Stalls on pedal liftout

[r1rider]

I have a 1997 Grand Prix GT with 3800. Girlfriend tells me the other day the vehicle stalled out while driving. When I checked I found the car idles (sat in driveway at idle for 1/2 hour) and accelerates with no problems. I did notice, however, that everytime I quickly lifted out of the gas pedal the RPM drops to about 500rpm. If I start the manuever at 1000rpm then it will definitely stall (drops to 400-500rpm then down to 0rpm). If I start the manuever at 2000+rpm then it still drops to 500rpm but recovers and doesn't stall. Doesn't matter if I'm in Park or Drive.

I've already removed and cleaned the Idle Air Control Sensor with no fix. Checked for a vacuum leak (using carb cleaner on hoses and fittings) and don't see any. Going to take it to Autozone to check codes tomorrow. Looking through forums I see MAF and TPS as possibles...but I got a feeling you guys are going to tell me crank sensor. Most of the forum posts regarding CPS sound like static problems (i.e. dies at idle). My issue is definitely related to transient pedal movement - only when backing out of the gas pedal. Just wanted to check. Any thoughts?

 

[Rather B Blown]

for sure sounds like a vacuum problem..check all the lines and t's

 

[r1rider]

I asked a couple people at work about issue (I work at OEM) and most thought backside of throttle body butterfly valve might be coated with carbon and choking air on pedal liftout. I removed throttle body and cleaned it out spotless. No fix. Then I rechecked vacuum lines and found a T connector that was bad. Ran up to dealership and replaced the connector. No fix. I then got a new Idle Air Control valve and replaced. No fix.

I will say that after all of these "repairs" I am unable to stall the vehicle in gear anymore. It only stalls in park/neutral. If I rev engine to anywhere between 1000 to 1700rpm and lift out the engine will stall. If I rev to 1800+rpm then no stall.

It has set a couple codes.....P0107 (MAF) and P0121 (TPS). Not sure if codes where set while I was screwing around. Going to look them up and see where it goes.

Update: although I'm not 100% sure....it appears the codes are setting due to the problem event (I don't believe the components are causing the problem).

This is frustrating. I know there's a technician somewhere who has run into this problem and solved it.

 

[r1rider]

I looked at a couple additional items on the car (TPS and EGR) with no fix. Believe it or not....the problem was caused by low trans fluid. I think air in the torque converter (due to low trans fluid level) allowed the engine to decrease speed too quickly (faster than the control system....i.e. idle air control.....could handle).

Makes sense.....lack of fluid between the torque converter pump and turbine would reduce the resistance on the pump side (engine).....allowing the engine to decrease speed too quickly.

I topped off the trans with fluid, performed a couple wide open throttle runs (to fill the converter back up with fluid), and the problem was solved.

Even though I didn't get much help here.....I figured I would post the fix in case anyone else runs into the same problem.

 

[SlowNA06]

Good to hear you're fixed!

 

[shawngtp]

I looked at a couple additional items on the car (TPS and EGR) with no fix. Believe it or not....the problem was caused by low trans fluid. I think air in the torque converter (due to low trans fluid level) allowed the engine to decrease speed too quickly (faster than the control system....i.e. idle air control.....could handle).

Makes sense.....lack of fluid between the torque converter pump and turbine would reduce the resistance on the pump side (engine).....allowing the engine to decrease speed too quickly.

I topped off the trans with fluid, performed a couple wide open throttle runs (to fill the converter back up with fluid), and the problem was solved.

Even though I didn't get much help here.....I figured I would post the fix in case anyone else runs into the same problem.

Good job yo:th_thumbsup-wink: