Misfire
Thread Starter
Burning Brakes
Joined: Oct 2005
Posts: 1,020
Likes: 18
From: Anderson SC
Misfire
What could possibly cause my 07 with only 4k miles to misfire on #7 cylinder? I've changed the plug wire, swapped the injector, checked the coil. It ran fine until I put Z06 exhaust on and it ran fine for awhile and just started doing it. It does it on idle but seems to clear up above 1500 rpm's. Any ideas?
Tech Contributor
Joined: Mar 2003
Posts: 12,094
Likes: 197
From: Colorado Springs, CO
St. Jude Donor '08-'09-'10-'11
The exhaust is hitting somewhere (transmission) and transmitting the sound along the pipes to the engine where the knock sensors hear it as a misfire. Its on the driver's side pipe.
Le Mans Master
Joined: Jul 2005
Posts: 5,328
Likes: 405
I'm betting the #7 spark plug ceramic was cracked during the exhaust swap.
Thread Starter
Burning Brakes
Joined: Oct 2005
Posts: 1,020
Likes: 18
From: Anderson SC
Wow. That's pretty specific Spin. I will check it this morning. I have a Tech 2 so that's how I knew it was #7. I have a manual tranny so I should be good on the over-the-axle pipes, right? I will check anyway. I'll also pull the plug and replace it just in case.
Le Mans Master
Joined: Jul 2005
Posts: 5,328
Likes: 405
Thread Starter
Burning Brakes
Joined: Oct 2005
Posts: 1,020
Likes: 18
From: Anderson SC
I just did a little research and it is the crankshaft position sensor that determines misfire. While vibrations can cause a misfire code to set, it has to be something that causes a variation in crankshaft RPM like an unbalanced driveshaft, rotor thickness variation, or rough road conditions. (Rough roads are ignored if the ABS wheel speed sensors detect rough road variations.) Regardless, all of those situations result in vibration amplitudes of sufficient order to be felt substantially in the driver's seat. The exhaust hitting something will not set a misfire code so don't bother looking unless you just want to double check your work. It's also doubtful that particular vibration would have sufficient harmonics to set the #7 cylinder misfire DTC every single time even if it had the amplitude. The spark plug is the only thing you haven't looked at and it's the most likely to have been damaged during an exhaust swap. Happy hunting!
Thanks. I will try the plug.
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