2006 Grand Prix GT Misfire

[Jag]

Hello,

I just bought a 2006 Grand Prix GT from a friend that got a new car. I noticed that it is misfiring bad at idle and up to speed. 3500rpm gets real rough them smooths out.
If I set the cruise, when going up a hill, the car will try to accelerate but then stutter real bad and the check light will blink on and off for about 5 seconds.
I had the car scanned and bought an OBD2 scanner. P0300 is the only code I get. I checked the live misfires and its only one cylinder 3.

I noticed that someone had wrapped electrical tape on the spark plug to #3. Found a cut in the wire.
Replaced the plug wire with another stock AC Delco wire. Still missing on 3.

Checked the coil pack on 3. The pack was producing a dirty red spark.
Replaced the pack and it now has a nice large blue spark. Still missing on 3.

Replaced the spark plug "which looked fine" with ACDelco 41-101 plug.
Had a compression test done on 3 which was within specs. Still missing on 3.

Used a couple cans of carb cleaner on vacuum lines checking for leaks. Didn't find anything.

I even replaced the injector for the fun of it. I noticed that the bottom of the injector, below the lower O ring was cracked. New injector in but still missing on 3.

I replaced the battery which tested bad with a new one.

I am wondering if there is a good way to see if the catalytic converter might be the issue, or a good way to see if the head gasket is leaking. Or a test I can do on the ICM. Other than that I cant think of any other thing to test. I am hoping someone might be able to point me in the right direction because I have exhausted my knowledge.

Thanks.

[Fivefingerdeathpunch]

The converter wouldn't effect just 1 cylinder that is the almost the farthest from the cat.

I'd start with the ICM, if you know anyone else with a 3800 you can just quick swap them and do a test before you buy one.

If compression was good, then the headgasket likely is not the cause.

[Jag]

I know someone with a 2000 GT but I don't think the ICM is the same. I could be wrong. I was wondering about the cat due to something I had read during my google searches. One guy had the same symptoms had me and his cat was dead. Replaced it and fixed the problem. I figured it wouldn't hurt to check it.

[Fivefingerdeathpunch]

The ICM is the same.

Part numbers are the same.

[Jag]

Sweet. Ill go see if I can borrow it.

[J57ltr]

Why didn't you just replace all the plugs and wires with new ones? I wouldn't trust that you don't still have an issue with them.

[Jag]

Why didn't you just replace all the plugs and wires with new ones? I wouldn't trust that you don't still have an issue with them.

I just did and its still only on number 3.

[Jag]

Im hoping to borrow the ICM this weekend and ill see if that fixes the issue.

[markpa]

Is #3 closest to the O2 bung? Can't remember. Might be arcing.


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[Fivefingerdeathpunch]

Cylinder 3 is on the front bank, middle cylinder.

[J57ltr]

I just did and its still only on number 3.

The way you worded it you replaced 1 plug and 1 wire. It's a waste spark ignition system so there are 2 plugs and wires on each circuit. I have had several times where I had a miss on one cylinder and it ended up being the other cylinder that had the issue even though it wasn't the one missing.

Jeff

[Jag]

The way you worded it you replaced 1 plug and 1 wire. It's a waste spark ignition system so there are 2 plugs and wires on each circuit. I have had several times where I had a miss on one cylinder and it ended up being the other cylinder that had the issue even though it wasn't the one missing.

Jeff

I did only do the wire and plug on #3. It had electrical tape wrapped around a gash in the insulation on the wire. When that didnt fix the problem I went ahead and replaced all the wires and plugs. Along with the coil pack that was producing a weak spark on #3.

[Jag]

Ok. Update time. I tried two new icms and still missfire on number 3.

While looking sensor logs I saw the cat was at 1100+ degrees Fahrenheit. Does that sound hot? I have no idea what the average temp should be.

[Fivefingerdeathpunch]

As far as I know we have no actual temp sensors in the cat converters, just regular o2 sensors before and after the cat.

Use a temp gun on the exhaust pipe before, on and after the cat. If the way way higher infront of the converter then it's likely clogged/failed. The temps will only show around 400-600 on most cats.

The exhaust exiting the head will be way hotter but cools down fast as it leaves, and inside the middle of the converter it will be really hot to burn the gases but that really won't spread to the metal case.

[Jag]

I've thought about drilling a hole in front of the cat and doing a pressure test and then another behind the cat.

Right now I'm down to the head gasket or cat.

Just an odd thought. I've not taken the car in to get the loose key in ignition recall taken care of. Is it possible that with a loose key in. It's not sending enough voltage out?

[Fivefingerdeathpunch]

It's easier to just remove the front o2 sensor and do a pressure check. Anything past the converter means nothing, "back pressure" is a myth and is not needed on a vehicle like this.

The key thing isn't going to cause anything like this, the motor is still running so it's still working so it's not something where it's losing power.

[Jag]

I'm running out of ideas. Lol I had another compression test ran and it all checks out still. So head gasket is my last resort.

[Scottydoggs]

swap the injector around, see if that moves it.

[SgtMarshal]

I would check the injectors and see if they are squirting like they should. lift the fuel rail up with the injectors attached, and put some cardboard under, then turn the key on, a small amount of fuel should come out of each if they are working, that is, if I remember correctly.

[Jag]

Injectors are good. Checked them all again. And swapped them around.