For some time I’ve noticed a puff of smoke from my car’s exhaust just after starting it after it has been parked for several hours. Figuring this might be normal for a car with 135,000 miles I didn’t take it too seriously until driving down a very steep road last week using first gear to hold the car back. When I reached the bottom of the hill, stopped and started, a huge cloud of smoke bellowed from the back of the car.

My first thought was I blew a head gasket, as this had happened once before, but I’ve been told a head gasket causes white smoke and this was more gray. Also, it hasn’t happened again since. What’s up with my engine? Is it terminal?

– Jackie Townsend, Santa Cruz, Calif.

Jackie, it sounds like your engine is suffering from a minor but annoying ailment – worn valve seals.

Your engine has two, or perhaps four valves for each of its cylinders. Imagine a new pencil with its blunt tip glued to the center of a quarter. The coin (valve head) seals a doorway (port) for air and fuel to enter or leave the engine’s combustion chamber. A mechanism pushes the eraser end of the pencil (valve stem) to open the valve, and a sturdy spring returns it to the valve-closed position.

The valve stem rides within a metal tube (valve guide), with its uppermost end and pushing mechanism bathed in engine oil and the valve head end of the stem within an intake or exhaust port. A rubber seal at the top of the valve guide allows a tiny amount of engine oil to lubricate the valve stem/guide confluence, but is supposed to prevent a greater quantity of oil from being drawn down the guide.

During engine operation the greater air pressure within the engine’s oil-bathed area tends to push its way through the stem/guide, into the combustion chamber, bringing oil with it.

The smoking you’ve seen during a warm restart is probably caused by oil dripping down the stems/guides due to worn, broken or dried out seals, and/or excessive stem/guide clearance, caused by wear. The same thing happens during a cold start, but there isn’t sufficient heat in the combustion chamber to burn the oil, causing visible smoke.

A lower-gear hill descent causes a greater-than-normal pressure difference between the crankcase and intake ports, really pushing the oil through.

The good news is this isn’t a major engine problem, but the vehicle’s exhaust hydrocarbon emissions are higher than they should be and increased oil consumption could lead to a dangerously low oil level if not checked frequently.

On many engines the valve seals can be renewed without removing the cylinder head(s). It’s perhaps a $200 to $300 job. On others, or if worn valve guides are found, cylinder head(s) removal and reconditioning will be needed. This could push the bill into the high hundreds or more.

Some vehicles are known for lousy or short-lived valve seals, and improved replacements are available.

Write to Brad Bergholdt, c/o Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190; or e-mail to under-the-hood@juno.com.

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