Lumpy running 2 minutes after cold start

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My 2004 E320cdi Estate has a strange problem. From cold it runs fine then after about 2 minutes the revs flick up by about 100 and I get about a minute of lumpy running then everything settles down OK. It doesn't matter if I go down the main A road where I'm doing about 55mph or down the lanes where I'm doing 30mph. This happens every day, any ideas?
 

Number_Cruncher

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Your Mercedes
1995 W124 E300D TE
The revs flicking up is when your glow plugs switch off, and the electrical load on the alternator is reduced. The glow plugs are kept active for a time, even after the dashboard lamp goes out.

What this means is that when the engine is cold, it isn't generataing enough heat to enable smooth combustion. Two possibilities spring to mind;

1) The compression in the cylinder is insufficient to produce the heat required
2) The injector spray pattern is incorrect, and the fuel is not sufficiently finely atomised.

It may also be the case that the glow plugs are turning off too quickly, before the engine is sufficiently warm - this will depend upon a signal from the coolant temperature sensor.
 
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Well, nearly six months on and the problem still persists. All the obvious parts have been replaced, gear plug, MAF etc. Latest news is water in the gearbox oil but we don't know how much long term damage has been caused to the gearbox. Strange thing is it starts the rev flicking at exactly the same point in my journey literally within a few metres, and it doesn't matter if I pull straight off the drive or I leave the engine running for a few minutes if I have been scraping ice of the windscreen. It's been to MB and nothing shows on the STAR system. The rev flicking lasts for about 5-10 minutes and is really noticeable when your foot is off the throttle when you cruise up to traffic lights. Any ideas?
 
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Bumping this up in case anybody can shed any light on it. Unusual not to get more comments on this, perhaps it's one of those problems you never get to the bottom of? New gearbox will be expensive though.
 
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Looks like this one has beaten the great and good of the forum. I get the impression gearbox problems are a bit of a black art and not easily resolved.
 

Number_Cruncher

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1995 W124 E300D TE
To move forward, you need to find out what's happening 2 minutes in to your journey that causes the revs to flick.

Earlier, I suggested the glow plugs turning off. This would be quite easy to verify using some wire and a multimeter, or possibly also via live data from Star.

Taking it to a garage when it's running OK will be very unlikely to find the fault.
 


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