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Diesel only cranks but doesn’t start until I apply starter fluid, it’s a mercedes benz 300sd 1983… also, it is normal that the engine is greasy? I change all the fluids regularly. thank you

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I’m not an expert in mercedes or how they are built but sounds like a weak fuel pump

The fuel pump on there old 300s I believe is mechanical, I know there is a manual purge/prime pump near the fuel lines that run into the cylinders. My strong advice, get a repair manual or better yet a service manual. They pop up on eBay cheap.

Does that engine have glow plugs? I have had glow plug systems stop working(bad controller, bad plugs, etc) and it wouldn’t start without ether.

I thought that glow plugs only matter when the environment is cold enough to prevent the ignition caused by compression. My diesel has a dashboard light that will turn on when it intentionally delays starting until the glow plugs have sufficiently raised the temperature.

Glow plugs are probably toast. I would replace them first, they aren’t expensive.

Is your glow plug relay working? Have you tried priming the lift pump on the side of the high pressure fuel pump before starting? Also there is a can fuel filter and a little inline one up by the pump, I suggest replacing these.

I have a 1994 vw t4, diesel engine with mechanical fuel pump. Had the same exact issue as you. Try replacing the ignition switch. 10 euros part and solved my issue.

I know it might not make much sense but yes, there was a fault on my ignition switch and even though my engine was cranking, it was not firing. Prior to that I replaced my glow plugs and starter.

Please also make sure you check all your fuses.

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