Reading Tailpipe Smoke

From How to Restore British Sports Cars by Jay Lamm

Originally published in the Austin-Healey Magazine, June 1992


Smoke from the tailpipe can tell you a lot about the engine. Some cars will smoke all the time, but others need a little hand-while the car's still cold, have a friend or the owner stomp on the gas while you stand in back. Look for:


White smoke: This is usually water; if it goes away quickly it's probably just natural condensation and nothing to worry about. If it stays with the car for a long time or seems particularly heavy, cooling water is getting into the cylinders-a bad head gasket or cracked head are the most likely culprits.


Blue smoke: This is oil. Oil usually enters the combustion chambers through bad valve guides and seals; it will mean a costly head rebuild at least. It can also indicate worn piston rings or cylinder bores (expensive) or a faulty crankcase breathing system (not so costly). Worn rings and bores should show up on a compression test; most valve problems will not (To check the bores and rings correctly, squirt a little oil into a cylinder and test its compression again it will rise significantly if that's the problem). A little oil smoke at the first startup that quickly goes away usually means a not-too-serious upper-end problem; the work will still have to be done, but not in a tremendous hurry.


Black smoke: This is almost always unburned fuel. Black smoke usually means the car is tuned poorly or there's something not quite right in the carbs or choke. It's possible that the owner has done this intentionally, however, an overly rich mixture will help keep temperatures down in a car that has cooling problems.


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