Is this possible? When I've been running my 1981 300d for awhile, a look at the clear fuel filter shows it to be 1/2 full of air - the car apparently is not drawing fuel through the clear filter. The end of the filter that the engine should be drawing fuel from is high and dry and there is no motion of fuel within the filter.

However, the car is still running! Clearly, it is getting fuel from somewhere. Not only is it running, but it is drivable.

Is it possible that the car is drawing fuel from the return line on the tank?

How is that possible? What part has failed to cause this reversal of fuel if that is indeed what is going on?

Any help would be great.

Thanks,
Jeff

Warm up the car with a short drive. Park it and shut it off. With the engine warm, you won't need the glow plugs to restart it. pull the line off the prefilter and have a buddy start the engine and be ready to shut it off. Hold a glass jar or coffee can under the fuel lines and see what happens. After you do this test, replace the fuel lines. Use your primer pump to bleed the air out of the system. I would crack the top bolt on the spin on filter housing and pump the prime pump until fuel runs out from under the bolt. A few paper towels topside on the filter will prevent a big mess.
BTW is your filter installed in the right direction? A picture might help us figure out what you're actually concerned about.

Thanks for the response. This is my concern: the engine appears to be drawing fuel from the line that it should be returning fuel to the tank with.

As a dramatic demonstration of this: If I take a pair of vice grips and crimp off the fuel line just after the clear fuel filter, I can start the car and run it no problem while the fuel line is crimped!

I am assuming that the fuel is coming up the line that usually returns fuel to the tank. How is this possible? What part is broken to cause this? Is the lift pump right after the clear filter shot? and then the Injection pump is somehow drawing fuel from the fuel return?

Air in the primary filter is normal as long as its a stationary bubble and not a stream of bubbles.

Thats because the engine uses very little fuel at idle.

Its not.

The fuel return line fitting on the pump has a check valve incorporated into it. Any attempt to get fuel to flow into the pump from that direction would be met by total failure. Not only is it a check valve going in from the return line direction it is also calibrated to open at about 14 lbs pressure I read reciently from the injection pump side.
The clear filter normally contains an air bubble as already mentioned and I agree the fuel appears static in there at times. Now if you find your fuel tank self filling and overflowing at the filler neck. Give me first chance offer on the car. The car is worth a fortune.