sag, hesitation, stumble off idle

[dnd_toolbox]

g'day fellow enthusiasts!

ok, got a problem child... 1999 pontiac Grand Am SE 4 dr sedan, equipped with 2.4L I4, 4 spd auto trans... Veh is also equipped with a mandatory immobilizer device.

Veh no start... assumed was run out of fuel as the fuel gauge was not working, filled tank with premium unleaded gasoline and towed to a garage.

what's been done: (DTC P0300 set)

1) Fuel flow test: failed - replaced fuel pump (also corrected the malfunctioning fuel gauge as it is an intergral unit), retested - fuel flow at proper levels. Fuel filter replaced with new pump.

2) Fuel pressure test: failed - replaced fuel pressure regulator. retested - fuel pressure at 58 psi (prime pulse) and 45 psi at engine idle. fuel rail and injectors inspected for dirt and deposits, cleaned then reinstalled. Injector pulse test confirmed injectors working properly with no leaking/dribbling.

NB: engine runs like a bag of hammers at anything above idle, then stalls.

3) Sensor + actuator tests: IAC motor removed, inspected, cleaned and re-installed - ok, Map sensor vaccum tested, voltage readings confirmed with both scan tool and Voltmeter - ok. TPS voltage and resistance test, confirmed with both scan tool and volt/ohm meter - ok.

4) Ignition system tests: engine crossover/misfire on cyls 1 + 4. Further testing concludes ign coils - ok, ign module - ok, ign coil housing, plug boots, and contact springs - ok.

NB: engine runs fine at idle, acceleration off idle, results in misfire condition, sag, hesitation and stumble, resulting in stalling.

NB: engine restarts after stall.

Summary: Spark, ignition timing, engine mechanical, all within mfr specs. Engine runs perfectly at idle. Transmission has no DTC set. engine continues to set DTC P0300. (confirmed with CRT Oscilliscope on cyl's 1 + 4)

Conclusion: Catalytic Converter? From discussions with other mechanics, an immobilizer installed thru the primary ignition circuit may result in momentary power loss (less than 1 millisecond) where the vehicle computer detects a misfire and compensates with an increased injector pulse width. Over an extended period, this can possibly cause the premature failure/clogging of the catalytic converter, resulting in the condition presently being experienced.

NB: this conclusion has yet to be confirmed with an exhaust system back pressure test.

Comments from other Master Techs are appreciated.

[skibum1111]

Check the cover on the coils. It's possible for a crack to have formed, given this is an extremely high heat location on the quad motors. If a crack has formed, its more than likely arcing between cylinders #1 and 4. There are 2 separate coil systems for the quad, one runs 1 and 4, the other runs 2 and 3. At idle, does the converter get hot?

[J-Ri]

Swap the coils and see if the misfire changes to 2,3. Even if they look good, they can be bad internally. I assume you checked the plugs? If the gap is excessive and the spark is weak, the increased cylinder pressure on acceleration could cause the spark to not jump the gap.

The cat outlet should be about 10% hotter than the inlet. I wouldn't think the cat would be plugged from the pulse width being 1 ms too high. If you Canadians use as much road salt as we do in Iowa, the cat's gonna rust apart long before it plugs up unless there is a serious ignition problem (plug gap shut, wire missing, etc). Get that pressure test done and let us know what you find.