoses, valves, reservoir, egr valve and gasket, HVAC controls & check valve. Low/rough/surging idle with TBI is almost always a vacuum leak, a very small one. Also, after replacing the IAC, make sure you do the relearn/reset procedure. Another possibility is base ignition timing set too low and causing low idle or too high and causing low vacuum.
If its running fine but there is noticeable vibration in the seat when idling in gear, check the motor and transmission mounts. Old rubber clam motor mounts get soft and squashed down over time and the bracket on the engine rests on the cross member causing a rough buzzing vibration mostly at idle.
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