First, my car is a 1990 Pontiac Grand Prix LE with a 3.1 MPFI V6 paired with the 4T60 Auto trans. The problem I am experiencing started out of the blue, for no obvious reason. When I turned the key, it would turn over and start for about 1-2 seconds, then die. If I give it just a little bit of gas, it would start just fine but soon as I let off the gas, it died. Had to drive it 20 miles home using both feet, lol. I thought it might have been the fuel filter, changed it and no fix. Thought maybe the Throttle position Sensor, replaced it and it helped a little, but did not fix it. Replaced the Idle Air Control valve, that helped with the incredably rough idle, but did not fix the problem. Since then, if I start it while giving it a little gas, then keep the RPMs at around 1,500 for 2-3 minutes it will idle on its own. But if I keep the idle too high (2,000 -2,500) the SES light comes on, but will turn off when I let the rpms go down under 1,500. Pretty sure that is just telling me the RPMs are too high. Occasionally the car will begin to climb in idle speed up to 2,600 +/- all on its own, for a while, then go back down to 1,100 +/- . This causes the SES light to come on as well. Other than that, the SES light DOES NOT come on.

I thought it might have been the ECM so I beat on the ECM and wiggled it around real good, which sometimes kills the car while idling, if the ECM is bad. It kept running. I also discovered that my ECM is not very old, and a re-manufactured Delphi ECM. Thought about the EGR as well but it is digital so I cant test the EGR valve without a diag. machine... I checked the Manifold Air Pressure sensor and the connector is broken. I was told that to test it, have the car running and un plug it, and if the car keeps running, its bad. And when I did this, it kept running. I also remembered that after giving the car a tune up, I took it down the freeway really fast, then slowed down to try out the cruise control and it didnt work. I pulled over and discovered that the vac hose to the map sensor was not plugged it. Cruise control worked after plugging it back in. After consulting my Dad (35 year mechanic) and he things it is probably the MAP sensor. But I figured I would ask you all about this as well. I dont want to spend the $60 in replacement parts just to have the problem remain. I apologise for the book, but I wanted to be descriptive to insure accurate answers