Misfire

Reply Subscribe

Thread Tools

Search this Thread

Jun 13, 2009 | 08:00 AM

2kvert

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?

Jun 13, 2009 | 08:03 AM

SpinMonster

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.

Jun 13, 2009 | 08:27 AM

glass slipper

Le Mans Master

Joined: Jul 2005

Posts: 5,328

Likes: 405

Quote:

Originally Posted by SpinMonster

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.

The knock sensors??? I thought the crankshaft position sensor was used to detect misfires.

I'm betting the #7 spark plug ceramic was cracked during the exhaust swap.

Jun 13, 2009 | 08:34 AM

2kvert

Thread Starter

Burning Brakes

Joined: Oct 2005

Posts: 1,020

Likes: 18

From: Anderson SC

Quote:

Originally Posted by SpinMonster

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.

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.

Jun 13, 2009 | 09:17 AM

glass slipper

Le Mans Master

Joined: Jul 2005

Posts: 5,328

Likes: 405

Quote:

Originally Posted by 2kvert

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!