85 300 SD....very hard to start

This starting problem has only developed this winter, so I am not sure if it will start better when we get warm spring weather or not. But It has
been very hard to start.... if the car has been sitting unused overnight. I was often cranking until the battery died, and then I could finally get it to start with a jump (even then taking a long time). There was one day that it wouldn't start even with a jump, and priming it manually with the hand pump got it to start...not on the first try after priming, but pretty quick. But the next morning, priming did not help. we had to crank the engine many times with a battery jump even after priming the fuel pump. All of the glow plugs were working except one. So I took the car to my mechanic, and he replaced the one bad glow plug, and replaced two bad fuel lines. You could not see any fuel leaking from them, but they were swelling up, and were always damp (with fuel, I assume). He also said the battery failed the load check, so I got a new 750 CCA battery for Mercedes installed. Got the car back yesterday. Started first try leaving the garage (warm afternoon). This morning it is near freezing, and although the engine does turn over a lot peppier now, I still had to crank it about 12 times before it would start. Dragged the charge down on the new battery a lot, but not nearly to "dead". Since all the glow plugs are working (all have 12 V at the terminal), and the engine is being cranked plenty fast, what can it be other than a fuel supply problem? My mechanic isn't equipped to do a lot on diesels, so should should I take it to a diesel specialist garage and get them to do a pressure check on the fuel injection rail? I can't see it being bad fuel injectors, as sometimes (other than when it has been sitting all night long), it will start up first time every time, as you drive around on errands. A clogged injector is clogged all the time, right? Worse yet, the mechanic replaced the little clear (actually blackened from fuel impurities) bowl on the fuel line going to the fuel pump& hand primer pump assy. The replacements are for some stupid reason opaque white, so I cannot see what is happening with the fuel AT ALL now! Unless there is high pressure there, could I just get some in-line fuel filter with a clear housing and install it in place of the new one?
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snipped-for-privacy@grandecom.net says...

(Snip)
It probably will. Sounds like high-mileage-engineitis. Did you have the valves adjusted in the Fall? How many miles on the engine? My old engine behaved in exactly the same manner. No start no matter what when the temperature went below about 15 degrees. I replaced the engine with a used one that had 137K on it. It starts instantly at 0 degrees. Well, not instantly. You have to keep cranking for a bit after it fires but it fires instantly.
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I doesn't have a lot of miles on it by diesel standards, about 230K miles....it doesn't even burn much oil. Last winter there was not a problem starting it , though.
says...

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A diesel is a compression engine, that means its combustion depends on adequate compression. The colder the engine the harder it is for its compression to achieve combustion temperature. That's why glow plugs were incorporated into its design.
The engine should be pre-glowed two or three times BEFORE cranking it. Glow - OFF, Glow - OFF, Glow - crank. That will warm the combustion chamber. My car's manual suggests flooring the accelerator, some owners say not to.
As you point out, it starts easily when warm so there's nothing wrong with the fuel system. Leave that alone.
You don't mention the miles on this engine, if 250K or more you're getting a clear signal from this hard starting that its compression is poor and an overhaul will be needed before next winter. If less than 250K miles it may only need to have its valves adjusted, especially if they haven't been adjusted in the last 20K miles.
Remember, it's a compression engine so when in doubt as to its remaining useful life a compression test is the only valid diagnostic test.
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Thanks, I will take it to the diesel specialist and get compression check and a fuel press. check while they're at it. One other question, if the valves are out of adjustment that could cause the compression to be low, couldn't it?

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Of course, its the valves that seal the cylinder during its compression stroke.
Over time the valve wears into its seat which brings its stem closer and closer to the cam until the cam prevents the valve from fully closing and sealing the cylinder. A safety "gap" is added to prevent such leakage. It's .004 inch for the intake valves and .015 inch for the exhaust valves. That's not much leeway so its important that the valves be adjusted periodically to restore their gap.
There's no need to test the fuel pressure, it's fine.
Have the valves adjusted; if they are OK (that the valves were NOT the problem) then have the engine's compression tested so you'll know its internal condition. There's a specified compression (about 320 to 350 psi) and a minimum (about 240) with about 45 psi allowed between cylinders. If this engine is more on the 240 side despite its valves being correctly adjusted then its time for an overhaul.
I suspect its compression is OK but that it needs its valves adjusted.
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Even when it's only moderately cold (3-5C), I find I need to do the same on my 1985. In addition, slightly depressing the accelerator pedal so that the engine starts at 1000-1200 RPM helps get it cranked over the first time (normal idle is 600-700 RPM).
(Thanks again for your earlier advice, TG. You have helped me quite a bit on the learning curve for these diesels.)

