c250 cold start trouble

jbond1978

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Hi

Suddenly my 1997 C250TD has developed starting problems after some time (several hours) in the cold weather. The longer it stands there, the worse it becomes. When hot, it starts perfectly.

I turn the key and it cranks over for around 5-10 times and dies - I have to do this a couple of times before it very slowly begins to work. Then idles roughly for a couple of seconds and everything is okay. It is almost as when I start without allowing time for the diesel pre-heating.

Checked the battery, and that is okay at around 12.55V.

I see two small air bubbles in the clear fuel filter hoses, but could such small bubbles really cause this?

Before this began, it always started at once and idling perfectly when turning the key. Just had minor problems with a bit of rough idling when slowing down to almost 0 in congestion lines, and a bit of jumping in rpm at around 80mph.

/Jens
 
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Cole@MBS

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If you can see air bubbles then i would suspect the seals on the pump have started to let air by, MB tool needed to replace them,

As merc convert, could be as simple as heater plugs starting to die!!
 

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Wouldn't the glow plugs throw up a warning light? How about the fuel filters?
 

turbopete

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Wouldn't the glow plugs throw up a warning light? How about the fuel filters?

usually but ive heard they dont if they are a dead short though. uneven idling sounds like air in the fuel to me. when cold starting does it sort of splutter like its trying to start every now and again, without it starting? if so its almost certain to be the air in the fuel lines
 

turbopete

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If you can see air bubbles then i would suspect the seals on the pump have started to let air by, MB tool needed to replace them,

can they be done without pump removal? how big a job are they to do?
 

Cole@MBS

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can they be done without pump removal? how big a job are they to do?

Not a bad job Pete, 1 clamp holds down 2 of the outlets, so you have to do the lot to be sure, as i said, you need the MB tool to undo them of the pump!
 

turbopete

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well if i find my pump seal are shot, there may be a job coming your way, if i cant do it myself! depends what the specialist says if/when i find out if the pump is at the root of my problems! i kinda have to sort it now. spent too much time/money on it now!
 
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jbond1978

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Good replies... thanks!

I'm now suspicious about pre-heating as well as air in fuel.

The pre-heating because I have done some experimenting - trying to give the pre-heating longer time, and doing it a couple of times before cranking the key to ignition. This morning it worked! But I will have to repeat the test once it gets colder. How should the voltage drop behave merc-convert?

The air in fuel I suspect because I had the sputtering at those troublesome cold start-ups, and, as I wrote, I have the sputtering in lines approaching 0mph (sometimes) and at 80mph on highway (actually it's very specifically 80mph).

I guess trouble start to appear when approaching the 370.000miles limit ;)
 
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turbopete

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the spluttering at 80mph makes me think its a tank strainer or a pre filter thats partially blocked and the engine is starved of fuel. the battery voltage with a voltmeter should be around 12.5 volts. when the ignition is switched on to pre heat, this should drop to around 11 volts. also, i cant remember on my c250 turbodiesel, but on my E300 turbodiesel, theres a plug on the heater plug relay. disconnect the plug and each wire is numbered (1-6 in my case, it will be 1-5 in yours) and carry out a resistance test, measure the ohms from a wire (whichever you choose to use first) to a good earth. there should be very little resistance. a high reading or open circuit indicates a fault on whichever number heater plug/wire your testing!
 
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jbond1978

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Today it happened!!! Pre-heating warning light is on.

Now, can I do this repair myself? - off I go to look in the Haynes book.
 

turbopete

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not too hard on a C250 turbodiesel. not an inlet manifold off job, unlike almost everything you touch on an E300 turbodiesel!:rolleyes::rolleyes::rolleyes::rolleyes:
 
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jbond1978

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For me it is an inlet manifold off job, as it is very impossible to reach at least two glow plugs on my engine (1997 or 98 W202.128) :(
 

Trevor Jones

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Cold Start with Deisel in older engines is probably due to fuel draining back from the fuel line to the tank overnight by gravity - the tank is lower than the fuel line. You can test this by parking overnight on an hill with the engine lower than the fuel tank. Gravity will keep the fuel in the fuel line and you should be able to start O.K. If this works, Mercedes now does a kit to fit to the fuel line preventing gravity emptying the fuel line. I am not sure how difficult or not it is to fit; you will have to enquire.

The Glow Plugs could be a problem, but if the glow plug warning light goes out and stays out, the plugs are probably not a problem - if the warning light goes out and comes back on after starting, it is one or more of the glow plugs that need changing - do the lot, not difficult.
 
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jbond1978

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Trevor: The warning light goes out and comes back on for approximately the afterheat period of 180 seconds. So I guess it's a glow plug issue.
 

turbopete

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For me it is an inlet manifold off job, as it is very impossible to reach at least two glow plugs on my engine (1997 or 98 W202.128) :(

im sure i managed to change mine without removing the manifold. i certainly did on my 220 diesel, and i think i threaded my arm through the gaps in the manifold on my 250 TD. then again, i do have very thin arms! theres no gaps in the manifold on my 300TD hence the manifold must come off. for virtually every job you do! ive had the car about 15 months and the manifold is coming off for the 7th time tomorrow, weather permitting! reason? air in fuel/poor hot starting checks!
 

Trevor Jones

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To change the Glow Plugs without taking off the manifold: purchase a thin, long, extension bar for the plug socket (Halfords do them). If you have not already got a socket, purchase one (Halfords) and ensure that it is the correct one: the socket must fit over the Glow Plug and be deep enough to locate the hexagonals at the bottom in order to turn it. Sometimes a little penetrating oil or WD40 will help loosen the threads. When fitting the new Glow Plugs (get the right ones, from, say, Euro Car Parts - cheaper than Mercedes Dealers) the correct socket will ensure the plugs are held in place so that you can locate them in the plug holes - tighten them, but not overtight which may crack the plug bodies if you slip.

Most work on cars these days is not easy because of cramped positions, but it saves a lot of money not having to pay dealers or cowboy mechanics.
 

turbopete

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To change the Glow Plugs without taking off the manifold: purchase a thin, long, extension bar for the plug socket (Halfords do them). If you have not already got a socket, purchase one (Halfords) and ensure that it is the correct one: the socket must fit over the Glow Plug and be deep enough to locate the hexagonals at the bottom in order to turn it. Sometimes a little penetrating oil or WD40 will help loosen the threads. When fitting the new Glow Plugs (get the right ones, from, say, Euro Car Parts - cheaper than Mercedes Dealers) the correct socket will ensure the plugs are held in place so that you can locate them in the plug holes - tighten them, but not overtight which may crack the plug bodies if you slip.

Most work on cars these days is not easy because of cramped positions, but it saves a lot of money not having to pay dealers or cowboy mechanics.

couldnt have put it much better myself!
 


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