2000 Grand Prix GT Intermittent Hesitation and Stalling

Hi all I hope you all can help me out. I have a 2000 Pontiac Grand Prix GT which had the dreaded P0171 code and the culprit was the MAF. Before I changed it it would stall sometimes while waiting at a stop light and it would seem to want to stall while driving and it would catch itself. We replaced the MAF and now a new problem has occured. When I took it out for a drive it hesitated and would not go above 25 mph and when I stopped to do a u-turn it stopped hesitating and was fine after that. I returned to the shop and I went out with the tech and had to do a long test drive for it to do it again. The car has plenty of power under acceleration. We went back to the shop and he hooked up a fuel pressure gauge and went back out on a test drive and it was in specs and no hesitation. Any ideas what it could be? It only hesitated twice and now seems to run ok.

I replaced the fuel pump in October 2010 and recently replaced the fuel filter and PCV. When the engine was rebuilt in Feruary 2011 all vacuum lines were replaced.

Does it still have any codes? Usually the p0171 is a vacuum leak.

There are no codes. Vacuum was checked and no leaks. Could it be the ICM?

It could be a great possibility of things but i wouldn't just go throwing parts at it. When you are having problems with it try to scan it for codes then. If your tach drops to 0 right before it dies or the tach quits working then it could be the crank sensor.

There are no codes. The shop was able to get the car to do the hesitation and they checked the harness out. They are thinking it is the ignition switch or ICM. I only saw the tach at 0 after it dies. The majority of the time the car runs great, has good acceleration. Wonder if it is the crank sensor? The cat was replaced a year and a half ago.

Just had an issue like this today... the p0171 code check for vac leak and cant find one. IAC's are good. Maf is good. but when i was looking at the sensor data i found the O2 sensor was reading 5 mv continuous! I had made an O2 extension harness for my SD headers and i guess either water got into the connector or there is an intermittent short in it. after tapping the connector the readings went back to normal and my trims started going back down from being maxed out.

I am still having issues with the car. It idles fine but sometimes when I go to accelerate it stumbles and it acts like it is not getting enough fuel. It will sometimes stall right after you start it. The security light is not on. Fuel pressure has been checked and it is within specs. MAF was replaced. The shop is thinking there is a short somewhere because it is intermittent. Any ideas??? This is so frustrating. Thank you for any help.

Do you have access to a scanner? Could be something throwing your fuel trims off at idle. Atleast seei g the IAC count, MAP readings, and LTFT'S we could get a better understanding of what is happening. If your over this way to Daytona let me know. I can scan it for you. Do you have any mods that might have effected he car? If you want a tune PM me and we can plan it out.

I do not have any mods, it is all stock. The shop thought the PCM was acting up and they replaced it but was still doing the same thing. They are checing the wiring at the harness to see if there is something electrical is causing the issue. Unfortunately, I do not have access to a scanner. It is so strange that it will idle perfectly but when you are in gear and go to accelerate it will sometimes hesitate like it is not getting the fuel it needs. Could a failing crank sensor confuse the system to the point it will do that? This car is really turning into a money pit lol. I am really nervous to drive it because I don't want to be stranded since I live in New Port Richey and work in Tampa. Next week I will be able to work from home full time. Help!!!