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The vehicle is a Mercedes-Benz T1 model 308D.

The engine is an old Mercedes-Benz OM601.940 4-cylinder indirect injection 58 kW (79 hp) 2299 dm3 diesel engine from year 1992. It's not a turbodiesel! - it's a classic plain naturally-aspirated diesel engine (neither turbocharged nor supercharged).

It never had a problem starting before, always started in seconds, and one day suddenly it won't start. This was April, and the previous time the vehicle was driven was December. So I try to start it in April, with the battery fully freshly charged, and it won't start, the starter is turning the engine like crazy, the engine makes sounds like it'd almost start, and then it doesn't. After many times trying to start, turning the engine with the starter for probably something like 5 minutes in total or more (this only works if the battery is fully freshly charged), it finally started, and once started it run fine without any issues.

After a few days I wanted to do another trip and tried to start it - same problem. This time I asked someone to pull me with a rope, we drove with me being pulled on a rope a few circles and it started.

Then I didn't use the car till July and wanted to start it a few times without success, again on one very hot day I finally succeeded after running the engine with the starter for several minutes. Then I made a test: I drove around for 1 hour so the engine became warm, then turned off the engine, and then I could start instantly without any issues. I then even turned it off again and left the engine off for half an hour - and even the it started instantly (the engine was still a bit warm). But if I left it off overnight and let it cool down completely - it won't start.

I thought its the glow plus, so I changed one, but then I thought of measuring resistance and all the remaining three had about half an ohm, so they're good as new, so didn't bother to change the rest. Also measured resistance of the lines from the glow plug relay and voltage getting there, and its all fine.

So then I thought its the starter, so I bought a new one, replaced it... and its all the same, still won't start.

What is the problem then?

What tests should I do to diagnose this?

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I have exactly the same problem, word-by-word. I don't think its the diesel "geling" because my other diesel Merc (same model) starts instantenously after 6 months winter garaging. It must be a vacuum issue in the fuel-valve or fuel-line? - (I believe vacuum opens and closes the fuel-valve) - because if I manage to start the engine, then it doesn't want to shut off! –  user2732 Feb 7 '13 at 7:11
    
having the same problem with a '64 190D. Tough on the cold start. Once started great all day though. Did you replace your fuel lines and have success fixing the problem? –  user3484 Jul 29 '13 at 3:36
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6 Answers 6

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You might have a slight leak in the fuel line this will draw air over time and cause you to have to crank till all the air is out .Then it starts fine .Difficult to locate those fine leaks .The fuel runs back in the tank

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This is what I believe now is the most likely cause, given these observations: the motor starts instantly even after not running for many days if the fuel tank is full. If I leave the tank with less fuel, then after a day or two the first time I start I have to run the starter for 1-2 minutes. –  miernik Feb 7 '13 at 11:03
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Four months went by between when you last ran the vehicle (December) and when you tried to start it (April).

Just like gasoline, diesel fuel can also "gel" and clog up your fuel filters. As a first, low-cost troubleshooting step, change both of your fuel filters. You can use some starting fluid to try and start the vehicle after changing the filters.

If this doesn't work, you may need to clean the fuel tank out by draining it and replacing it with fresh fuel. If it's low enough, you may be able to just add some fresh fuel to the mix.

If you still aren't able to start the vehicle, it's possible you've clogged the injectors. These may need to be replaced or cleaned. (I've heard this is more expensive; hence, this is further down the list of troubleshooting steps).

Lastly, be mindful of your starter and that you don't burn it out by cranking on it too hard. In my experience, starters for these cars aren't cheap, so good prevention is to use Sta-bil Diesel Stabilizer, which helps prevent sludge and other buildup that will make it hard to start your diesel.

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thaks for all info, I will try that. I also read elsewhere that heating up the fuel pump and/or injectors with a hot-air blower can help, and it makes sense as the vehicle starts without problem till the engine is still warm. Does it make sense? Then I could use that method to start it until I use up all the old fuel (unfortunately I have 3/4 of the tank full), and then change the fuel filters, as if I change them before I get rid of the old fuel, they'll just become clogged again, no? P.S. I already just (needlessly) changed the starter for 100 EUR so now I have two good ones. –  miernik Jul 30 '12 at 7:57
    
If you can run the old fuel out of it, it makes sense that it should help. Good luck! :) –  jmort253 Jul 30 '12 at 8:06
    
I mean does it makes sense to heat up the fuel pump and/or injectors with some portable hot thing to be able to start the vehicle that way until I run out of fuel. That way I don't waste 3/4 tank of fuel which is worth half of the price of the starter. –  miernik Jul 30 '12 at 8:12
    
You could probably change the fuel filters with the 3/4 tank of fuel, fill it up the rest of the way with clean diesel and the Sta-bil and go from there. Your goal should be to do the minimum to get it started, then you should have a clearer picture on whether or not more steps need to be done. In other words, don't do all 4 steps at once. Do 1, try to start, if no luck, move forward with step 2. If that doesn't work, move to step 3. Hope this helps. –  jmort253 Jul 30 '12 at 8:17
    
To clarify, I don't know about the "heating up the fuel pump" trick. BTW, I forgot to mention starting fluid. –  jmort253 Jul 30 '12 at 8:19
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I like the fuel idea; cheap and easy fix, but also think about the fuel pump/return. If your pump is going bad you'll have low pressure which causes a hard start. Also if the fuel is allowed to flow back into the tank you'll have an issue.

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I can't imagine fuel going bad in only 4 months. My summer cars (granted, none diesel) regularly spend 6 months in winter with a full tank (to prevent tank corrosion) and come back alive in april with the same tank of fuel.

It might help to check the engine compression. Diesel engines don't have spark plugs and ignite fuel with the help of very high compression. An old diesel engine having cold start problems usually has to do with engine compression issues.

But as jmort said, first make sure that the fuel filters are not clogged up.

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How about buying an electric fuel heater which heats the fuel before it gets to the injection pump? There are various on ebay intended for running vegetable oil instead of diesel and are easy to install, though I would seek advice on which would be the best for your application. They're not cheap but would still be cheaper than replacing your starter motor which you are caning. Running a starter moter for two minutes is not good for it and it will give up eventually. Hope this helps and isn't too late.

Guy

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The original poster of this question said he replaced one glow plug, but didn't say whether he checked if the glow plugs were actually working. A diesel suddenly becoming hard to start, only starting on warmer days, and/or after long cranking times, are all consistent with the glow plugs not operating at all.

I'd suggest looking at the glow plug relay or controller. I expect you'll find they aren't turning on at all.

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I am the original poster, and now I know its nothing to do with glow plugs or glow plug relay. If the fuel tank is full, it starts instantly even after sitting many days in the cold. And if I leave little fuel in the tank, it takes 2 minutes cranking to start even when kept in a heated garage. The observation of starting on "warm days only" was because on warm days the old battery had enough juice to keep the starter running long enough, now I have a new battery and can start it even in cold winter, with little fuel in the tank if I keep the starter running for 2 minutes. –  miernik Feb 7 '13 at 18:31
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