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Old 10-30-2000, 01:53 PM
John Hamilton
Posts: n/a
I currently have an intermittent starting problem with my 1994 C250 diesel (in case this model isn't available in America it's a non turbo 5 cylinder engine).

The problem is -

For the past 3 weeks, and only first thing in the morning, and about twice a week, it will start but immediately stop. It then takes about 20 seconds of cranking for it to start to splutter into life, then go smoothly. It's only in the morning as it's never done this when I leave from work. It sounds very much like air, as it's the exact same symptoms I'm getting when I changed its fuel filter about a year ago. If it was its pre-heaters I would have thought it would be lumpy from first turn of the key, then go smooth, unless the majority of my pre-heaters are shot!

Since the fuel pipes going into and out of the fuel filter assembly are clear I can see about an inch long air gap in the fuel-in pipe (the pipe from the fuel tank pump to the fuel filter assembly). Do you know if this is normal, or is this a clue? This air gap has been there for months and months, but is it possibly playing catch-up? I park it at night nose up on a slight hill; At work the car is parked on the level, if this helps.

I've been in correspondence with another C250 owner who's had the exact same symptoms. They were advised to change the pre-heaters, but this didn't make the slightest difference. They then took the car into their local MB garage who connected up to the car's 'black box' which indicated it had 3 failed starting problems logged in the computer. These registered faults were cleared from the computer's memory and the owner hasn't had any problems since. I smell a red herring with this as my car's diesel pump looks pretty mechanical. The only wires I can see coming from it are for the fuel solenoid. Also, surely if it was a computer problem the car would be more consistent with its starting problem, rather than the current lottery.

There are no manuals apart from MB authored ones (that will cost the earth) so it's down to intuition or advice from other owners. The small MB owners' manual I got with the car says the diesel system is self bleeding with about a minute's worth of turning the starter key. I feel pity on the starter motor for doing it this way, or can you advise if the starter motor is made of stronger stuff than what I credit it? If it is just air ingress bleeding my dad reckons cars with in-line diesel pumps should have a bleed screw, but access on the Merc is limited with the inlet manifold getting in the way. Do you know of a bleed screw or would you recommend bleeding the car by loosening the metal high pressure pipes?

Any advice?

Apologies if this mail is too long winded.

John Hamilton
Great Britain

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Old 10-30-2000, 05:48 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
We have only the non-turbo 6cylinder version of this engine, but the fuel system is simular. On all of the plastic fuel supply lines there are o-rings that can leak air in w/o leaking fuel out. Try parking with the rear of car higher than the engine & see what happens. Replacement of the o-rings may solve your problem.
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Old 10-31-2000, 05:22 PM
John Hamilton
Posts: n/a
Response to Marshall's questions,

(Apologies if this isn't the way to reply, but I couldn't see any other way using the mail digests)

I've asked the question before about the difference in the 190 2.5l diesel and the engine in my C Class. The answer that came back was it was much the same except mine has 4 valves per cylinder. I asked this question as I can get a 3rd party service manual for the 190 series here in Britain, but not for the C Class (yet).

I can't say if there's a link between the problem and the level in my tank, but I'll now take note. My drive is on a very slight hill, probably 5 - 15 degrees, but nearer the 5 degrees, and I park it nose up. Until I can get a clear picture on where the problem lies I'm currently leaving it parked in the garage or nose down. It hasn't happened since last Thursday, although it does get used at the weekend - we use the wife's car then.

I've been told by a friend who owns and runs a general car repair garage that his money is on the pre-heaters. I'd argue it can't be this because it sounds totally dead until about 20 seconds of cranking. If 1 or 2 of the heaters were faulty I would expect it to be lumpy since the cylinders that were OK would be firing. Any comments on this?

I also suspect it's air since the symptons are exactly what I experienced when I changed the fuel filter last Christmas. While I'm on this subject can you explain to me what's wrong with this logic - When changing the fuel filter, or removing the filter's retaining bolt, I would have thought the diesel that's in the remaining pipes, and pump, would have been sufficient to start the engine. Once the engine is running with this 'trapped' fuel it would keep the engine going long enough for the fuel-tank pump to replenish the removed pipes and filter. All I get is instant non-starting until it's all been primed again! Why does the fuel in the pipes and pump not allow it to even run for a few seconds?

Another correspondant to this message has suggested the rubber O-rings on end of the fuel pipe junctions could be allowing air ingress. I haven't checked these, but I suspect it'll be difficult with the naked eye to see any minor wear and tear. Changing them would be a preventative measure.

I've seen another possible clue when I checked the cavity where the fuel goes into the filter assembly. The pipe (from the fuel-tank pump) goes into a gauze (spelling?) filter. This cavity was bone dry when I opened it up. I would have thought it would have been brimming with diesel, unless internal fluid pressures drained it when I opened it up.

I think I've rabbited on for long enough, but I hope this answers all your questions.

John Hamilton
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Old 12-31-2008, 04:44 AM
Registered User
Join Date: Dec 2008
Posts: 1
Cool 1999 c250 turbodesiel fuel problem

I also have been having this problem all week, the car was looked at by an insependant mechanic over the last two days. Heater plugs and fuel filters have been checked and cleaned. The same problem keeps reocuribg.
Last night i was told that one of the fuel tanks was full the other was bone dry , to try filling the car to the brim and it started first turn this morning. I think the problem relates to the cross pumping system between tanks, there must be some kind of sensor feedback to the computer , it may pump all fuel back when swiched off because it isnt a problem when the car is running. Hope tyhis makes some sort of sence to someone.
Cheers, happy new year from scotland
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Old 12-31-2008, 09:17 AM
Registered User
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
You will have better luck over at the Diesel section of the Forum..

A Dalton
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