W124 300 TD Diesel engine shut off valve

[John Turner]

I have a 1995 W124 300 diesel estate (606 engine). I have diesel leaking from a small hole in the shaft of the shut off valve on top of the fuel filter on start up. The car has twice failed to start, requiring 2 batteries worth of cranking. I have received some excellent advice regarding replacement of o rings in fuel hoses, and possibly even the plastic hoses and their connectors, but am not yet convinced that these are the problem. I suspect the shut off valve is the problem, but apparently replacement is expensive (£65) and, unless seated well, may cause running on after replacement. It may be that the o rings which should be changed when the fuel filter is changed are shot, but these and the filter were only changed last year during a main dealer service. Can anyone advise whether the hole in the shut off valve shaft is some sort of bleeder hole, and fuel leaking here is indicative of another problem, or whether simply replacing the o rings will solve the problem? I obviously do not want to replace the fuel shut off valve (how can it be so expensive?), or start on the hoses and o rings if the problem has a more simple solution. Any help much appreciated (& apologies to anyone who has responded on the other place form!)
John

 

[simon_wall69]

Had this done on mine recently; the valve was leaking, left me with no option but to replace (I had replaced all the o-rings previously). It is a ridiculous price, though!

It could be that the pre-filter and its o-rings weren't changed. You may as well eliminate all the cheap alternatives first.

 

[Number_Cruncher]

>>Can anyone advise whether the hole in the shut off valve shaft is some sort of bleeder hole,

No, it isn't, it should be bone dry. There is a route for self bleeding, but that's an internal drilling inside the filter head, and nothing at all to do with the stop tap.

>>fuel leaking here is indicative of another problem

No, it just means that the tap is leaking.

>>whether simply replacing the o rings will solve the problem?

If you mean the 2 o rings lower down in the filter itself, they're cheap to buy, and easy to fit, so why not try it? (although if your description of where the fuel is leaking from is right, I doubt it)

 

[Alan42]

Hi, I had the same problem, leak from the stop control and sometimes difficult starting. As a temporary fix I bunged up the hole and the underside of the nut with an instant gasket compound. Of course, I now can't stop the engine when I'm under the bonnet, but then I can't do that on other cars either. Not all gasket compounds are suitable for diesel. The 300D is my son's, not mine, I think it's a wonderful car and that 24 valve engine is a fantastic piece of engineering.

 

[Number_Cruncher]

but then I can't do that on other cars either.

John, the OP, will have seen this before, but, I post it for Alan42

The valve is there on MB diesels for a particular reason - other cars don't *need* it. Unlike virtually all other modern engines I know of, the diesels in W123s and W124s rely on the vacuum system working properly to enable the engine to stop. Vacuum must be applied to the shut off valve to stop the engine rather than the other way round.

On non-multivalve engines, the pump is accessible enough to have a manual stop lever as well as the vacuum shut-off. OM606 engines have the pump hidden under the manifold, so, there's no access to a lever.

OM60x engines in W210s are OK, because there is the electrical stop solenoid on the side of the pump which fails safe - i.e., it stops the engine.

It's the lack of a failsafe engine stop that makes it necessary to have a backup method of stopping the engine - especially in a runaway situation (which is thankfully rare).

John - the diesel is leaking out of your stop valve because your stop valve is faulty - it's not a bleed hole, and diesel should nver come out of it. There are only 2 options open to you if you want to effect a permanent repair;

a) Buy and fit a new stop valve

b) Take your existing stop valve apart and repair it (given the cost of new one, this is what I would probably try first)

 

[John Turner]

Today I removed the shut off valve, and gave it a thorough clean, before replacing with a new fuel filter (topped up with diesel) and 2 new O rings. The upper (larger) O ring was dirty, and the piston in the valve looked contaminated too. I took the opportunity to remove the pre filter and clean this too (although it looked pretty clean - I only run on diesel and not veg oil!). On reassembly, the car started immediately, and ran without any diesel leakage. The shut off valve stops the engine when turned, so all looks OK. I will leave the car overnight, and check again tomorrow, but so far it looks as though cleaning the shut off valve has worked. I have not disassembled the tap (I understand that there may be a 3rd O ring in there, but some very tiny alan keys would be needed I think, and anyway, cleaning accessible parts seems to have stopped the leak). I have on order all of the new fuel hoses (about £26 + Vat), and so will probably do these as a precautionary measure - mine are all original and after 13 years, I think they deserve replacing before a problem occurs! If all is well over the next few days, then I will cancel the shut off valve (which is on pre-order at £55 + vat). Thanks for the guidance Number Cruncher and others - really useful advice.
John