If a 1988 Grand Prix runs for five minutes then dies how do you figure out if it is an ECM or ignition module problem?

Answer 1

You need to determine what you're losing first. Is it spark? fuel? or compression? I would suspect the first 2. Lets eliminate fuel by getting it when it is cranking but not starting. (I am assuming you don't know if fuel is or is not the problem). Spray some starting fluid into the throttle bore for a few seconds, then go try and start it. If it starts, loss of fuel is your problem. Say it doesn't start, your down to probably loss of spark. Without seeing the chart for troubleshooting, I would suspect that the answer for testing the ECM is "Replace with known good part". Many times in that era, test procedures would lead to an ecm, but was not the problem. Typically, the control module was the answer. I am assuming you have a 3100 motor. I think you can check the resistance and compare it against a new one as far as the module goes. But I may be mistaken on that. Since you mention another problem of an intake leak, is it internal or external? Is the coolant low? The only way I see that as possibly being related is if the coolant had been drawn into the cylinders and caused hydrolic lock. But if it starts up after setting a short time, then I would disregard that. One note about intake leaks is that if it leaks internally, you oil will turn to a milkshake or coffee color. My 88 GP had a short in the thermostat in the radiator the dealer had to run a new ground wire from the computer to the thermostat to fix the problem. now the car runs but the tac buries itself.