95 E320 Running Rich-Why?

[TROVERMAN]

The car is pre-OBDII, so no MIL lit, and no easy diagnosis. The obvious culprit is the H02S. The car has 105k on it, the last sensor was done by me at 59k miles. The car has been running rich for a couple of thousand miles now, anyway. In town gas mileage is poor, highway isn't great. I just can't see the o2 sensor needing replacement until 120k.

So, what could cause this? Car has been running a K&N filter since 40k miles. What about the air temp sensor on the air intake?

Also, what would happen if the air pump ran all the time? When the car shredded a serpentine belt last summer, it cut the wires to the pump. I spliced together as best as I can, not sure if the pump runs all the time or none at all, or correctly.

What about the MAF? How do you test?

In very hot temps, the car needs to cool down before it restarts.

Any ideas?

Thanks.