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Mazda Ed
Mazda Ed, Auto Service Technician
Category: Car
Satisfied Customers: 2223
Experience:  ASE Master Technician, Mazda Master Technician, GM and Ford factory trained. 15 years experience!
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1999 pontiac Grand Am SE: sedan..equipped..2.4L I4, 4 spd auto trans

Customer Question

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.
Submitted: 8 years ago.
Category: Car
Expert:  Mazda Ed replied 8 years ago.

HiCustomer

First let me say that it sounds like you have a high skill level and none of my comments are meant to offend.

That being said, how did you test the ignition coil housing. It is impossible to test using resistance tests or any other method besides swapping a good one in it's place. I find that most often these fail internally where you can not see it. The spark arcs through the closely spaced firing tubes at the bottom of the housing.

Also, I recommend swapping the position of the 2 coils to see if that changes the cylinders misfiring. We have several of these cars come into the shop with a misfire condition. Typically the customer has replaced many of the sensors/components EXCEPT that coil housing.

Also, if you are suspecting the aftermarket device as the source... Can you monitor the ignition voltage with your scope to look for a dropout? Can you temporarily bypass the device. I have done this before on the state installed DUI boxes to test the vehicle control system.

Expert:  Mazda Ed replied 8 years ago.

HiCustomer (attempt #2)

First let me say that it sounds like you have a high skill level and none of my comments are meant to offend.

That being said, how did you test the ignition coil housing. It is impossible to test using resistance tests or any other method besides swapping a good one in it's place. I find that most often these fail internally where you can not see it. The spark arcs through the closely spaced firing tubes at the bottom of the housing.

Also, I recommend swapping the position of the 2 coils to see if that changes the cylinders misfiring. We have several of these cars come into the shop with a misfire condition. Typically the customer has replaced many of the sensors/components EXCEPT that coil housing.

Also, if you are suspecting the aftermarket device as the source... Can you monitor the ignition voltage with your scope to look for a dropout? Can you temporarily bypass the device. I have done this before on the state installed DUI boxes to test the vehicle control system.

Customer: replied 8 years ago.
Reply to Mazda Ed's Post:

Hey Ed:

The ignition system has been fully tested, including hooking up an old CRT osilliscope to watch for any peaks/drops caused by internal shorts. The coils have been swapped, and no change found...

Once again, the engine runs perfect at idle, but drops off immediately at any speed above idle. (misses, surges, then stalls). WOT results in immediate stalling of the engine. DTC P0300 is the only code being set.

?Plugged catalytic converter???

Drop me a message back thru PM.
Cheers, Jay

Customer: replied 8 years ago.

Attention: all automotive experts.

FYI - My problem child, a 1999 Pontiac Grand Am SE (2.4 L with 4 spd auto trans).
The problem/fault has been solved... The culprit is a plugged Catalytic Converter causing excessive back pressure resulting in the engine's exhaust gases not being properly vented from the cylinders, which in turn is causing the HO2 sensors to have false readings of a 'lean condition'. Overall this fault is causing the ECM to over fuel the engine.

How was it found?

After exhausting all the diagnostic procedures associated with the DTC P0300 to be set, which included the entire fuel system, and ignition system. I removed the Catalytic Converter from the vehicle and...

Low and behold, the engine responded with a roar! (I remembered to wear my hearing protection, but it was loud enough to be heard in the shop office.)

2.4 Hrs later, the vehicle was fitted with a generic replacement catalytic converter, that had to be welded into place, and the absolutely joyful customer was merrily settling his invoice and headed towards a vacancy withing a stream of traffic.

This particular fault had 2journeyman mechanics stumped for a week... I received the vehicle from them and spent a total of just over 20 hrs in diagnostics.

Something to watch for in the future,

Cheers, Jay

 

Expert:  Mazda Ed replied 8 years ago.

Now you need to find out WHY the convertor went bad! Typically it is an outside influence such as a misfiring cylinder or skewed oxygen sensor, possibly even excessive oil consumption. I can't recall finding a bad convertor without an actual cause.

Customer: replied 8 years ago.

After consulting with the customer, the vehicle was driven repeated short distances without the engine reaching full operating temp... The suggestion was made to the customer to give the car a good long run of both city and highway driving. He returned with the vehicle later in the day, and reported he was very pleased with the way the car was handling and running. He also noted the exhaust system vented a lot of carbon...

Cheers, Jay

Customer: replied 8 years ago.

FYL: Further to the problem child... After checking the vehicle history file from the immobilzer installer company, this vehicle has been recalled by them to have the immobilizer installation modified.

The Immobilizer installer used the primary ignition circuit, which for all GM products is a 'No - No"... They are correcting the wiring for the immobilzer to the secondary ignition circuit to correct a known drivability fault. This Vehicle is also being sent to the local GM dealer to have it's ECM chip reprogramed with information that includes the immobilizer functions.

Information for everyone to file away for future reference.

Cheers, Jay