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<misc> Re: Blue smoke question

Bill Peters asked:
>I thought my car didn't burn oil at all, but recently I've noticed an
>increase in the puff of smoke that comes out of the tailpipe immediately
>after starting the car.  Otherwise there is no smoke whatsoever, and it
>only seems to happen after prolonged periods when the car is parked
>(overnight). What causes this to happen?

The usual cause of the symptoms you describe is worn valve-stem oil seals.

When the engine is stationary one of the exhaust valves is often open (it
has something to do with another cylinder being on the compression stroke
and stopping before top-dead-centre when you switch off). A little oil
drips down the stem into the cylinder, and when you start the car it gets
burnt: hence the smoke, and a little oil makes a lot of smoke!

The common test for this (an old 2002 buyer's check) is to rev the engine
to about 4500rpm in neutral, back off the throttle, and then floor it again
briefly when it has wound down to about 1500rpm. The strong vacuum caused
by closing the throttle at high revs tends to suck oil past the inlet
valve oil seals, and causes a short puff of smoke when the throttle is
opened again. You could try this test. Another way to check the same thing
is to select a low gear and descend a steep hill with the engine spinning
above 3500rpm with a closed throttle. Get back on the power at the bottom
of the hill and check how much of a grey cloud follows you!

I had your symptoms as an "intermittent" problem on a newly-rebuilt
engine (Datsun) that had one leaky exhaust valve seal. If stopped
overnight with THAT valve open, it would smoke in the morning, otherwise
not. It took a while to diagnose.

Andrew West, '83 318i, BMW Club Queensland, Australia