Ask the Mechanic

Steve Thompson, Contributing Columnist

READER: I have a 3000 Ford diesel tractor that has started blowing a gray vapor out the pipe that runs down the side of the engine. It appears to be connected to the valve cover. Where is that smoke coming from? Is the engine about to blow? It seems to get worse after the engine warms up.

STEVE: What you are seeing coming out of the pipe is called blowby. This is a combination of combustion gases that have leaked between the piston rings and cylinder walls, and into the crankcase area.

This blowby must be released somewhere, or it can blow out the front and/or rear seals of the engine. So, it vents back through to the top of the engine and exits through the tube on your valve cover.

All engines have some blowby. When it gets extreme, you will see it exit the engine. Eventually, it will get so much blowby that the tube can actually drip engine oil out of it. It gets worse when the engine warms up because the viscosity of the engine oil gets thinner and is less able to seal the leak by the rings.

An engine overhaul is the fix for this engine to stop most of the blowby. Newer engines recycle the blowby back through the engine for environmental reasons.

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-- Email Steve Thompson at mechanic@progressivefarmer.com, and be sure to include your contact information and phone number.

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