Pontiac Repair: 1990 pontiac grand prix reving and speedometer problem, pontiac grand prix, classic symptom

Question
I have a 1990 Pontiac Grand Prix that has not been driven for about 6 months except to be moved little way. It runs like a dream. No miss or anything but revs when started and dies but only when warm. The tach and speedometer always reads at least twice what it should. We have tried changing iac and throttle sensor. What could it be and would the ecm make the speedometer and tach go nuts? There is no carbon build up that I can find. have also put fuel injector cleaner in it. Please help as my van blew up and need to use this car now. Thanks. It is a 3.1 6 cyl. le

Answer
Hi trudy.

That year car is a noted for bad ecm's. a classic symptom is the speedo wacking out and stalling. So I would say your right in your guess. If you choose to replace it your self do a goodle search for 3.1 Liter idel learn proceedure. If you don't follow it the idel will be erratic and you will have stalling issues with the new ECM due to not learing the idle speed. good luck :-)