Ford Repair: Oil blow by, freeway miles, piston rings

Question
'98 Mercury Mistic
144K Freeway miles
Runs good Always has. No abuse.

I changed the oil & Filter on my car. Two days later I discover that it is leaking oil on the driveway. The oil level is 1/2 quart low. I topped it off and tightened the filter (assuming that might be the problem). The next day the CHECK ENGINE light comes on. I have my wife park it until I have time to look at it closer. When I do, I find that the oil is coming from the air cleaner. I think that the oil is shooting out the carburetor and into the air cleaner. I find about 1/2 quart of oil at the bottom and the air filter is saturated.
I change the oil again so that I can replace the filter. I did not actually see any oil shooting into air cleaner. Afterward the CHECK ENGINE light is still on.

Here is my theory: The Motor Craft filter that I installed was defective. It was clogged and the pressure backed up the oil so bad that it made it out the carburetor. The CHECK ENGINE light is on because the sensor in the air cleaner was saturated with oil.

Does my theory hold water? Do I need to change the sensor? What type of sensor is it?

Bubby
MCBubby@msn.com  

Answer
Oil blow-by is caused by excessive pressure or a build-up pressure in the crankcase.  It it usually caused by combustion pressure leaking past worn or sticking piston rings, or a clogged PCV system.  I don't think that a defective oil filter could cause this. The filter has a bypass valve inside that allows oil to flow if the filter gets clogged.  The oil filter is cheap and wouldn't hurt to replace for peace-of-mind. Stick with Motorcraft, they are very good quality.  I would check that PCV Valve first.  Remove it and shake it, it should rattle freely.  If it doesn't, it can be cleaned out or replaced.  The oil in the air filter housing could possibly effect the emission sensors and cause the Check Engine Light to come on.  I would have the system scanned before replacing any parts.

I hope this helps,
Dave