Rough idle when cold OM606 E300 diesel W124

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3 phase

Active Member
Nov 2, 2008
Estepona, Spain
A bloody Opel
I have posted about this before but the problem has returned.
I have a cold to warm rough idle problem that disappears once the engine is hot. The rough idling feels like it's only running on 4 or 5 pots. Once above 1000 rpm it's smooth running on all 6 and pulls as well as a 300 non turbo will pull.:eek:
Over the past year I have replaced all the plastic pipes and viton rings, the delivery valves were leaking 6 months ago and they have been renewed. There are no diesel leaks and no visible signs of air bubbles in the lines. Fuel filters are new.
When the engine is hot it is smooth and running fine, but once the temp falls back to below 40 to 50C after being parked then the rough idle commences but ceases when it's hot again.
I wonder whether this can be caused by any of the vacuum operated valves on the inlet side, are these actuated at higher temperatures or could it be EGR related, the EGR is standard and connected as normal. Can anyone tell me what the valves on the inlet manifold actually do, and I honestly don't know what the heat exchanger does on the IP either.

Thanks in advance for any advice.
How many miles has it done? How much blow-by do you get at idle with the oil filler cap off?

Nick Froome
290,000 kms so around 182,000 miles, no blow back, does not use any oil it is a lovely runner when it's hot Nick, smooth and quiet, just rough only on idle until hot.
Sorry Nick do you mean blow by when cold to be honest I don't know so will do that when I start in the morning and report back.
Just started the car, then with the oil filler cap off absolutely no blow at all.

It's much warmer here this morning at 18 degrees and it isn't as bad but still not smooth, although interestingly as I manoeuvre the car the idling gets smoother then rougher as the car moves around, this is why I wonder whether the problem may be related to an air valve opening or closing incorrectly but as I don't know how they are supposed to operate I don't know where to start.
Take the inlet manifold off and watch for leaking inlet valves.
Take the inlet manifold off and watch for leaking inlet valves.

Ok, what symptoms should I look for will that be air being pushed out of the ports? Reason I ask is when we fired it up checking for fuel leaks following delivery valve resealing, without the manifold on we didn't notice anything puffing.
Manifold off and engine cold and it idles smoothly no blowing from any inlet valve ports. So now the manifold will go back on and I'll report back.
It is likely to be a poor injector, then.
It is likely to be a poor injector, then.

Had injectors checked at a diesel service specialist and they are all reported as being good and serviceable. So they're back in and still the problem is there when cold.
However, the other night the car was parked nose up next morning when it started it idled smooth!! But then after driving away and back downhill to a junction it was rough again and this continued til it was hot when it all smoothed out again.
Then it's the usual leaky fuel pipe scenario. It's O-ring time!

Nick Froome
Then it's the usual leaky fuel pipe scenario. It's O-ring time!

Nick Froome

I agree with you Nick but all the o rings have been done, new pipes as well and the delivery valves along with the rubber hoses further back under the washer bottle. What I cannot work out is the temperature issue, when it's hot it's silky smooth, when it's not the whole car shakes.:wallbash:
What changes when the engine's cold? The thermostat on the IP will adjust the rack position and some clearances will be somewhat different

Maybe it's a valve that's marginal on sealing when cold or a cracked manifold - something that cures itself when hot

I had a 606 that would run rough up to 1700 rpm then smooth out. After a £1200 top end rebuild it was returned to me with exactly the same problem... My thought was that it might have a bent rod - but why then would it run better at higher revs?

Of the couple of hundred E300 Diesels I've driven, and the thirty-odd I've sold, I've only had one engine that ran at all rough. All the high-milers have some valve seat recession and blow-by but they still run fine. They tend to be flat and gutless but they run OK

Nick Froome
My 95 e300d does the same as this. It started doing it at about 150k miles after the glow plugs were changed - not sure if that was a coincidence. Like you I replaced all the fuel lines, filters, shut off valve, eventually replaced the injectors at about 180k miles. Still did the "stumbling" when cold - bit like a misfire on a petrol car - absolutely smooth idle when warmed up a little - only "misfires" on the first start of the day after that it's fine.
I took the manifold off (again) and started it from cold - mine has blow by on one or more of the inlet valves on cylinder 5 or 6 (one of the rear ones anyway) when cold - once the head warms up a bit it expands and no more blow by.
The car is now coming up for 275k miles and it has not got any worse which has surprised me.
I did have a problem with a fractured fuel delivery pipe to one of the rear injectors at about 240k miles - the plastic studs holding it in place on the top of the head had broken - possibly due to the shaking when cold and worn out engine mounts - eventually replaced at about 220k miles.
Sorry don't have a simple solution but it may not get any worse - keep an eye on the plastic studs holding the fuel pipes in place though.

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