What is excessive "blowby"?

Dear Car Talk

Dear Car Talk | Apr 01, 1991

Dear Tom and Ray:

I have a 1983 Ford F-250 with the five liter engine. The filter inside the air cleaner becomes saturated with oil every two to three weeks. Why? What can I do about it?

RAY: Gee, Bob, that doesn't sound good. You have what we call excessive "blowby."

TOM: Blow by is the small amount of exhaust that normally gets past the rings and winds up down in the crankcase. When everything's working right, the blow by gets sucked out by the crankcase ventilation system. But if the ventilation system is plugged, or there's too much blow by for it to handle, then the excess gasses escape the only other way they can--up into the air cleaner.

RAY: The first thing you should do is check the PCV (Positive Crankcase Ventilation) system and make sure you're getting suction. If the system is plugged and can't suck the gases out of the crankcase, unclogging it should solve your problem.

TOM: If you ARE getting suction from the PCV system, then the news is much worse. That means that the engine is producing too much blow by--that the pistons, rings, or cylinder walls are all worn out and that too much exhaust is getting into the crankcase. That means it's time for an engine rebuild.

RAY: Our advice? Kneel down, face Dearborn, and pray hard for a plugged PCV, Bob. If that doesn't work, pray for a $1,500 bank error in your favor.

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