Vortec 8100 Issue...

[Tobynine9]

It's stuttering and hesitating randomly at various throttle openings. It starts and idles just fine.

Sometimes it stutters right off idle like a carb that's way too rich and is bogging out. Sometimes it doesn't.

It's starting to stutter and hesitate and higher speeds now. Example - rolling along at 60, accelerate to pass, 4L85E downshifts and the motor starts hiccuping.

I think it's getting worse.

I already swapped out the Crankshaft Position Sensor - apparently that's a common problem for '02 8100 equipped 2500s. No change.

I swapped the fuel filter. No change.

I cleaned the MAF and made sure the intake plenum was properly secured. No change.

Anyone got any ideas?


Edit: The vehicle is a 2002 Avalanche 2500 - 4x4, 8100, 4L85E, etc. Just hit 100k miles.

[00ls-1]

no ses light?

[00ls-1]

the platinum plugs usually go between 80k and 100k miles

[Too Fast]

Sure sounds like a plug/wire issue. Try looking at them at night, see if you see any light shows, there should not be any. Or try misting them with a water bottle while idling. If it's all golden, nothing will happen when the water hits it.

[Tobynine9]

No SES light.

Plugs are new NGK V-powers.

Wires are young Taylor 10mm.

[00ls-1]

what about coil packs? air filter clean?

[Too Fast]

This engine has the coil-near-plug setup like our LS1s? And it just started after or before the new plugs/wires? If after, my bet is a wire or two is not snapped on all the way. They will feel like it, but take them apart and try again. Many here have been fooled.

[00ls-1]

you coulda got bad gas too

[0rion]

I'm going to agree with the others that said it sounds like an ignition issue or it could be leaning out too I guess. Have any way to log it and look at things like fuel trims and misfire counts? Would at least put you on the right track.

[00ls-1]

i had a 6.0 in a 2500 truck.....just rememeredit had an injector go out and thats what it did 2 different times
they also make 2 different length wires for those motors you might have got the short and need the long so they arent making good contact

[Tobynine9]

Air filter is clean.

The 8100 does have coil-near-plug ignition. It's basically a 454 stroked to 496 with a few tweaks here and there to allow a Gen-III style ignition.

I re-gapped the plugs just now. We'll see tomorrow if that helps. For some reason I thought I read that opening the gap from stock was the "plug-gap mod". Turns out, you're supposed to narrow the gap. I dropped the gaps from .65'ish to .45'ish.

[Tobynine9]

If that doesn't help or clear it up I'll check fuel pressure at the rails. If that's good then it's off to someone with a proper scan/diagnostic machine to get the full run-down.

[Tobynine9]

Meh, it's still doing it.

I think I've described the problem somewhat wrong. It's not really a misfire or a stutter. It's more of a loss of power. As you accelerate it sort of bogs out - rrrrRRRRRR buuuuuu RRRRR.

At higher engine speeds it just feels like a stutter as its happening quickly - RR bu RR bu RR bu RR bu, etc.

Gonna check fuel pressure tonight.

[konigandy6]

Meh, it's still doing it.

I think I've described the problem somewhat wrong. It's not really a misfire or a stutter. It's more of a loss of power. As you accelerate it sort of bogs out - rrrrRRRRRR buuuuuu RRRRR.

At higher engine speeds it just feels like a stutter as its happening quickly - RR bu RR bu RR bu RR bu, etc.

Gonna check fuel pressure tonight.

How many miles on the vehicle? Possibly a plugged converter?

[Tobynine9]

100k miles.

I guess it maybe could be a problem with the catalytic converter... 8100s use a lot of oil. This motor is no exception.

[00ls-1]

I'm still gonna say it's probably an injector

[Too Fast]

Did it just start after you changed the plugs/wires, or is this something you did to try for a fix?

Is your MAF clean? That would cause some drivability issues you (newly) describe.

[Tobynine9]

MAF is clean. Did that already.

I changed the plugs in response to the problem. It hasn't helped.

[jiveass]

put your hand up to the tailpipe and have someone rev on it...if you cant feel much air movin, then most likely the cat is plugged.

[Tobynine9]

It's not a clogged cat. You can feel a strong pulse on both exits just by walking past them.

I'm starting to fear it may be a dying fuel pump.