Why does my car release a big cloud of blue exhaust smoke?

Dear Car Talk

Dear Car Talk | Oct 01, 1989

Dear Tom and Ray:

I have a 1982 four-door Subaru GL with a little over 40,000 miles. I get good mileage...an average of 27 miles per gallon. My problem is that approximately every tenth start, there is a big cloud of blue exhaust smoke. What causes this?

TOM: This year's exhaust smoke comes in three basic colors. Each color is indicative of a different type of problem. If you see blue smoke, that means that you're burning oil. Black smoke indicates that your fuel mixture is too rich. If you see white smoke, that means a new Pope was elected.

RAY: Actually, white smoke means that you are turning coolant into steam, and you probably have either a cracked head or a blown head gasket. In order of expense, black is the cheapest smoke, white is the next most expensive, and the stuff you say you have--blue--is the high-priced smoke.

TOM: Intermittent clouds of blue smoke are difficult to explain mechanically. One possibility is that you only notice it one out of every ten starts due to wind conditions. Get Willard Scott to stand behind your Subaru for the next couple of weeks. I'm sure Bryant Gumbel won't mind. Have him put together a satellite picture of the wind patterns in your driveway. Maybe conflicting high and low pressure systems are throwing off your diagnosis? If it really is blowing blue smoke every time you start the car, it's probably due to worn valve guide seals, or even stuck piston rings.

RAY: Another possible explanation is that what you think is blue smoke is actually white smoke. It's not that easy to tell them apart in your rear view mirror. White smoke is more likely to be intermittent. If you have a blown head gasket, for example, one of the cylinders could be filling up with anti-freeze after long drives. This would create plumes of white smoke the next time you start the car.

TOM: Get a mechanic to correctly diagnose this for you. If you have a bad head gasket, fixing it right away could prevent further damage to the cylinder head. If you're burning oil, either replace the valve guide seals, or buy a pair of dark sunglasses so you won't see the smoke anymore.

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