Thread: 02 GTP Mysterious Intermittent Idle Surge

Hey Guys,
My 02 GTP is experiencing an intermittent idle surge when warm (fluctuates between 300-1500 RPM at worst, sometimes stalls) which will usually last about 30 seconds. It happens when I'm coming to a stop in gear, and sometimes when I press the throttle for the first time after the car has been started if I leave it alone to warm up. If I rev it up, it revs just fine, and giving it a rev usually stops the surging once the engine returns to idle. The car acts like its hunting for the right idle speed, but drops too low, then compensates by revving up too high, then drops too low again (this is sometimes where it stalls), then revs up high, and so on. It's like an unstable control system, and its not shaking or running rough. In the meantime, I checked the short term fuel trim and it goes down to -15-20% during the surging behavior and it smells rich. I thought it could be the idle air control valve since the TPS was reading 0% while surging, so I replaced the IAC valve, but no luck there. What was really strange was that the MAF sensor was reading pretty steady values during the surging, and there aren't any check engine lights. Please help!

Current DIC's: none
Ongoing DIC's: P1844-Torque Reduction Signal Circuit Desired By TCM (predates the surging issue)
CEL: not illuminated

Reccent repairs:
Battery: 2yrs old
MAF Sensor: 6 months old
Vacuum lines: Checked 6 months ago. Some replaced.
Fuel Pressure Regulator: 6 months old
Fuel Filter: 6 months old
Injectors: Cleaned 6 months ago
O2 Sensor: 6 months old
TB: Cleaned 8 months ago
Air Filter (Cone): 6 months old, pretty clean still
Plugs and Wires: 6 months old
IAC: Just replaced


Engine Modifications:
Cold air intake
ZZP 1.0 PCM
ZZP U-bend delete (w/out downstream O2 sensor)
SLP Catback

When I unplug the MAF, it is even more likely to start surging. I'll check the lines again, but I just replaced the one on the TB last summer, and at the time the one under the snout wasn't leaking.

Did you unplug the MAF before you started the car to test? If not, try it that way. May need a little pedal to keep it running until the ECM figures it out then it should stay on its own.

Recall work been performed? Have you changed the LIM/UIM gaskets yet? Bad gaskets can be fine at cold/idle and as the car warms up air/fuel can get fubar'd

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check the vacuum line under the MAP sensor. It can come loose and is not visible until the MAP sensor is taken off.