engine looses power after about an hour of driving
 

I just bought a 1983 300DT. I took it for a long drive this weekend and after about an hour I couldn't get it over 75. There was still about an inch or two of throttle left at 75 but when I pushed on the accelerator nothing would happen. The car shifts through all gears smoothly and the turbo works. What could be wrong?

paleosol,

The car only goes 90 or so anyway, so you are nearing the end of the rpm range available. Losing power here can be either a breathing problem or a fuel problem. There are other possible problems but if it runs ok up to 75 mph, most of them are probably not the issue.

The air cleaner and valve adjustment may need attention, as they contribute to the breathing of the engine. Check the air cleaner, and replace it if it is dirty. The valves need to be adjusted and timed to do their job correctly, so these features should be checked and addressed as necessary.

The fuel system is pretty simple, and there are two filters on the inlet to the pump on the driver's side of the engine, plus a non-vented cap on the fuel tank. The filters need to be changed regularly as Diesel is often dirty. And if the venting system is not working the tank can develop a pretty strong vacuum that will limit the flow of fuel to the engine.

All this stuff is constantly under discussion here. Your problem description is not too clear so as you read the information about the valves, filters and other parts of the engine that may be suspect, be a little more specific about your question. For instance, I was expecting a car that would go fine when you started, but as time on the road built up, it would slow down, based on the title. Your description indicates the car just doesn't go more than 75mph. To give you much more specific help at least I need some more specifics about the problem.

Congratulations on you purchase, and if you are persistent I am sure we will be able to help you get more use and fun from the car than even Mercedes-Benz expected when they built it for you. Good Luck, Jim