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.
