check engine light goes on and off on 1991 Mitsubishi 3000GT SL

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For the last three years the check engine light has been going on and off. When it first started coming on it would run rough then run fine when it went off. Now when the check engine light comes on it runs fine and rough when it goes off. I've had a diagnostic test done four times and it is not tripping any codes. Also the car is only doing this when it gets to running temperature. This car has always had scheduled maintenance done and has never been neglected. The mechanics are stumped as to why this is happening. Any ideas?
(1) Answer
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1991 only had what now seems primitive on board diagnostic capabilities, it looked for opens or shorts in key engine sensors around 1996/97 on board diagnostics level 2 (OBD2) became standard. OBD2 monitors sensor opens, shorts and performance on a lot more sensors and has lots more diagnostic capabilities and problem capture ability.
Check out this Mitsubishi specific link it may help
I have replaced two computers on these 3000GT that had computer issues. A company called Fuel Injection Corporation
can test the computer but I would want to see trouble code information and test wiring ans sensors prior to condemning the computer as being defective.