Ask Away! with Jeff Smith: Compression Looks OK—So Why is My SBC 350 Engine Using So Much Oil?

I have a small block Chevy 350 that uses a terrible amount of oil. I did a compression test of all the cylinders with the results ranging from 155 psi to 172 psi. That would seem to me to be within reason but the engine still uses a lot of oil. Any suggestions before I tear this engine apart?

J.T.

There are a ton of different ways this can go and we will likely not be able to cover them all in this short tech question and answer format.

…But we’ll give it a shot!

Right from the beginning, we can’t assume you have experience with this engine. It is more likely that this is an engine in a vehicle you just purchased, so its history and usage is a mystery. If the engine oil looks nasty and black, the first thing we’d do is change the oil and filter.

Is this a flat tappet engine? If so, we’d recommend a hot rod oil like Summit Racing ZDDP Performance Motor Oil or any of the other boutique oils to protect the cam and lifters. A good selection would be the 10w30 oil as this is a common viscosity that would work across the board.

Clues for Diagnosing Engine Oil Consumption

From your question, you’ve probably already done this and measured the consumption. With decent compression indicated by the compression test, it would not appear that the rings are the problem, but again, that’s a rather broad assumption. There are other clues that you should look into. For example, is there oil on the threads of the spark plugs? If so, this could be an indication that the oil is coming from the top end of the engine instead of past the rings.

Oil on the plug threads is often a clue that liquid oil is entering the combustion chamber. This occurs because cylinder pressure pushes the oil (after thousands of piston cycles) into the threads where the oil is not subject to combustion.

This oil could be coming from several different places or a combination of sources. For example, if the engine has bad or broken intake valve guide seals then oil could be leaking past the intake valve guides and entering the combustion chamber. It’s also possible the intake valve guides are worn and need rebuilding. Just adding new valve guide seals will only mask the real problem.

Another source could be an aggressive PCV valve system that is pulling liquid oil into the intake manifold because of a non-existent vapor separator in the valve cover. Early LS engines with high oil consumption were thought to have poor ring seal—but the real cause was a too-small oil separator which allowed the PCV valve to pull liquid oil directly into the intake manifold. One solution here would be a separate vapor separator canister between the PCV valve and the intake manifold. This allows the PCV valve to work as intended but will keep liquid oil out of the intake manifold.

Another source of high oil consumption could be a poor fitting intake manifold. We’ve seen this happen on at least two of our own small blocks where the intake manifold sits at an angle on the heads that does not seat properly at the bottom of the intake manifold. This lack of gasket crush on the lower portion of the gasket allows engine vacuum to pull oil directly from the lifter gallery into the intake ports.

This pulls oil from the engine almost continuously and can burn up to a quart every 400 to 500 miles!

The solution is to first measure to see if the intake manifold is in fact parallel to the head when bolted down. Jim Hughes Engines in Washington, Illinois offers an inexpensive wax string kit that will evaluate your package and let you know if the intake will need machining and how much to remove.

Of course, it could be that the rings are a fault. While many people think of the second compression ring as backing up the top ring to seal cylinder pressure, the real purpose of the second ring is to remove oil from the cylinder wall that remains after passing by the oil control ring as the piston moves down the cylinder.

Since the second ring is not a compression ring, it’s possible that the top ring is doing its job but one or more of the second rings are not performing. It is possible, depending upon the engine’s condition that they’re stuck or gummed up. It’s also possible that one or more second rings may be broken.

This is not an entirely uncommon occurrence. If the engine has detonated badly, this uncontrolled cylinder pressure will crack the bottom of the top ring land which forms the top of the second ring. This will cause the oil ring to stick in the groove and therefore fail to prevent oil entering the combustion space. If this is the case, the only solution is to fully rebuild the engine.

Testing & Troubleshooting Engine Oil Consumption

The overall approach to deciding which problem or areas may be contributing to high oil usage is to systemically evaluate each of these possible areas. The smart thing is to look at the systems that are easy to repair first. Don’t assume that you have to rebuild the engine only to discover that the intake manifold was not sealing. Repairing the intake manifold can be done with the engine in the car. Only after you’ve run through all the easy repairs should you consider rebuilding the engine.

Another point is that there could be multiple areas contributing to this high oil consumption problem. This means it may take more than one solution to reduce the oil usage problem. We had a similar problem many years ago and it took us almost eight months to finally find the problem which turned out to be the poor manifold seal between the intake and the head. We machined the intake, replaced the gaskets, and oil usage went from one quart every 500 miles to more like one quart every 2,500 miles.

Good hunting with your search for the culprit(s)!

Engine Cylinder Pressure Test Results

This is the cylinder pressure test performed on the engine. The generally accepted standard is a 10 percent variation in pressure between the highest and lowest cylinder. The difference between 172 and 155 is 17 psi which is very close to 10 percent  It depends on which pressure standard you choose but it’s safe to say that there would appear to be no evidence of a single or pair of under-performing cylinders.