This is regarding a 1975 240D (W115).

The engine was rebuilt 10,000 miles ago, and the whole time she ran like a top, with no oil use between oil changes.

Then suddenly she started burning oil (no leaks) and we noticed air bubbles in the fuel return line.

The other day we took her out and as soon as we started driving down a steep hill it started burning oil (engine started speeding up by itself). Then we leveled out and it smoothed right out. We drove it around and it rode like a dream. Then we were facing downhill again and it started speeding up by itself.

This has our mechanic with 50 years of mercedes diesel experience pulling his hair out.

Thanks for your time!