Question: I own a 1979 Mercedes Benz Turbo Diesel. Several weeks ago, I added a bottle of fuel additive to my tank and repeated the procedure for the next four tanks. Then, I began to get dieseling, in which I could not stop the engine after turning off the ignition key. The engine would shake violently before finally dying out. What causes this? Should I quit using the additive?--E.S.
Answer: Although the term dieseling would seem to be a problem affecting diesel engines, it is actually a condition that affects gasoline engines. It is very unusual to get dieseling in a diesel engine, because turning off the engine stops the supply of fuel, and that should kill the engine instantly.
In a gasoline engine with a carburetor, dieseling occurs when the engine continues to run even after you have shut off the ignition. The hot spark plug continues to detonate the fuel, and the carburetor simply supplies the fuel, because it is not always controlled electronically.
But a diesel engine does not have a carburetor. It has a pump that squirts small amounts of diesel fuel into each cylinder. The mixture is detonated by the heat and very high compression created by the piston.
The dieseling that you are experiencing could be caused by a problem with the fuel injection system. Possibly the turbocharger is leaking engine oil into the intake manifold, which gives the engine a supply of fuel to burn.
As for the fuel additive, it seems unlikely that it has caused your problem. It is possible that the additive may have eroded a seal that has somehow caused the turbo charger or the fuel-injection system to malfunction, but it would be unlikely.
Q: In your recent column about timing belts, you said if a belt breaks it can cause serious engine damage in some cars. I wonder if that is the case with my new Ford Escort. My mechanic told me that a broken belt would not seriously damage the engine.--I.I.
A: The most serious damage that can occur to an engine if the timing belt breaks is that a valve hits the piston, either bending the valve or puncturing the piston. This occurs because at least some valves on the engine will be stuck in an open position if a belt breaks and the pistons continue to reciprocate.
Ford Motor claims that after 1983, it designed a depression in its pistons so that they will not touch a valve even if it becomes stuck in the open position as the result of a timing-belt break. This is definitely not the case with other manufacturers, whose engines remain extremely vulnerable to major damage as the result of a simple break of the timing belt.
Q: My new Chevrolet owner's manual recommends that I use a 5W-30 weight oil in the summer. I am worried that that is too light and will not provide enough protection on hot summer days. I have always used 20W-50 weight oil. Should I ignore the owner's manual?--T.O.
A: The lighter-weight oils are being recommended because they flow much more quickly when cold, providing an engine with better lubrication on starting. Much of the wear on an engine occurs in the first seconds when you start up in the morning, because the oil is thick and takes some time to begin doing its job.
The new 5W-30 oils, which are rated as SG, are vastly improved over the SF-rated oils they replaced. About 85% of the 1989 cars carry a recommendation for the light 5W-30 oil.
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.