I have a 1993 GMC sierra SL 1500 4.3 litter it will start right up and runs fine for twenty minuets or more then it quits and wont even try to start its getting fuel and spark. if i wait till it cools of it will start up no problem.i cant find the problem.?
Sometimes an ECM, or Engine Control Module aka Ignition Control Module, when it is beginning to fail it will work when cold, then normal engine heat will cause it to fail. Then when it cools, it will be OK. That is a guess, but you describe just how it would happen
Thing is, I see you said you have fuel and spark, but with the ICM going whacky the spark will be there, but timing and./or fuel trim will be way off
Can you get the codes read? That might detect which system or sensor is failing when it warms up. --- I would suggest starting fluid but if the timing is off, you don't want explosive gasses floating around and popping when it's not supposed to. If I had to guess, I would suspect the crank position sensor. You might be able to spray it with a canned air duster to cool it off and see if it fires.
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