What Dirties My MAF?

After you've finished chuckling, here's my question:

I just fixed a CEL by cleaning the wire of my MAF. What did I clean off? After all, there is no oil, gas, coolant, grease or brake fluid that far forward in the intake tract. It should just get clean air going over it. So why does it get dirty? Cleaning was a few gentle swipes with a Q-tip soaked in rubbing alcohol.

Vehicle is a 1999 Toyota Tacoma V-6 4X4 with manual transmission and about 173K miles. I drive it WOT and shift early to improve mileage. Error code was P0171. Truck did the exact same thing, on the same interstate drive, almost exactly two years ago. What gives?

The first explanation I came up with is that maybe after shut-off, engine heat breaks down the oil (and maybe gasoline) remaining on the cylinder walls and the resultant gases leak out an open intake valve and float up the intake tract. Or maybe the same process but up from the bottom end and through the PCV valve.

What say ye mighty GRM minds?

David

Do you have a K&N filter? An over-oiled filter will do it every time. Any air filter isn't perfect, either, so over time you will have MAF issues, no matter your change interval.

Ever look inside the intake tract? When you shut off the engine there is almost always at least one intake valve open with atomized oil and fuel, not to mention the atomized oil from the PCV and the carbon from the EGR are free to swirl around in there. It seems like the airflow is all clean air from the filter into the engine, but valve overlap, reversion, EGR, PCV, blowby... they make things a mess.

everyone about summed it up.. but honestly... "What dirties my MAF" sounds like the start of a rant