theoutlaw30

I have an '08, stroked to 6.6l with a big cam, long tubes, etc. No codes. Car was tuned at my local performance shop and was running fine until a few months ago when it started to stumble at light throttle when cold. When it did that, a quick hit on the gas made it go away and it wouldn't come back. Last week it began stumbling at part throttle all the time, won't go away. Idles fine, still has all the power as before under hard throttle, but stumbling bad at light/part throttle, including maintaining constant speed on highway. No backfiring or codes. I replaced all the plugs (half the electrode was missing on one of the plugs). It made no difference. This constant stumbling did start after getting a new tank of fuel -- just coincidence? Motor used to have a supercharger so its relatively low compression, and I do get a bit of blowby at high rpm. Old plugs removed were mostly tan colored, but a couple were quite black with soot, but no real buildup. Any ideas are much appreciated.

heli-vette

If you run a stock air flow sensor , give it spray cleaning .

Chevy-SS

How would half the electrode on one plug ever go missing??? Maybe running lean? I'd start by replacing coil and wire that were on that plug, perhaps injector too. Then get engine running and check each exhaust up by the head with infra-red temp gun to ensure all cylinders firing - temps should be fairly consistent.

Sounds like you've done a good bit of engine work, so don't overlook the obvious. Check every nut, bolt and clamp to make sure nothing's loose.

Keep us posted. Good luck..........

timd38

Vacuum leak?

theoutlaw30

going to give that a try too, thanks.

theoutlaw30

you got me on that one...never seen part of an electrode disappear. The problem is, all cylinders are idling fine and run good after about half throttle on up, its just partial throttle where I have the issue.

theoutlaw30

yep, did that right after the plugs, plus it was cleaned just 4 months ago too.

theoutlaw30

Update: after much trouble shooting, my shop determined the passenger side #1 O2 sensor was shot and giving no signal. this caused all the injectors to go to wide open all the time, flooding all 4 right side cylinders at anything below wide open throttle. New sensor and reset the ECU and runs better than it ever has.

theoutlaw30

moral of the story, check those plugs from time to time if you have a heavily modified car. I only have 34k miles, and one of my plugs was missing most of its electrode -- I definitely wasn't burning all 8 cylinders. Car is much smoother and more powerful now than before the O2 sensor issue came up.