Looking to the collective brain trust here on the forums to help solve a subtle issue I have.

History:

2002 C5 Z06, bone stock, purchased few years back with 17,000 miles on it. Car currently has 69,xxx miles.

Car has always run well and has been well taken care of. It is still bone stock (stock air cleaner, battery, intake, tune, exhaust, T/B etc. etc.) No performance mods whatsoever.

The Problem:

I've had the chance to make a few runs in the 1/4 mile and am getting what appears to be a misfire or engine break up during WOT pulls in 4th gear on most runs. The car doesn't nose over but you can hear the sound and in fact I've noticed in on GoPro footage audio. I believe it's knocking my trap speed down as max I've seen is 112 with an overall average perhaps 110 (discounting runs with blown shifts etc.) Some days I was struggling to break 109 mph (it was a 95 degree 4500+ DA however)

The break up is intermittent, and doesn't happen at the same time or same amount on each pass, and only happens in 4th gear WOT pulls. Initially I attributed it to high temps and DA on one session. I ran 100 octane (no-lead street legal stuff) at one session but that didn't solve the problem, so I assume it's not detonation. All other events and daily driving uses Chevron 91 octane (best we can get).

Other than those very brief episodes in a limited regime the car drives perfectly. It still gets great mileage when not standing on the gas pedal.

What I've checked:

There are no codes. No PCM, misfire codes etc. Nothing.

I hooked up an inductive timing light to check each plug firing at idle. No issue there but that was to be suspected since that wasn't the regime where I had the issue.

The plugs and plug wires are fairly new (about 1 1/2 years ago, OEM GM plug wires) and I re-checked them last night. Pulled plugs, gap good, no cracks, wires looked good, 'clicked' on correctly etc. (Gawd they are a PITA!)

I did a cursory wire inspection and didn't see anything out of the norm. This is a SOCAL car and hasn't had any introduction to water, snow, ice etc. So corrosion isn't an issue.

YES I did a forum search!

While I did find a few hits with kinda / sorta the same issue they were from many moons ago with no definitive solution other than what I've done.

I suppose it could be a coil pack but I have yet to be able to determine which one, if any. I'd rather not start throwing coil packs at the problem unless I had some more intel.

I plan on testing again this weekend but before I go would like to hear from the crowd on anything I may have over looked or suggestions for troubleshooting, OR better yet a definitive answer to my dilemma!

Thanks for looking

 

A mis-fire will set a code - if you don't have codes it must be something else. I'd take it to a tuner have have him data-log to car to see what's going on. What you're feeling may be causing the knock-sensors to pull timing...

 

I understand a mis-fire will set a code but used the term for lack of a better description.

If the issue persists I may take it to get some data logged. Thanks.

 

Any other ideas?

 

Log with HPT and WB, for under $500 this is the easiest way to diagnose what the car is actually seeing...

I'd bet you are going lean or someone may have done a custom tune and added timing which your fuel can't support at those temps. I can hear audible knock on the tune in my '04 Tahoe when running 87 octane and temps are 90-100 and it doesn't set misfire codes...brand new plugs/wires.

My bone stock '02 would trap 116-117 all day long - damn I miss that car!

 

This car has never been tuned or touched. It is stock as stock gets. I have yet to see a C5 Z06 at this track hit 117 in stock form. Not to say they don't, but I haven't experienced nor witnessed it. Even raced a C5 Z06 with H/C/I and LT with DR and that car did hit 117.

I might have to drop it off to CMS and see what they can find.

Thanks.