You asked:

Why would a car engine start smoking? — Enrique, Victorville, Calif.

Marshall Answered:

Assuming that you mean “smoke coming out of the tail pipe”, what you are seeing almost always is a side effect of burning oil. Somehow oil is getting into the combustion chamber and burning, and it creates smoke.

So the next question is, “how is the oil getting there?” There are four possibilities, and if you have read How car engines work it can help to visualize all four:

1)I once took my pickup truck to the dealer to have the oil changed. It was a new truck, and the first oil change was free. About a mile down the road I looked in my rear view mirror to see a huge cloud of smoke. What had happened in this case was they has put in way too much oil (like 8 quarts instead of 5). Draining the excess oil out solved the problem.

2)The pistons have rings that create a seal between the crankcase and the combustion chamber. As these rings and the cylinder walls wear, oil can start getting past the rings and into the combustion chamber. This usually takes many miles, and it usually starts slowly and gets progressively worse. The normal solution is to get the cylinders rebored, and then install new pistons and rings. Or you buy a new engine and have it installed.

3)There is a gasket that sits between the block and the cylinder head. If the gasket rips or gets a hole in it, it can let oil into the combustion chamber. The solution is to replace the head gasket.

4)There is also oil lubricating the valve stems, and seals that keep this oil from leaking. If one or more of these seals fail, it can cause an oil leak that creates smoke. The solution is to replace the valves and seals or the entire head.

How do you figure out which it is?

If it is #1, then you have just recently changed the oil or added oil. Check the dip stick and see if the engine has too much oil.

The normal way to check for #2 is to do a compression test. One or more cylinders won’t be able to hold pressure if the rings are leaking.

If it is #3, there are often other symptoms like water in the oil.

#4 is less common, and would be suspected if the other three have been ruled out.

Here is a nice explanation, with special emphasis on head gaskets: