My 1999 Pontiac Gran Prix GTP (3.8 Liter Supercharged) has 217,000 miles. For a year now the mechanics have been unable to successfully repair the problem. The engine starts normally in the morning, but after about 5 minutes, it will stumble, then stall. Often, the tach dies but the engine stumbles & continues to run. When the car dies, I can put it into neutral and crank the starter, but it will not restart. When I stop, I put the car in Park and close the ignition, & usually it will re-start on the first try. Sometimes I must try several times before it will restart. When the mechanics look at it, the computer has no record of the stall. I am thinking that perhaps the crankshaft position sensor is bad, because I don’t think that goes through the ECM. I hate to replace the ECM if it’s a sensor. Except for the intermittent stall, the engine continues to run smooth and strong, with mpg in the 24 overall range for me. Does anyone have advice for me?
The crankshaft position sensor is one of the primary inputs to the ECM. So if the signal is lost from the crankshaft position sensor the engine will shut down.
There’s a resistance spec for the crankshaft position sensor. So what should be done is test the sensor when cold, and then take a heat gun and heat the sensor up and measure it’s resistance. It’s here where a defective sensor will show up.
When it stalls and won’t restart you have to be ready to check for spark and fuel.
Get a spark tester, when the engine doesn’t restart install the spark tester on a ignition wire to see if there is spark.
Next check for fuel. Find the fuel test port and depress the schrader valve to see if there is pressure. Ideally having a fuel pressure gauge would be the way to go.
You could also see if the injectors are being pulsed. Purchase a test “noid” and install it at one of the fuel injector connectors, crank the enigne and see if it pulses.
These are some of the basic steps your mechanic should have performed.