After a little freeway driving last weekend, I have a fresh concern. 1983 300D, on biodiesel. Fuel filters are fresh, alda line has been cleaned, diesel purge done recently.

When driving on the freeway for an extended period of time (20min +), the car starts to lose power. At first I have good acceleration, getting up to 75 mph without much effort. Once the car has been holding 60 or 65 for a bit, it starts feeling weak. There is a significant power loss, and it gets harder and harder to accelerate. When I get off the freeway, the engine rocks a lot at idle, like a panting dog.

This got so bad the other day that I was having trouble accelerating up to 40mph. I pulled into a gas station, checked the oil. It needed about a quart of oil, and a little fuel, so I added both. The car rested for about 15 minutes.

When I started up again, the acceleration was good again. But, sure enough, after about 15 minutes of driving, it got sluggish again, and was pretty weak by the time I got home.

Normally I drive around town, staying under 45mph, so I don't get to experience this problem. What is this a sign of? Why is it intermittent? All my gauges read normal when this problem occurs.

Have you cleaned your tank screen? It sure sounds like fuel starvation. Which generally leads to filters, tank screen, or an air leak. Try switching the supply and return fuel lines where they connect to the steel lines in the engine bay. This will turn the supply line into the return and the return into the supply, which will bypass the tank screen. Take if for a drive and try to repeat the symptoms, if they go way you know you have a clogged tank screen, if not they something else is up.

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Pop your fuel cap and try the car next time out. You might have the tank vent line obstructed and after running for a time the lift pump is pulling against a fairly strong vaccum. Of course some degree of fuel starvation is happening at that point. Or if f you stopped the car when experiencing the power loss and removed the fuel cap a large intake whoosh would be heard if the tank vent is obstructed. Just a possibility and easy to check out. Easy is usually good. There are of course other possibilities as well.

You didn't mention whether you have checked your fuel lines for leaks recently - I had similar symptoms with a leaky fuel line on dino diesel.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

This is a good test but be sure and have your fuel level above 1/4 tank too be safe.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"