Grand Prix running issues
Asked by Jeff Mar 15, 2023 at 08:13 AM about the 2004 Pontiac Grand Prix GT1
Question type: Maintenance & Repair
I have a 2004 Grand Prix. After it idles for a while it will start running rough.
The fuel trim will go to -25%, the MAP voltage will slowly climb to over 3
volts, the downstream O2 sensor will bottom out, and the MAF values will
start climbing but the idle speed doesn't change. I replaced the downstream
sensor, cleaned the throttle body and the MAF. I replace the EGR valve
because it would not open with my scanner. Nothing I've done changed
anything. What should I check next?
7 Answers
Is the ECM out put power stable at 5 volts? Check sensor and computer grounds, I can’t remember just how many running problems I’ve fixed due to a bad ground in the electronics. You may actually have a bad MAF. Do the tap test when it starts running bad and see if the engine has any kind of response, it if does then replace the MAF.
5 volts is steady, sensors are grounded. I have figured out that it starts acting up as soon as it goes into closed loop. The upstream O2 goes to 1107mv and switches to around 650mv back and forth but the downstream drops to 30mv and stays there. ST and LT trim goes to -25% and stays there. Tapping MAF sensor does nothing. I can read MAF sensor reading and it appears that it is functioning normally
Make sure there are no unmetered air leaks or vacuum leaks. Get it to operational temperature then electronically disconnect the MAF with it running, if the engine smooths out then it’s a bad MAF sensor. If nothing happens then it could be a misread on the ACT sensor. Something is causing it to lean out after it warms up.
