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Moving Forward Against Blow-Back

March 05, 1987|RALPH D. VARTABEDIAN | Times Staff Writer

Question: What is engine blow-back? How is it caused, and how can it be corrected? I have a 1963 Cadillac with 150,000 miles on it, which I bought new. The car uses oil, particularly on sustained drives, and will need a quart every 200 miles. It does better in city driving. I don't see any oil leaks or blue smoke, but the engine is covered with slimy oil. I have been told the problem is blow-back, and only a complete engine overhaul will fix it. The compression on all cylinders is between 125 and 145 pounds per square inch.--E.M.B.

Answer: Engine blow-back is a condition that can certainly cause excessive oil consumption and does affect engines as they grow older. In many cases, blow-back can be fixed only with a complete overhauling of the engine, but often you can live with it by minimizing the problem.

Blow-back is caused by the escape of exhaust gases from inside your engine's combustion chamber to the crankcase or valve area. When the air-fuel mixture inside the cylinders fires, it creates tremendous pressure that drives the pistons downward. To contain that pressure, each piston has several rings, which are circular metal seals that ride against the cylinder walls.

When the rings become worn, they allow combustion gases to pass and blow into the crankcase area. The gas usually escapes the crankcase through the oil filler cap, the breather cap or the dipstick hole. Blow-back is worse at freeway speeds, because the engine is working faster and generating more blow-back gas.

The exhaust gases blowing out of the engine take along minute particles and droplets of oil, which causes your oil loss. In addition, your engine may be burning up some oil, even though you don't see blue smoke.

All engines have some amount of blow-back, because it is impossible for the rings to contain all the combustion gas. Modern engines have what is called crankcase ventilation, which captures blow-back gases and burns them in the engine. Your 1963 Cadillac simply vents these pollutants to the atmosphere.

Before you invest in a costly overhaul, you should first check to make sure the breather cap on your engine is clean. I have seen oil loss caused by blow-back sharply reduced on a number of 1960s-era cars by just cleaning the breather cap. Soak it in kerosene. Make sure the dipstick fits snugly in its hole and doesn't allow oil to blow out.

If your car has a draft tube for crankcase ventilation, make sure it is operating properly. You may want to consider using a heavier-weight oil or an additive that will increase the viscosity of your oil. That sometimes provides a slight improvement in the performance of worn rings.

Q: I took my car to a dealer because the engine would race constantly. Now I have to keep my foot on the gas pedal to keep the engine from repeatedly dying when it's cold. When the engine warms up, it will not idle down and continues to race. What's causing this problem?--A.B.

A: Several possible conditions could create the problems you are describing, but it sounds as though you haven't found a mechanic who's capable of diagnosing and repairing a modern, complex engine.

One strong possibility that would cause both problems is a vacuum leak in the various tubings for the emission control system. Such manifold vacuum leaks are not easy to find, but they can cause erratic engine conditions. And with all the plumbing on modern cars, it's a wonder more vacuum leaks don't develop.

Ralph Vartabedian cannot answer mail personally but will respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, You section, The Times, Times Mirror Square, Los Angeles 90053.

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