We're stumped on this one.

We rebuilt the engine on a '75 240D (W115) and it drove beautifully for over 10,000 miles. Didn't use oil between oil changes.

Then it began to use oil rapidly, blew smoke, and we could see air in the return line. The smoke and rough running was intermittant however.

We fed fuel from a gas can on the car's roof to ensure there wasn't an air leak before the fuel pump. Then we replaced the seal between the fuel pump and the engine, but there was still some air in the return line.

We took her for a test drive and as soon as we drove down a steep hill, the engine started knocking and began speeding up by itself (burning oil)... we drove around on the level and it smoothed out, stopped smoking, drove like a dream. As soon as we were facing downhill again it started running rough and speeding up by itself.

Both the sucking air and burning oil confuse us... and the coorelation between the two.

If oil is getting in the fuel then why not run bad all the time? Again, if the rings somehow went bad suddenly and are sucking oil... why only when facing downhill?

Our mechanic with 50 years of diesel mercedes experience is pulling out his hair on this one.

We appreciate your time.