91 VANAGON LOSS OF POWER/BLACK SMOKE
 

I have a 91 vanagon westy. All was well until driving up a long hill. Power declined significantly (loaded semi trucks passed me easily) and black smoke issued from the rear. Fuel consumption skyrocketed. Our mechanic replaced the O2 and coolant temp sensor as well as the air flow box. Problem solved...for about 10 miles. Same problem (on the same hill!) started again. The engine runs smoothly, temp guage normal, plenty of oil on the dipstick. Just loss of power, black smoke, especially on full throttle, and gas being guzzled like a V12!

Any suggestions welcome.

Cheers.

Andre