My friend is on the rode with his 81 CD. he tells me the car will not go over 50 mph. He changed the fuel filter and there is no change. anyone have any idears? thanks

It's usually the smaller plastic one that's the culprit.

Fill with fuel, the screen in the tank could be clogged. Check small bent vacuum hose on top back of injection pump. Check vacuum hoses at turbo boost switch on firewall near brake booster.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Likely the pre-filter is clogged. Change that. It's called the "Secondary" filter.

If it clogs up again shortly after, you have a bacteria issue that needs to be remedied with a Biocide or tank removed and cleaned. Also, the tank screen will be dirty as well.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

I had a 300D with the same issue years ago. Turned out to be the Fuel Pressure Relief/Overflow Valve (this is what holds the pressure in the Fuel Injection Pump Housing) Its located on the injection pump between the pump and the engine with a fuel line attached to it. Its a simple ball/spring valve. Disassemble and measure the spring, if it not 27 mm long then stretch it evenly until it is and reasssmbe

Thanks everyone for the info.I will pass it on.