Misfires On Deceleration

Pete94Silverado · May 23, 2009

Yesterday, my Pacesetters decided to eat a couple of plug wires and caused a TON of misfires...so much so that I went into limp home mode...blinking SES and all (drove it about 8 miles like that). Today, I picked up another set of the same plug wires and put boot protectors on them. Fit well, slid right on...seemed to lock in place. Took it for a test run and it ran good did not seem to misfire. Tonight, I figured I better hook up AutoTap just to make sure that nothing was going on that I couldn't feel or hear. Well, I did not have any misfires at idle and up to about 2k rpm (just revving and holding the throttle open) but, I noticed when I let up on the throttle, it threw misfires on almost all cylinders. Even the ones that were not misfiring before.

Thoughts on what could cause this? I am almost thinking it could still be hold over from the limp home mode...but, I cleared the DTCs from the ECM. Should I unplug the battery and let the ECM reset itself? I just don't see how it's anything major...I didn't drive it far enough or hard enough in limp home mode...kept it below 2k rpm.

BTW...what plug wires are y'all running with your long tubes? I used the Summit brand but, they are basically resting on the headers on plugs 4 and 6 (I think that's right). I figure with the fiberglass Summit boot protectors resting on the headers shouldn't be a problem...but...we'll see.

Thanks,

-Pete04Silverado

svensk · May 23, 2009

Catalytic converters might be damaged from the mis-firing from before. When multiple cylinders go out like that you are essentially dumping raw fuel into the engine and out the tail pipe.

Pete94Silverado · May 23, 2009

Well..that thought crossed my mind. But, I don't have rear O2s. So, I don't see how the ECM would know if the cats were plugged or not. I would think that if the cats were plugged bad enough to throw misfire codes, performance would be in the toilet.

I searched around on the internet last night and found some information that could point to a lean condition. A lean condition would surprise me but...I suppose anything is possible.

EDIT...actually a lean condition would make sense. If the engine is at a high RPM and you lift off of the throttle, you bascially cut fuel flow immediately. I would suspect that the fuel cut event happens quicker than cutting spark. Yes?

svensk · May 23, 2009

Why would you have a lean condition happen after plug wires were damaged? Unless this was happening even before that?

Also check for intake gasket leak. Run a vacuum test for that. Very common issue unless you already changed the factory original at one point.

Pete94Silverado · May 25, 2009

Why would you have a lean condition happen after plug wires were damaged? Unless this was happening even before that?
Also check for intake gasket leak. Run a vacuum test for that. Very common issue unless you already changed the factory original at one point.

I don't know...hence why I said...I found some information on the internet. I didn't know if it was accurate or not. And...I have not changed the intake gasket. Do you mean the ones at the heads or is there some other one that leaks (e.g. between the throttle body and intake).

Yesterday, I hooked up the Autotap again and went out for about a 45 minute ride. Today, I looked over the logs and found that I am throwing a handful of misfires here and there...but...nothing like I was seeing while increasing the idle in my driveway. I noticed that the misfires happen at lower RPMs instead of WOT. With the throttle position angle registering 99.2%, everything looked good. No misfires, no knock retard, ignition voltage 13.7+, ignition timing advance was 22 degrees+ and the front O2 sensors were registering high 800s to 900. So, I would say that it seems like everything is alright.

The only thing I saw of any concern was the trans temp. It hovered from 125-145 throughout the drive. Is that warm enough? While not trailering, should I put a piece of card board over it?

-Pete

04Silverado