Ever heard of fuel flowing back into the fuel tank when the car has sat overnight? This was suggested to me as the cause for my difficulty starting the car after it has sat for several hours up to overnight.
New battery, new glow plugs, new fuel filters, diesel kleen and lucas added to last tank of fuel, fresh oil in oil bath, no clog in air intake.....
But everytime the car has sat overnight or for more than 3 hours it takes about 3-4 tries and about 30-45 seconds pulling the start knob.
If it is fuel flowing back into the tank, what would cause this to happen and how do I fix it?

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Anthony
'76 W115 240D
'67 IH Scout 800
'61 IH Scout 80

Yes, this is definately possible with diesels since they have a "sealed" fuel system. If there is a leak anywhere in the system.....and by leak I don't mean an actual drip......this will allow air to enter the fuel system and then the fuel will by syphoned all the way back to the fuel tank. It's like sucking a drink up into a straw and putting your tongue over the end of the straw. The liquid will remain in the straw. But if you poke a hole in the straw, the straw has slight cracking, etc. there goes your liquid. Same deal on your dielsel and that's why there are clear plastic fuel lines from the filter to the injection pump. So you can see if you have fuel at the read, or air in the line, etc.

Remember that diesel is compression ignition. This is especially important cold! If your engine has compression that's on the low end of spec, or you have the wrong weight of oil for your cold starting conditions, or a slow cranking starter due to marginal battery, or battery cables that are just too small in diameter or poorly installed, you'll have hard cold starting.

Typically if a diesel is syphoning it's fuel, it will not start at all, no matter what. That's why you have that priming pump on the side of the injection pump...so you can prime your own system should it ever syphon back, either due to a leak or due to being taking apart for repair work of some sort.

If yours starts on it's own, you probably don't have syphoning. You probably have either low compression or slow cold cranking speed as I mentioned above.

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I forgot to mention it has a new Bosch starter...

It does eventually start...so its not a siphoning issue...
I dont think its a weak cranking issue (given the new battery and starter)

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Anthony
'76 W115 240D
'67 IH Scout 800
'61 IH Scout 80

Well, new starter and battery are good. But what about the battery cables and the ground from the bellhousing to the firewall? If you want her to really crank over fast and start quick, replace all these with some larger diameter cables. If you've got a NAPA auto parts store in town, they typically have a large selection of premade Beldon cables in diameters down to "2" and "0" gauge. Those big cables make a nice difference!

I would certainly do a compression test on this engine hot, all injectors removed, throttle plates wide open, crank until the compression test gauge needle stops rising. If it's not within spec, adjust the valves and try again. If it's still not within spec you can do a leak down test to find out if you're loosing compression past the valves or past the rings.

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Thanks...this is a good starting point...this will keep me busy for awhile...

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Anthony
'76 W115 240D
'67 IH Scout 800
'61 IH Scout 80

Be sure and take a look at the plastic diesel fuel line and see if it's empty, or only partially full during cranking when cold. It could be that you have a slow leak and that it's only slightly sucking fuel back to the fuel tank.

Always start simple and work toward the more complex solution.

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Heck, the last gorilla knob and salt shaker glow plug mechanism Benz I'd seen (Jim Harris w110) took about 4 minutes of glowing and reglowing the plugs before it would start stone cold. What makes ye think 30-45 secs is enough? With primitive diesel starting mechanism you can pretty much pull the knob, return to house and pour yer coffee then return to find plugs glowing.

not a diesel guy, but I've heard that valve timing on these beasts is even more critical than on the gas counterparts. This could also cause the problem you're seeing.

Whe was the last time the valves where adjusted.

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72 250 M130 W114

If these have good compression, they fire off with a nice crack and come right to life, even cold. Multiple attempts to start and quite a bit of cranking indicates the engine is either trying to build up enough compression to fire, or enough fuel to light.

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Mr. Miller is right...check all fuel lines for leaks,install the transparent in-line pre-filter(2$),prime the system,open the bleed screws on the filter cannister and on top of the IP(one by one,and screw them while you`re pumping the primer pump).If the lines are sound ,then the valves in the lift pump are suspect(cheap and easy to replace,or they`ll need just some cleaning). All lines from tank to IP should be full of fuel(yes,just like a straw). Add a good starer+battery,the upgraded glow-plugs,and the starting is MUCH easier.
Good luck!

http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=115119&M=616%2E916&GA=722%2E108&GM=716%2E002003&L=760%2E101++++++++++++++765%2E700&CT=M&cat=004&SID=07&SGR=045&SGN=01
valves-fig.431