8.1 Misses At An Idle

djv1951 · March 14, 2008

I have a 2004 silverado with a 8.1 and allison with 36,000 miles on it. It has developed a sparatic miss at an idle. It feels like a spark plug is miss fireing and you can see the tac drop abour 500 rmp. The service engine light has not come on and when I take it to the dealer no codes are stored. I've had it to the dealer 3 times now for the same problem and they have cleaned the injectors and replace the crank sensor but it has not corrected the problem. It seem to happen more when the engine is warmed up. Some times it idles very smooth then it will start missing and shakes the whole truck. It runs fine above an idle and performs well pulling my 5th wheel. Any one have any ideas?

dewfpo · March 14, 2008

I have a 2004 silverado with a 8.1 and allison with 36,000 miles on it. It has developed a sparatic miss at an idle. It feels like a spark plug is miss fireing and you can see the tac drop abour 500 rmp. The service engine light has not come on and when I take it to the dealer no codes are stored. I've had it to the dealer 3 times now for the same problem and they have cleaned the injectors and replace the crank sensor but it has not corrected the problem. It seem to happen more when the engine is warmed up. Some times it idles very smooth then it will start missing and shakes the whole truck. It runs fine above an idle and performs well pulling my 5th wheel. Any one have any ideas?

My '05 8.1L never idles smoothly, dealer says "it's normal", yeah right.

Has the dealer checked the spark plugs? coils? wiring to the coils? I'm surprised the ECM didn't pick up a code for a misfire, even if inconsistent.

DEWPFO

L29er · March 15, 2008

not to hijack a thread but my 98 454 does the same thing haven`t been able to pin point it has done it since new, just did plugs wires cap and rotor, now it seems worse, no codes, considering crank sensor or cam sensor, miss kinda annoys me, only happens warm, most noticable in gear and brake on (not moving).

Haulin' 8.1 · March 15, 2008

For the 8.1 have the shop perform a crankshaft sensor variation relearn procedure and clean the throttle body. My 8.1 idles so smoothly that with the radio playing and the windows up you have to look at the tach to know it's running. I'd look for a vacuum leak on the 7.4.

Vernon

c4racer · March 18, 2008

For the 8.1 have the shop perform a crankshaft sensor variation relearn procedure and clean the throttle body. My 8.1 idles so smoothly that with the radio playing and the windows up you have to look at the tach to know it's running. I'd look for a vacuum leak on the 7.4.

Vernon

I agree - my 8.1 idles very smoothly too - barely can even tell it is running. And it has 85K miles, assuming on the original plugs. Which I will be swapping soon btw - just because.

L29er · March 18, 2008

For the 8.1 have the shop perform a crankshaft sensor variation relearn procedure and clean the throttle body. My 8.1 idles so smoothly that with the radio playing and the windows up you have to look at the tach to know it's running. I'd look for a vacuum leak on the 7.4.

Vernon

i`m pretty sure wouldn`t i have a raised idle if i had a leak? or maybe i don`t have a MAP sensor, hmmm maybe not... that is possible i have had the upper plenum off many times checking injector leak down, maybe they are not sealing well, i have new gaskets and injectors, will soon replace and see!

10.90 hot rod · March 26, 2008

You could set an egg on a golf tee on the top of my 8.1 when it is idling without it falling off !!!!

dewfpo · March 26, 2008

You could set an egg on a golf tee on the top of my 8.1 when it is idling without it falling off !!!!

I'm very jealous. You could sometimes mix margarita's in your lap with mine. Often in gear it is smooth, but in Neutral is shakes quite a bit, sometimes more than others, once in a blue moon it will idle fairly smoothly in neutral, but rarely. Dealer says it is normal, but most all difficult problems that don't bring them much warranty money are considered 'normal' IMHO.

DEWFPO

Daubs · March 28, 2008

At first start up in morning mine sounds like a Duramax! Then settles down to a nice smooth idle. No problems here...

BigJayDogg3 · March 28, 2008

It isn't nearly the same application, but in my LeSabre, I had a miss that sounds almost exactly like yours. I found out it was the spark plug wires. The spark plugs were changed, and the dealership, instead of changing out the wires as well, put the same ones on and the cap and the wire separated. Might be your problem, might not be. It is just another thing to look at.

dewfpo · March 28, 2008

It isn't nearly the same application, but in my LeSabre, I had a miss that sounds almost exactly like yours. I found out it was the spark plug wires. The spark plugs were changed, and the dealership, instead of changing out the wires as well, put the same ones on and the cap and the wire separated. Might be your problem, might not be. It is just another thing to look at.

Thanks I'll see what I can do. The truck only has 10K miles on it.

DEWFPO

ATMINF · March 30, 2008

I have heard the 496 have issues with their crank positioning sensor, or something like that. I don't remember where I saw it but it sounds similar to so other threads I have read on the Web. I hope this helps a little.

blujay_02 · March 31, 2008

Mine had the same prob and we cleaned the throtle body and now its idleing smoothley.Everything runs good except for that knocking when i rev the engine.All the vidals seem fine i think it might be the gas.

L29er · April 1, 2008

hey so this might work, buddy of mine with his 454 changed plug gap to 45 from 60 says it idles perfect, i`m gonna try it but maybe 50 thou, dropping 15 thou seems like alot, but i may try both any ways till its good!

n0dih · April 7, 2008

My 99 2500 Sub 454 does the exact same thing.

1. Stock injectors are problems and leak, and just plain need to be replaced. Heck, they are too small for any power upgrades anyway.

2. In 99, probably 98 too, GM had a idle miss PCM tune fix that upped idle to 750.

I do tuning so if you need help with it or want to much to the 454, let me know.

not to hijack a thread but my 98 454 does the same thing haven`t been able to pin point it has done it since new, just did plugs wires cap and rotor, now it seems worse, no codes, considering crank sensor or cam sensor, miss kinda annoys me, only happens warm, most noticable in gear and brake on (not moving).