[TopperT]

Seems to only happen when it's warm.
The main issue is pulling up to a red light for an extended period of time. Idle will be normal. Then it'll settle a bit to 5-600RPM. Idle air corrects normally. OR sometimes bump to 1000+ RPM>lurches forward before settling to normal. The odd time after idle correction it'll just stall out completely. Takes a couple tries to restart.
I parked in the driveway, in drive(daughter on the brake) Seemed to happen at the moment the A/C cycled on or off.
The only other symptom is, once rolling, if I hop on the throttle hard, it'll sputter for a moment between 2nd and 3rd then take right off like it should.
I've checked over everything I can think of. Cleaned and tested everything. MAP is good. TPS is good. Idle air valve was dirty, so just changed it. Seemed logical given the "idle" issue..
Any feedback would be awesome. A huge thankyou in advance.

 

[1999 White C5 Coupe]

Seems to only happen when it's warm.
The main issue is pulling up to a red light for an extended period of time. Idle will be normal. Then it'll settle a bit to 5-600RPM. Idle air corrects normally. OR sometimes bump to 1000+ RPM>lurches forward before settling to normal. The odd time after idle correction it'll just stall out completely. Takes a couple tries to restart.
I parked in the driveway, in drive(daughter on the brake) Seemed to happen at the moment the A/C cycled on or off.
The only other symptom is, once rolling, if I hop on the throttle hard, it'll sputter for a moment between 2nd and 3rd then take right off like it should.
I've checked over everything I can think of. Cleaned and tested everything. MAP is good. TPS is good. Idle air valve was dirty, so just changed it. Seemed logical given the "idle" issue..
Any feedback would be awesome. A huge thankyou in advance.

Did you clean the inside of the throttle body bore?

Did you replace the PCV valve, two o-rings, spring and the black plastic fitting?

Did you check all vacuum lines for cracks / splits - especially on the ends where they connect to fittings (and check the plastic fittings to see if they are cracked)?

 

[Brhatweed]

My money is on the idle control servo on the side of the throttle body, they will stick and get sluggish when the BCM commands idle up for the AC compressor as carbon junk gets packed behind the servo piston and the engine will surge when the servo finally moves back.

 

[TopperT]

Just soaked and cleaned throttle body. I'll grab a pcv and associated fittings tomorrow. Will get back to everyone when I have more news. Good or bad.
Many thanks thus far.

 

[1999 White C5 Coupe]

Just soaked and cleaned throttle body. I'll grab a pcv and associated fittings tomorrow. Will get back to everyone when I have more news. Good or bad.
Many thanks thus far.

When removing the PCV valve fitting, the MAP sensor is snapped in to the top of it - and must be transferred to the new fitting.

ACDeco sells a PCV valve kit (o-rings, spring, black plastic fitting and PCV valve). It is inexpensive - buy it and not an aftermarket one (Made in China.....).

Be carefully removing the MAP sensor from the fitting. They are easy to break.

Let us know how things go.

 

[TopperT]

Appreciate all the suggestions. BUT..... Alrighty folks.
So far, I have replaced the Fuel filter, Mani pressure sensor, Idle air valve, Mass air sensor, and Throttle position sensor as well as the PCV valve. Cleaned the intake, butterfly and idle air ports. There are no air leaks on the intake anywhere. Confirmed with the old starter fluid rick (with a fire extinguisher on standby). I still had the odd stumble when idling, most notably when the A/C clutch engaged/disengaged..
Then today while driving in town at 30-40 MPH, the engine cut out, sputtered, and carried on as normal twice.
Then in the old Walmart parking lot at 10-15 MPH, she died completely twice. Restarted no problem.. > Inserted some colorful language<
So now I'm thinking:
Fuel pump?
Fuel pressure regulator??
Coil pack??
Cam sensor??
Or the A/C compressor?? < The coincidental stalling and revving at idle has me scratching my head.
Time for some deeper digging, and a lot more in depth testing.

 

[1999 White C5 Coupe]

Appreciate all the suggestions. BUT..... Alrighty folks.
So far, I have replaced the Fuel filter, Mani pressure sensor, Idle air valve, Mass air sensor, and Throttle position sensor as well as the PCV valve. Cleaned the intake, butterfly and idle air ports. There are no air leaks on the intake anywhere. Confirmed with the old starter fluid rick (with a fire extinguisher on standby). I still had the odd stumble when idling, most notably when the A/C clutch engaged/disengaged..
Then today while driving in town at 30-40 MPH, the engine cut out, sputtered, and carried on as normal twice.
Then in the old Walmart parking lot at 10-15 MPH, she died completely twice. Restarted no problem.. > Inserted some colorful language<
So now I'm thinking:
Fuel pump?
Fuel pressure regulator??
Coil pack??
Cam sensor??
Or the A/C compressor?? < The coincidental stalling and revving at idle has me scratching my head.
Time for some deeper digging, and a lot more in depth testing.

It has been a while, but my son had a new 2004 Pontiac Gran Prix GT with the non-supercharged 3.8 V-6. It had an external fuel pressure regulator valve. Perhaps check the line to it? You may want to change it.

You have done a considerable amount of work to the engine. Have you had it scanned for any trouble codes?

 

[TopperT]

Yes sir. No SES light. No codes. I'm running a realtime scanner on it right now to keep a birds eye view of the realtime sensor values.
Unplugged the AC relay this morning just to see if that is in fact the issue.

 

[TopperT]

PROBLEM SOLVED. Many thanks for your input along the way.
Watching realtime data i found some questionable MAF feedback. The voltages coming back from the sensor would be stable, increase on acceleration as is nomal, but drop to near zero (as the car sputtered). Finally SES light came and i had a P0102 Mass airflow low frequency.
MAF tested fine. Input voltages were on par as well as the sense voltage at the connector. Dug a little deeper.
CRACKED AND CORRODED sense wire terminal in the MAF harness connector itself. Adressed the other 2 pins while i was at it. She runs like a brand new car. Peppy. Responsive. Smooth acceleration.
CHEERS ALL. Happy times.
Thanks again.