Mechanics

PONTIAC GRAND PRIX COMPRESSION PROBLEM

2008 Pontiac Grand Prix • 85,000 miles

I have a 2008 GXP and is started to fun rough and check engine light came on and flashed, had the code read and it stated that number one was misfiring. Had the plugs and wires changed (never had that done yet, cost me 320) and drove it home. After it heated up a bit, did the same exact thing again. He did a compression test in that cylinder and said it still had compression. Mechanic said it might be the coil. I really need help. Single parent not made of money =)

also the mechanic said it might have something to do with the Active Fuel Management system?
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Joemka
January 26, 2013.



Kinda need to know the engine size when talking about an engine problem. If yours has separate ignition coils, you can try switching two of them to see if the misfire goes to a different cylinder. You can do the same thing with injectors. GM has more trouble with injectors than any other manufacturer. They don't necessarily fail, but unlike Chrysler, GM does not buy their injectors in matched sets. They just grab a handful out of a big bin and stuff them in. They are not flow-matched. When the engine is relatively new that usually doesn't cause a problem, but over time the difference in flow rates shows up as a lean condition in one or two cylinders. The unburned oxygen is detected by the oxygen sensor, and the Engine Computer responds by commanding more fuel to all the cylinders on that side of the engine. No matter how much extra fuel it commands, there will still be that unburned oxygen. You smell raw fuel at the tail pipe but the computer sees a lean condition. The fix for that is to install a set of rebuilt injectors that have been flow-matched.

That injector flow problem usually results in the computer detecting and setting a misfire code but you typically won't feel it or even know it's running too rich. What you described sounds more severe. As far as compression tests, it's not whether or not a cylinder has compression. It's how much compression does it have and is it the same as all the other cylinders. From the way you described how it acts, it doesn't really sound like a compression / valve / piston ring problem. My first vote is for a coil.

Caradiodoc
Jan 26, 2013.
The GXP has the 5.3, AKA 327 in it with the Active Fuel Management on it

Tiny
Joemka
Jan 26, 2013.
Thank you for the info. It is running a lil rich, very smelly. And I will check the compression in comparison to the rest, thanks for the info =)

Tiny
Joemka
Jan 26, 2013.