My 1990 300E has 122,000 miles and has an oil consumption problem. Oil will appear on the top of the air fliter box around the breather tube from the valve cover. The rate of consumption is about 1 qt/500 miles. Any ideas why this is happening?

 

Most likely valve stem oil seals. Very cheap to buy and about 1.5-2hrs to fit (not a DIY job though).
Look for smoke from the exhaust on start up and when changing down a gear on a long road.

 

Yuck, that nasty blue smoke.

 

After some more investigating, I found the crankcase building pressure. I removed the breather tube that connects the valve cover to the airbox and placed my thumb over the outlet on the valve cover. I removed the valve cover and found the passage in the valve cover blocked where it connects to the small tube that leads to the intake manifold. The passage wasn't clogged, it plugged as if they failed to completely drill out the passage during manufacturing. Very strange. I drilled it out with 1/8" bit, hopefully this will correct the problem.

 

Do you not think this might have given you problems during the first 122,000 miles?
122K is about right for valve stem oil seals.

 

I just bought the car a couple of months ago. Don't know what it was doing prior to that.