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If you haven't kept up with regular valve adjustments, doing one may help but it sounds like low compression.
Gene

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Concur on the valve adjustment. Diesels clearance get tighter with time so they don't clatter when out of adjustment. Check each glow plug too. Should be .5 ohms. Check resistance at the glow plug relay connector and that way you also check the wiring.
Try waiting about 10 seconds after the glow plug light goes off to crank. Once started, get RPM up to 1200 or so and maintain for 15-20 seconds. If you are not using synthetic oil that will affect cranking RPM. Slow crank speed equals hard start too.
Dick
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Well, if it started last winter and won't start this winter either you've lost compression badly in one year (unlikely if you're a careful owner) or something else is wrong.
First, plug in or get a block heater and you won't have to worry about it ever again. If you're away from any place to plug it in for more than 5 hours, take the car for a 5 minute drive every 4 hours. End of problem.
If you havn't switched to full synthetic oil yet do so immediatley. It is the single most effective thing you can do to help winter starting. If you're in a very cold place (like *here* -40 last month and at that temp C and F are the same) then you'd use 0W40 which Mobil makes; Petrocan also makes a semi synthetic 0W30 that works as well. Use 10W30 Mobil 1 if your colds aren't this extreme.
Adjust your valves. This is fairly critical.
Hows the chain stretch? You can read this off when doing your valves. If it's off more than 5 degrees replace the chain and your injection pump timing will be correct again.
I think repacing one bad glow plug is a bad idea. The other ones are pretty weak now and here's the scoop on these: BOSCH are OEM, BERU are just as good, Monarch are cheaper and dno't last as long... BUT they burn much hotter and if I had this problem I'd put a new set of Monarchs in every year. Don't forget to ream the prechambers when yo uchange the glow plugs.
Make sure all the filters are clean.
If you do all this then yo ushoulc be able to start it down to 0F, and while I assume you know the cold weather starting procedure I'll sell if out anyway: glow it util you hear the realy go *THUNK* then glow it again and again. After you've glower it 3 times crank it until it starts or 90 seconds which ever comes first. You only get one chance, just crank it and crank it. The starter is utterly massive and while this would burn out say, a Toyota starter I've cranked thes ehtings for 3 minutes on 5 minutes rest then back on for 7 cycles and mine wasn't even fazed (I had 1 working glow plug and started it in -40 with a boost this way 3 years ago)
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(Re my 85 300SD) The car seems to be starting fine after putting in a new battery, and I found a tiny leak on the return fuel line between #2 and #3 injectors and temp-fixed that. Maybe that was where air was getting into the fuel system. I ordered new return fuel line from THEBENZBIN.COM so I can replace all of it as it is brittle from age. Hard to believe the mechanic missed that, but he replaced the one remaining bad glo plug (#3) which I couldn't replace as it was stripped, and so was working in that area--- maybe he jostled the line around enough to induce a crack. Anyway the question is: I have not noticed any reduced performance like poor acceleration in the two+ years I have had it, seems as good as ever. I am thinking that if compression is down significantly, then the acceleration performance/horsepower would be down also, is that correct? Makes sense....more compression, more horsepower to some extent, right? And I noted from your response to a Mercedes buyer that your opinion is that at 250K, the car is probably ready for engine or tranny overhaul. Gosh....I have been repeatedly told that MB diesels are just getting broke in at 200K....you can get 400 K out of one if it is well-maintained. SO was I being given "a line"? That was the whole reason I bought a diesel....the longevity of the engine as opposed to a gas engine! Regards, Jim

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