w124 diesel smell/not starting

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Active Member
Aug 26, 2008
'96 W124 300D

The car has smelt of diesel for a few days and then on Saturday wouldn't start. Well it sort of started but never really ran properly and then stopped. For the next 5minutes or so it just wouldn't go. I had a look around and nothing obvious so concluded it would be a fuel blockage somewhere and to persevere. As such after leaving it for a minute or two and trying again it did start and it's been fine since.

Arrived at work this morning and openend the bonnet and one of our technicians came out and said "that smells of diesel". You could immediately see a small puddle (red bit in the picture) but other than that nothing obvious.


Are they related? Any thoughts anyone?
That's the leak off return line, which is probably perished. It isn't the correct pipe anyway so buy 2 metres of decent pipe and and injector blank and replace it all.
In addition if you look at the main filter return pipe the end looks split. that could be leaking in air when the engine is off, although that should purge if the main feed lines are sound.
I agree with DMs assessment, and can only add that the clip for the high pressure injector pipes is missing - this probably means that the other such clips under the manifold are also either broken or missing.

Fatigue failure, brought on by vibration, will be the result, and, these hard pipes aren't cheap to replace when they begin to leak - certainly not cheap when compared to the cost of the plastic clips at any rate! (Edit: about £30 each)

Not only the type of leak off pipe, but also its routing is a bit odd - it looks like someone left it a bit too long, and it has been rubbing on the black plastic engine cover, and possibly also on the nut/bolt end for the poly-vee belt spring anchor.
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its sucking in air due to a leak - very similar to the problems i have had shortly after running on SVO (straight veg oil).

replace the leak off pipe (you might be able to do this carefully by not removing the manifold, but as others have said, the clips are also missing - i'd be tempted to set some time aside and remove the manifold and clean it up and replace the clips and pipes - if you do it yourself, no more than about £10 worth.
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Yes, to get the leak off pipes done, you need to remove the black plastic engine cover. To remove the plastic engine cover, you need to take off the crossover pipe. Once the cross over pipe is off, it's only about a dozen easy bolts, and the whole manifold is off.

You'll need more than 1m of leak off pipe, but, less than 2. As it's usually sold in metre lengths, you'll probably end up with 2m anyway.

Taking the manifold off also allows you to;

- replace all the low pressure diesel pipe O rings
- replace all the injector high pressure pipe clamps
- check on the condition of the engine wiring harness (mine failed near the coolant temp sensor
- check the operation of the resonance flaps - these can get gunged up from the inside of the manifold
- check the routing and leak-tightness of the vacuum pipes (some of mine were splitting)
- change any duff glow plugs
- check the top of the injector pump for leaks - the delivery valve seals.

Removing the manifold usually entails damaging the rubber bungs for the crankcase ventilation system, the rubber goes hard, and cracks, so, it's worth having a few of those to hand before you begin.
Afternoon folks,

A colleague has just replaced the pipes and looks OK now but there is diesel in the little alcoves where the injectors are/pipe goes? I don't think it's supposed to be like this but i could be wrong....?



Could this not just be spilt diesel that happened when the pipes were removed and re-installed?

check all the leak off pipes are fitted OK and then dry the area.... see if it stays dry. If not then there is a leak somewhere.

are the bolts that hold the high pressure metal pipes tight?
Number_Cruncher - i don't know, i didn't do it, wouldn't have thought so though...

There was diesel in there before the pipes were removed but this it is normal to have some spillage apparently.... I'll clean them out next week some time and see if any excess diesel comes back.
>>wouldn't have thought so though...

It's the leak off pipes which need 2m of pipe to be bought - 1m just isn't quite enough (however, if you've changed the leak off pipe between No1, and the fuel filter, 1m may be enough for the pipes between injectors). There's also the bung in No 6. The pipe costs very little, and old leak off pipe is notoriously leaky.

As I've written a few times on this site, it's best to get all the cheap consumables to hand, and fit them all at once, while the manifold is off. Leak off pipes, leak off bung, crankcase breathers, injector pipe clamps, low pressure fuel lines & o rings, perished vacuum pipes,... the lot!
Apparently i didn't have a bung, but all leak-off pipes have been replaced.

I don't doubt doing everything at once would be the best but since i wasn't doing it and he did the lot in just over 40mins i didn't like to push it.

I'll clean up the excess diesel next week, then check again a week later. Providing that is OK then i'll leave it alone.

Make a new bung from a bit of leak off pipe and the pin out of the old bung.
it would be wise to take a look at the crankcase breathers to check that they are still intact.... if reasonably old they will break when the manifold was removed but it could quite easily be bodged back together afterwards. the parts need to be got from a merc dealer (as far as i know) so unless the mechanic went to a dealer or carries these in stock (i doubt) then i reckon they would get re-used, even though they may be past their best.

just a thought. no idea who did the work so i am sure he would of done this

the parts come to a couple of pounds in total so no need to re-use old parts.
Apparently i didn't have a bung, but all leak-off pipes have been replaced.

where did the last leak off pipe go then if there was no bung? or was it just empty?

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