Thread: 2000 Grand Prix GTP Supercharged, Idle surging and stalls at idle

2000 Pontiac Grand Prix GTP, SC 3800 with a idle surge / stall problem and occasional P0300 code. Multiple parts replaced in an attempt to repair. Initial problem was bad fuel pump relay controlling high speed of fuel pump along with FP resistor. This GTP uses a 2 speed fuel pump where the resistor is used for slow speed of the pump and is switched out of the fuel pump circuit when higher fuel pump speed is needed due to fuel demand.
Next problem was random misfire P0300 and P0301, P0303 codes. This was isolated to defective coils and replaced ICM (ignition control module and all three coils with new).
Symptom: Engine starts, revs to about 1500rpm, slows down to about 600-1000 and then surges back up in RPM, stalls almost completely and speeds back up and eventually will stall out while it attempts a stable idle speed.
Fuel pressure at 43 PSI (range should be 48-55). Stable but a little low. Fuel flow for 30 sec at fuel rail test to about 1 liter, so fuel flow not restricted. Ultimately replaced fuel pump and pressure now at 50PSI. NO CHANGE in symptom.
Scanner data from an ACTRON Elite scanner indicated higher than normal LT fuel trim and we located an intake vacuum leak. Intake gasket replaced along with gasket for supercharger.
While intake was off, throttle body and egr valve and associated areas cleaned of any carbon (very little showing)
Fuel trims now mostly negative ( -8 to -10%), O2 sensors (has 2 heated sensors) are reactive. Short term fuel trims are showing positive and negative values (normal), so fuel trim appears to be reacting normally and is within acceptable range (+/- 10%).
IAC (Idle air control replaced. MAF (Mass air flow) replaced, fuel pressure regulator replaced.
CKP (crank sensor) and Cam sensor output are good and stable.
If the throttle is used, I can keep the engine running but it will not idle without stalling.
Any ideas or suggestions welcome... THanks.

Check again for vac leaks, especially the "T" under the S/C snout. Did you replace the MAF with an OEM one or aftermarket? Most aftermarket MAF's don't seem to play well from what I've seen around here.

Thanks FordMan77 - yes, changed the MAF sensor with OEM and all vac lines were replaced including the plastic "T" connectors. The S/C snout connector and tubing tested. Normally with a vac leak, the fuel trims will be positive, indicating the PCM is adding fuel to adjust for un-metered air entering manifold and this was the case early on before intake and SC gaskets were changed. The leak wasn't terrible but changed gaskets and all vac tubing while at it. MAP readings on scanner are within specs and even substituted another known good one.
Now, the fuel trim is negative but not terrible, indicating the PCM is calling for reduced fuel from the injectors.
One other symptom I forgot to mention is the check engine light and ABS error light dim when the engine RPMs slow and engine is close to stalling out. I don't know if this is normal for this vehicle (it belongs to my grandson). ABS error is un-related to engine problem by the way since the error light has been on for a few months.
Since I'm not able to confirm a "good" cam sensor signal, I may replace the camshaft sensor in case it is causing erratic timing advance, like shifting 20 degrees, which would stall the engine.
Still open to suggestions... thanks. (Reggie)

Scottydoggs, I think I've tried with MAF sensor unplugged, but I'll give it a try tomorrow AM. I wish I could attach the scanner readings to this post for a run cycle (cold start to stall), about 30 frames, and see it it shows something I'm missing, but I don't see a way to do that. Thanks. I'll get back on it tomorrow AM.

Tested with MAF unplugged - starting and idle were much worse with MAF disconnected. Test was done at cold soak (after overnight). Next, brought the engine up to operating temp and disconnected MAF. No improvement, actually ran worse. As mentioned before, the MAF has been replaced and the MAF readings in Gm/S on scanner are relatively the same with the orginal MAF and replacement.
I can't find a specification for the MAF readings in gm/s (grams per sec) for this engine, but have been informed that the MAF frequency should be about 2400 hz at idle and about 8500 hz at WOT. Would need a frequency meter to confirm or an expensive scanner, etc. Anyway, I'll continue investigating. I'm beginning to think I may have a erratic CKP or CMP which is causing the changes in engine idle as well as the random misfire (P0300).
I mentioned in my original post that the signals were good and stable, but the word "good" is relative. I should have said the signals appear stable and waveform amplitude for both were what was expected (0-10v) in amplitued given that the sensor is fed with 12V. What I don't know is if there is a drop-out or erratic output from either one. So....I'll try more testing of those sensors.

Fixed. The problem was with the exciter rings (reluctor) that are attached to the rear of the harmonic balancer. These two rings are used to activate the crank position sensor. One ring has 18 "teeth" and the other has 3 large teeth (sensor produces the X3 and X18 signals sent to the ignition control module. There were 2 of 4 missing screws holding these rings to the harmonic balancer and the remaining 2 screws were loose and almost falling out. The mounting hose in the rings were worn oblong, thus the loose exciters (rings) were causing variation in the signal coming from the crankshaft sensor. The harmonic balancer was replaced (the exciter rings are not available a a separate part). Engine started and idle is stable an smooth at 750 RPM. Next is get the PCM "educated". There is crankshaft position variation learn procedure that needs to be done and an advance scan tool is needed.
I would attach photos but there seems to be some restriction.

My Grandson said the noise I was hearing was from Ac compressor, it needs replacing and pwr is disconnected, but now we know where the rattling was really coming from ( I wear hearing aids for a reason ). . LT fuel trim at idle is -18 right now, but purring like a kitten. We'll check the evap system and other fuel/air stuff to make sure we didn't overlook a line. He know a guy with a Verus scanner that can do the crank relearn step. Glad to get it out of my driveway. I'm getting too old to crawl around on the concrete, turning 69 soon.

Originally Posted by Regval

My Grandson said the noise I was hearing was from Ac compressor, it needs replacing and pwr is disconnected, but now we know where the rattling was really coming from ( I wear hearing aids for a reason ). . LT fuel trim at idle is -18 right now, but purring like a kitten. We'll check the evap system and other fuel/air stuff to make sure we didn't overlook a line. He know a guy with a Verus scanner that can do the crank relearn step. Glad to get it out of my driveway. I'm getting too old to crawl around on the concrete, turning 69 soon.

Just an update on the status - after making a few short drives down the road, the scan of the engine reveals the LT fuel trim is averaging around -2. No codes, starts up immediately whether cold or hot, very stable idle, no hesitation on acceleration. Good to go. Final opinion is that just checking the CKP sensor for signal or resistance may not reveal the problem with an engine that stalls and idles erratically, given all other sensors appear to be operating normally. Access to a professional level scan scope may have pinpointed the CKP signal as abnormal, but alas, us non-professionals have to use what we have. Hope this thread helps someone.