I have a 1994 S350 Turbo Diesel with just over 200,000 miles. I been having a problem with the turbo boost for several years, during which the problem has become progressively worse.

When the engine is cold, it seems to boost normally. After driving for a while (30 minutes when the temp is around 0, maybe 5 minutes when the temp is 40 and above) the turbo stops boosting. If I pull over and shut the car off and immediately restart it, it will boost near normal for a minute and then boost somewhat for the rest of the drive (apparently regardless of the remaining duration).

I've removed and cleaned the EGR valve (it seems to operate smoothly). No help. I've also removed, cleaned contacts, and re-installed the ECU. No Help. I'm wondering if the MAF sensor could be the culprit.

I've taken it to the local Mercedes Dealer more than once but they have no idea what the problem is.

Any suggestions?