Here's some questions you might be able to shed some light on if you've ever rebuilt the venerable Chevy 350 small block: According to my mechanic, my Vette is burning oil in 4 of the 8 cylinders. With new plugs it runs fine, but then not so good once they get fouled. The car has 95,000 miles on it. I bought it used with 20,000. I've babied it all along, as I did with a Buick Century that was too rusted out but running strong when I got rid of it at 117,000 miles. So my questions: 1. Is the Chevy small block "known" for some particular failure mode, either related to the rings or the valves ? 2. Is the unknown first 20,000 miles more important than all my efforts for the next 75,000 miles ? 3. Was I simply unlucky with my Vette ? 4. Alternately, was I just plain lucky with my Buick ? Thanks for your input.
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Re: oil burner
Bill----
You didn't say which 4 cylinders are burning oil; it could be important to a diagnosis. But, on the assumption that it's the four cylinders of one cylinder bank, I'll offer some comments.
Small block Chevrolets suffer from a few notorious maladies which produce excessive oil consumption in some engines. One is valve guide/valve stem wear. Another is the fact that, prior to 1981, no POSITIVE type valve stem seal was used. Instead, the valve seal consisted of a rubber o-ring and cup-shaped shield to minimize, to one degree or another, the amount of oil which was allowed to get to the valve stem. As "clumsy" a system as it was, it actually worked pretty well.
Another problem with Chevy engines involves those fitted with forged pistons. Forged pistons require more cylinder wall to piston clearance, which, consequently, produces more clearance for oil to escape the crankcase into the combustion chamber. Also, the increased clearance causes some piston ring "instability", which further aggrevates the problem. And, to top it all off, forged pistons wear much faster than cast, again producing the potential for greater oil consumption. Chevrolet hasn't used a forged piston in an engine intended for a PRODUCTION VEHICLE since the L-82 went out of production after 1980.
But, I don't think that the above are what your problem is, especially if your "oil burning" is mainly occurring in all the cylinders of one (or both) cylinder banks. That's because the problem that often causes this is the improper fit of the intake manifold to the cylinder head. In Chevrolet V-8s, the intake manifold serves as both the induction manifold as well as valley cover. If there is even the slightest misfit, which can be caused by a even a slightly improperly machined block, cylinder head surfacing, or distorted/mis-machined manifold, the cylinder head to manifold surface will not seal properly. This fact, coupled with the high vacuum present in the intake manifold especially under certain operating conditions, will cause oil mist present in the lifter valley to be drawn into the intake manifold-to-cylinder head interface.
I can assure you that NO amount of gasket replacement, high-performance gaskets, or any type of gasket sealer will remedy this problem which is VERY DIFFICULT OR IMPOSSIBLE TO DETECT VISUALLY. The only "cure" is correction, by machining, of the underlying mis-fitting parts. And, this is VERY difficult to get right. This problem is why many Chevys burn oil. Also, certain camshaft configurations exacerbate the problem due to the vacuum profile which they exhibit. The "300 hp" 327 and 350 camshaft, GM #3896929, is especially bad in this respect.In Appreciation of John Hinckley -
Re: oil burner
Bill,
You may want to do some further diagnostics on your motor before you spend alot of time and money on a rebuild. Is it evident that oil is being burned as the motor is running (blue smoke, burned oil smell)? Or is it only upon initial startup after sitting for a period of time? I have a '79 Z28 that burned oil from warn valve guides after 65,000 miles. After the car sat for a day it would burn oil for a couple of minutes as it warmed up, after it warmed up it was fine. The oil leaked down through the seals/guides and into the combustion chamber while the car sat. You should be able to pinpoint if it is valve related or piston ring related through the use of a compression tester and/or vacuum pump.
Good Luck!
MikeComment
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