240D ran fine, now won't start... glow plugs? primer pump? starter? fuel system?

At age 27 I just bought my first ever car, a 1975 Mercedes 240D. It was running and driving fine for about a week with just a burst of black smoke on starting. Then the knob on the pull starter (that you use to turn on the glow plug and turn over the car, the proper name eludes me,) came off and my mother drained the battery trying to keep ahold of the shaft long enough to start the car. After a duct tape knob fix, we had AAA jump it. It would grind but not catch and sounded dry. It did almost catch once, but not being used to diesels I released the starter too soon because it sounded like I was grinding it. We tried using the primer pump but it didn’t feel like there was any resistance. AAA tested the starter and battery and said they were fine. Thinking maybe we needed more fuel in the system for the primer pump to work (we were just below a quarter tank,) we filled the tank and tried again with the same all around results. A few days later we tried one final go and couldn’t get it to do anything but make a kind of flippy-clicking noise my mother (a backyard mechanic of modest ability,) said sounds like knocking “but not the normal knocking.”

The last owner was not big on maintenance (but was honest about that) and had it “converted” to “biofuel” by getting the fuel lines and filters replaced a few years ago. This “conversion” uses a single tank for all fuels and didn’t add any extra heating elements. He used mainly (80% of the time) commercial diesel, the usual B5 and some up to B20, but also put various forms of veggie oil fuels in up to and including filtered WVO. I’ve only put in B5.

My goal is getting this Craigslist car to run for six month. I’m learning fast about the benefits/pitfalls of the 240Dog and reliability/money pit older Mercedes situation. If this needs a number of smaller fixes I’m all in, but I can’t afford a hobby car at the moment. If it looks worthwhile I would be willing to sink proper amounts of cash into it later. How serious do my issues sound and what is this most likely to be?

-Glow plug needs replaced. There isn’t any light in the indicator light (or is that actually the plug itself i should be seeing) on the dash. From what I’ve read this is definently part of the problem, but is it all?

-Primer pump. There’s no resistance (and yes I pumped it like 500000000 times). Like the glow plug this is obviously in need of replacement, but could this be indicative of compression or other big problems?

-Filters. Should I replace all the fuel filters? I’ve seen that recommended as a first step for this car, especially considering the fuel “conversion,” and will do it anyway if I can get it running, but would the filters have caused this particular problem?

-Fuel system, particularly fuel pump. Because of what I’ve learned about proper conversions, and especially because I don’t see how a dead battery/bad primer pump/ dead glow plug could lead to the flippy clicking noise, I’m suspicious of the fuel pump. It will die eventually, but is that time now?

-Other stuff. What else is likely to have caused the initial problem of not being able to start post battery jump?

I know a little about older VW diesels but don’t know how much of that translates to a Benz diesel but I’ll make a suggestion. Make sure that your glow plugs are getting power with either a voltmeter or a 12 volt lamp. Non TDI VW diesels need help starting from the glow plugs with a cold engine.

It might be risky to change the fuel filter at this time because that introduces the possibility of another reason for failure to start, that being air bubbles in the fuel line. A couple of ways to deal with this possibility are the use of a small hand operated vacuum pump and prefilling the new filter with diesel fuel. If the former owner did not change the filters and threw whatever he pleased into the tank, you may have no choice but to change the fuel filter(s).

There are plenty more things that might cause your non start problem. I suggest that you find a Benz diesel site or get what you can from the Technical thread at VWVortex when old VW diesels are discussed.

You should have ordinary diesel fuel in your tank to eliminate another potential source of a non start.

If the car has a manual transmission, you might want to get some experienced help to attempt a push start.

I second the push start idea…especially with an old diesel like this…once the engine is turning (when you pop the clutch) it will catch and run if it is still capable of running in its current condition.