Dear Car Talk:

The mass air-flow sensor on my beloved 1999 VW Eurovan camper has been replaced three times, and now it's gone bad again. The mechanic who looked at it said there was oil on the sensor. What would cause that? And any idea what it would cost to fix? -- Meredith



You know all those quarts of oil you've been adding to your engine? Now you know where they're ending up.

It sounds like you have an affliction we call "blow-by": When enough motor oil sneaks by worn-out piston rings and then gets combusted in the cylinders, oily vapors can get blown back into the air-intake area, where the mass air-flow sensor lives. And if you get enough oil on it, you can muck up the sensor's electronics and cause it to stop working.

If you do have blow-by that's that serious, an oily air-flow sensor won't be the only piece of evidence: Your mechanic undoubtedly would see an oily film all over the air filter, too.

The best-case scenario for you, Meredith, is that whatever blow-by your engine is producing is being exacerbated by a bad PCV valve. The PCV is supposed to remove combustion gases from the crankcase, and recycle them through the air intake so they don't build up -- and blow back.

If your PCV system isn't working anymore, that could explain why those gases, and the oily vapors, are getting blown backward and fouling your airflow sensor. A PCV system might cost you a couple of hundred bucks to repair.

The worst-case scenario is that your PCV system is working fine, which means it just can't keep up with the massive amount of blow-by your engine is producing. That would mean you're on a countdown to an engine rebuild. That's thousands.

If you're looking for a shorter-term solution, you also could try cleaning the sensor that failed. Normally, they fail because a wire breaks. But if yours is just smothered with oil, you can try using contact cleaner (not the Bausch and Lomb stuff for your contact lenses, the stuff that cleans electronic contacts) to clean off your sensor and see if you can get it working again.

Or you can simply invest in mass airflow sensor futures, Meredith. Maybe you can get a case price from Murray the Airflow King. Good luck.