#8 fouled oil since new

droptail · December 13, 2004

My 1990 5.7L has always deposited oil on the #8 plug. Chevy rebuild heads first (seals, guides, Manley valves, springs), slowly still oiled. Then they said they found the #8 ring gaps aligned so the rerung just #8. Still oils. Has a little rough idle but has otherwise run very well. Any ideas what it is?

budhayes3 · December 13, 2004

There could be a number of things causing this problem. Valve seals, valve guides, Piston rings. Do you see a cloud of blue-grey smoke when you first start it up in the morning or during deceleration? If so, the problem is most likely the valve seals/guides. If you experience smoke upon accel, then the piston rings are to blame. You could have oil return holes that are plugged up, keeping the valve seals submerged in oil and allowing oil by.

Check the PCV system also...make sure that the PCV valve isn't stuck open and allowing oil to get through. If too much oil gets sucked into the intake, plugs will definitely get oil fouled. If the intake PCV port is in any way close to the intake runners for the #8 cylinder, that could be your problem.

Good luck!!

GM-Tech · December 13, 2004

Hmmm, that's quite a few years ago and my memory is not that great.... Does that trans use a vacuum modulator? If so, I'd pull that line off the manifold and see if there's trans fluid in it.

Modulators that leak through the diaphragm fouled out #8 cylinder because the line plugged into the manifold in the back runner and towards the #8 side I think. Gravity would help direct the trans fluid mostly to #8. This problem was also good for trans fluid mysteriously going low with no visible sign of external leaks.

PeterbiltChrome · December 13, 2004

the previous owner of my truck put on 78k miles on my 94' 5.7

I put another 10k on it and I was getting a few green plugs (#8 the worst) and water in the oil. I was burning anti-freeze. Turned out to be a combo problem of the exhaust valve guides, intake manifold gasket leak, and head gasket warp. Figured the valve seals were shrinkin too... I figured the previous owner drove the pants off of it to work cold sooo many times.

I had re-man heads put on with all the other fixes. No gravy dipstick anymore. And the last time I checked the plugs, they seemed normal. I have a slight vibration yet while it warms up at the higher idle, but it smooths out all throughout the rpm band when warmed up.

droptail · December 20, 2004

Thanks for the feedback. No modulator, its a 5-speed. I'm going to look at the above symtoms more closely and I'll let you know what I determine. I was suspecting PCV venting into #8 intake runner as well.

Anyone have a closeup picture of the AIR plumbing on a 90-ish manual 5.7? I need to resmog it and got rid of the some of the stuff.

Great looking rig above. Did you lay down the rubber?

PeterbiltChrome · December 23, 2004

That Pete is one of Panelite's show trucks out at the Las Vegas Motor Speedway for a calendar shoot... I wish I had keys to it, truly I do. Thinkin about this--- if I had a $120,000 truck would I want to do donuts with it?? ONLY IN AMERICA!!!!!