Ford Repair: 1992 Ford Econoline E150, oil filler cap, ford econoline

Question
I have a 5.0 E150 Conversion van, I have noticed that when I get to about 65 mph, the check engine light comes on usually when its going up an incline, and goes off once its back downhill, it doesn't come on around town just on highway. Also I hve noticed brown deposit in oil filler cap and excessive oil in the oil breather, it has just had a new serpentine belt at the main dealer. Any Ideas. The mileage is 142K

Answer
replace your pcv valve and make sure your vacuum hose is not restricted

as far a check engine light it is probably a egr flow problem but you will need to have it checked

clean your throttle bore and iac motor since you have oil in breather.