E300 diesel intermittent rough idle

[3 phase]

Hi and a belated happy new year to you.

Have a problem which baffles me. When the engine idles sometimes it´s as smooth as silk the next it seems as though it´s running on 5 or even 4. It is fine as soon as the revs increase and driving the car normally is ok as well, no problems under load etc etc. I have replaced the fuel filter. Have read about issues with fuel pipes and air leaking in, it´s the intermittent nature that confuses me. Appreciate any advice-

 

[SilverSaloon]

do you have any bubbles in the clear fuel pipes that run to the filter etc?

is it ok when warm or is it always a rough, cold and warm

what grade of fuel are you using? any additives added recently?

 

[3 phase]

No I don´t see any bubbles, it happens when hot or cold, no fuel additives, regular shell diesel. This afternoon driving through town, it was fine and smooth until I reversed into a parking space then it was rough again. On the journey home all ok until I stopped at a pedestrian crossing, it was rough again, then smooth at the next junction. Really strange.

 

[Number_Cruncher]

Check for low pressure fuel pipe leaks first. Then, take the inlet manifold off, and run the engine. There should be no smoke being pushed out of the inlet ports - inlet valve seat recession on the rearmost cylinders is common on these engines.

 

[Smiley]

NC I think that the inlet valve regression will show up when the engine is cold and then goes away when warm as the head expands a little. Mine has been like that for the last 80,000 miles and has not got any worse - it has a slight rough idle at cold for a couple of minutes then it is smooth after that. The intermittent nature of this one is a bit strange. Maybe seals on the diesel lines or even the engine loom breaking down (not sure what that would do)
Have you replaced both of the fuel filters and the o-ring with the pre filter?
Is the fuel decent quality - contaminated fuel could do this.

 

[Number_Cruncher]

Smiley, yes, I agree, it would be worse when cold. However, as the valves do rotate, intermittent good and bad seating is a possibility. Electrics is an interesting one - the loom can fail in the area around the coolant temperature sensor, and this is an input to the idle speed controller - however, this would just be an idle speed problem rather than a misfire. To eliminate this as a possible cause, the OP could temporarily disconnect the plug from the idle speed solenoid on the back of the injector pump - the idle speed would then revert to being mechanically controlled.

 

[3 phase]

The fuel is, I think, of decent quality, comes from the shell garage I always use. I did run it very low before xmas when I had gone out without my wallet and the red light was on for a good while. I then filled the tank right to the top the following day, maybe it pulled some crap through? I replaced the fuel filter at the front of the engine with new ring. Would engine wiring loom do this? surely to all intents it is mechanical when warm?
I will take all your comments on board but will be travelling to the uk this week to work at my Dad's place in Hampshire for a few weeks so it will probably have to wait til my return. I have been out this afternoon and it was absolutely fine all the way......until I parked up outside the house.

Thanks for your input much appreciated.

 

[Smiley]

Have you replaced the diesel fuel lines? The clear pipes - if they are yellow then they are old and that is where I would start - my bet is that this is an intermittent leak of air into the diesel.
Have you ever run the car on veg? The other thought would be (as you suggested) would be crud pulled through with the low tank - but the only time you get crud in there is using veg oil - I took my tank strainer out after over 200k miles and it looked the same as the new one - always been run on diesel.
As you say I doubt the loom is causing this - unless there is some interaction with the ECU somehow.
I wonder would a sticky valve cause this?

 

[Smiley]

There are two filters at the front - the small pre filter (strainer) with o-ring and the screw on main filter that is changed every 54k miles.

 

[3 phase]

No never run on veg, and no haven't changed the lines but they do look a bit yellow. I'll set to it whenever I get back from the uk.
I only run my trusty old Renault Trafic van on veg, that seems to love it.

 

[Smiley]

Those lines (or rather the o rings on the ends) could well be the problem. Well worth changing them all in one go; IIRC I think the mainfold has to come off to get to all of them. They are not too expensive. One of the lines has changed from a push fit to a threaded banjo - you simply pull out the old fitting and you will find that there is a thread in the recess to take the new line.

 

[SilverSaloon]

of you run the renault on veg, why not the Mercedes? I've done 50k on veg

 

[johnsco]

Does this diesel have a MAF ?
The E300TD does, and they can be a bit temperamental ?
The MAF can be responsible for general poor running on the E300TD

 

[swannymere]

Link to the How-To guide to replacing the seals.

http://www.MBClub.co.uk/forums/howto/46736-howto-diesel-delivery-valve-leaks.html

 

[Smiley]

Does this diesel have a MAF ?
The E300TD does, and they can be a bit temperamental ?
The MAF can be responsible for general poor running on the E300TD

nothing so sophisticated....they are gggrreeat cars

 

[3 phase]

if you run the renault on veg, why not the Mercedes? I've done 50k on veg

To be honest Derek, it's the van that does the mileage, the Merc would need some mods to the plastic pipes wouldn't it? (though it sounds like it might need that now in any case!!) and it's also the supply of the stuff, I can only just get enough locally from the restaurants and bars here to run 50/50 with normal diesel. Other than that I would run it on the stuff, plus the ambient temp down here is much more favourable for veg.

 

[3 phase]

So I am now back in Spain after some 3 and a half weeks in the uk. Car has been stood in the garage unused in that time. Started first time as always. I have driven the car each day for the past 5 days and it is running as sweet as can be, not once has it suffered any rough idle symptoms. Totally strange, but at the moment if it ain't broke .....We'll see what happens.

 

[Olivier]

This could be a case od the CPS , crank position sensor?

 

[bolide]

Is this a W124 or W210?

Nick Froome

 

[3 phase]

W124 E300 diesel OM606.