I’m having a problem with the small block 383 in my Impala. The engine was new about two years ago, but I keep having problems with the spark plugs loading up with oil. Some of my friends say it’s a valve guide seal problem but the heads were new when we put the engine together. Others say the rings never seated. If the rings never seated, wouldn’t the engine smoke all the time? The engine doesn’t seem to smoke either accelerating or coming down in gear. It uses about a quart of oil every 500 miles and it keeps fouling the spark plugs. Do you have any ideas?
Jeff Smith: You’re right, a quart every 500 miles is way too much, especially for a relatively new engine. At first, I was in agreement with your friends who suggested valve guide seals, but if you don’t see clouds of blue smoke from the exhaust on deceleration, then likely there is another situation. The above can often happen when the valve guides are worn or when improper valve guide seals are used. I still see unenlightened engine builders using those old, hard plastic PC seals. The problem with these seals is they are very stiff. Even normal valve guide clearance can quickly elongate these seals allowing oil to leak past – especially on intake valves that see manifold vacuum. That’s why the engine will smoke on deceleration –the high manifold vacuum pulls the oil right past the guide seals.
The better valve guide seals are the ones made with Viton or other fluoresastomeric synthetic seals that remain pliable under high temperatures. These can often be metal bodies that fit over the guide bonded with a rubber seal. The more pliable material will allow the valve to move inside the guide yet maintain a proper seal.
But assuming that your valve guide seals and the rest of your engine is in good shape, there could be another explanation. I actually had this same high oil use problem happen to me many years ago and I at first assumed it was poor valve guide seals. I replaced the seals with high quality Fel-Pro pieces but the problem persisted. Like your engine, I did not notice any telltale blue smoke on deceleration yet the engine continued to burn oil at a prodigious rate.
Eventually, I yanked the intake manifold and discovered small dots of oil on the floor of nearly all the intake ports. It was clear that the engine was suffering from a vacuum leak and the air was pulling oil from the lifter valley into the intake ports anytime the engine was running at high vacuum. For a street engine, this is just about 100 percent of the time!
At first, I blamed the Fel-Pro Print-O-Seal intake gaskets – the ones with the silicone seal around the perimeter of the port. I tried a different, paper style gasket but met with the same result – the engine continued to use oil. Finally, after a discussion with now-retired Fel-Pro gasket engineer Greg West, I discovered that my intake manifold was the real culprit.
After carefully measuring the angle of the heads and the corresponding angle of the intake manifold, we discovered that the intake created a diverging angle at the bottom on the intake ports which meant the gasket was not sufficiently preloaded at the bottom to prevent oil migration into the intake port. Subsequently, Greg developed an interesting test that I have outlined in a story in Power & Performance News. This story covers in detail how to do your own testing with no specialty tools required. All you need are some soft, lead shot gun pellets and a dial caliper.
After we measured the mismatch, we discovered the intake needed to be machined, removing roughly 0.015-inch from the top of the manifold at a tapered angle to zero at the bottom. I took the intake manifold to my machine shop and they milled the intake to create more preload on the gasket at the bottom of the port than at the top. Once that was completed, we re-assembled the intake manifold with those same Print-O-Seal intake gaskets and a new set of spark plugs and the engine immediately stopped using oil. Then I dribbled about 16 ounces of water through the carburetor at fast idle which steam-cleaned the chambers to eliminate all the carbon buildup on the pistons. Eventually, we were able to also add a degree or two of part-throttle timing that beforehand had only resulted in a bad case of detonation. Oil has a nasty habit of creating detonation so our now-clean chambers could accept a little more timing and the engine immediately responded with crisper throttle response and slightly better fuel mileage.
So the point is that even a small thing like a leaking intake gaskets can have a big effect on the performance of your engine. It’s worth checking if your engine uses oil. While small block Chevy was the engine in our case, we’ve seen this same problem with small block Fords as well. This will be true with any engine that has a common area between the lifter valley and the intake ports. Pontiacs or LS engines, for example, will not suffer this problem because the lifter valley is separated from the intake ports. However, they may still have a problem that will show up as a simple vacuum leak.