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Discussion Starter #1
I have a an odd misfire issue on my 2.8 V6 that is really puzzling me, and I am hoping someone on here will be able to provide me with some insight. The car on misfires under low load conditions, i.e. sitting at idle, or while maintaining a set RPM in neutral. The misfire can be felt, mostly I think due to the stiffer torsion mount I installed on the trans. You can also hear it if you walk behind the car, and stand near the exhaust, but it is only a gentle/quiet noise. Under load the car pulls hard, and is smooth as silk throughout the entire band. The only other odd behaviour is that it will occasionally stall, or nearly stall if I am accelerating, and push in the clutch, letting it "fall" back to idle. - Seems worse in warmer weather, but that may just be because I don't run the A/C compressor as much (windows down).

Vehicle work includes 19T with tune, and upgraded injectors, de-catted, new MAF, cleaned MAP, removed/cleaned TB, new O2 sensor, new plugs, gapped to ~0.75, new coils. I have searched over, and over for vacuum leaks (at least one can of carb & choke cleaner, probably more like two), smoked the intake to check for vacuum leaks, disconnected, and plugged the high pressure crank case breather, EVAP system, and brake booster. I also ran a can of GM top end engine cleaner through her followed by an oil change.

My MAF responds as I think it should, my O2 sensor (Lambda) reads 1+/-0.02 with little throttle blips (even with the misfire actively happening). My short term fuel trims are often quite a bit out running -12 while cold, and settling to around 0 when hot/warm. Long term trim is about 12-13 (pretty stable over time) which is why I have been looking so long, and hard for a vacuum leak. The lower the total trim is the better it runs.... Short term trim seems unaffected by engine speed. I have not checked the output voltage of the Lambda sensor, and am wondering if the ECU is reading an artificially low voltage thus the high LT trim, and causing a rich condition. Coolant temps are as expected, so I don't think that is the problem.

The car seems to be quite rich as I am getting significant build up on my rear bumper, and my exhaust tips are BLACK. I have had a lot of cars/trucks without cats, and never before have I had one where the exhaust fumes made my eyes water quite like this before.

Any input would be greatly appreciated as I am starting to pull my hair out, and haven't had time to check the Lambda voltage. I am hoping there is something stupid I am missing that someone will point out! ;ol;
 

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I had a similar issue after replacing the spark plugs on my 2006 9-3 Aero. Turns out I had created a tiny crack in one of the tubes near the firewall. Once I covered it with some fiberglass the check engine light and misfire went away.
 

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Discussion Starter #4
I had a similar issue after replacing the spark plugs on my 2006 9-3 Aero. Turns out I had created a tiny crack in one of the tubes near the firewall. Once I covered it with some fiberglass the check engine light and misfire went away.
How did you find the cracked tube? I have gone through so much carb/choke cleaner I am hard pressed to believe that I could have missed a cracked tube. It is always worth checking again though.

I've also had that issue when I gapped the plugs wrong.
Plugs are cheap, I will have to buy another set, swap them in, and see what happens I suppose.

Thanks for the feedback.
 

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Discussion Starter #5
How did you find the leaky tube? I have disconnected all of them independently. I really don't think this is the issue, plus all the carb, and choke cleaner would have found this I think. Also, when your plugs were gapped incorrectly, were they under, or over?
 
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