Help, my diesel has died.

[fivebucksa]

Can any one sugest why my old Mercedes 636 will ony run for 30 mins, then stop. I then have to manually prime it, then its ok for 30 mins.
I then bleed what seems to be a lot of air out a round thing called a knecht? I have checked all the joints, the only leak i have is a very small damp patch round one of the injectors, could it be that? i am not very good with diesels.

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[snowleopard]

can't be the injector leak. more likely to be air getting into the fuel line before the lift pump. here are a few possibilities:

blocked fuel filter or muck in the pipe

low fuel level in tank allowing pipe to suck air as fuel sloshes around

leaking union in the feed line

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[Anchorite]

Also check breather pipe which if squashed or sealed with salt crystals can
produce this sort of effect.

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[duncan]

when checking for a blocked fuel pipe go right back to the pick up in the tank

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[oldharry]

Check 1. (Most likely) Tank breather . 2: Leak in the fuel lines - particularly on the low pressure feed to the injector pump. 3: Fuel lift pump diaphragm(s) and valves.

If the system is gravity fed from the tank to the fuel lift pump, then any leaks will show up, but if the fuel is being sucked up by the lift pump, leaks will not show up as the system is under suction, and air will be leaking into the pipe, but fuel will may not leak out.

A quick way to check on the tank breather system is to release the fller cap after the engine has stopped itself. If there is suction - air whooshing in as the cap is removed - then the pump has been unable to draw any more fuel against the suction for the engine.

The slight leak round an injector is a slight compresion leak which will not affect the running, but should eventually be dealt with as it may damage the seating. Usually there is a copper washer that seals the injector base, and this needs replacing if it leaks.

If still in trouble, re- check each joint in the fuel line. Remeber that the line from the lift pump to the injector pump is also under suction each time a dollop of fuel is pumped under high pressure to the injectors, so again leaks may not be showing up. A tiny amount of fuel poured over each joint on the low pressure side will show leaks as tiny bubbles appearing at the joint when the engine is running.

What you described is fuel starvation - the fuel supply is restricted in some way and after 30 minutes there is not enough to keep things going.

You have of course changed your fuel filters this winter..... ?

Good Luck!

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[fivebucksa]

Thanks guys, as usual you lot are full really useful information, when i disconected the fuel supply pipe from the tank, there was a suction of air, as if there was a vacuum in the tank so i think the connections are ok. What about the fuel pump?.

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[paulrossall]

Before we can tell you whether there should have been a suction of air when you undid the pipe to the tank we need to know whether (1) the level of fuel in the tank is higher up than where the fuel goes into the pump on your engine or whether (2) it is lower down.
This is because we need to know if the fuel is fed to the pump by gravity (as in (1) above) or whether the fuel pump sucks the fuel up from the fuel tank (as in (2) above)
You also need to say when you disconnected the pipe at which point you disconnected it, was it where it comes out of the fuel tank or was it where it goes into the fuel pump.
Is there a fuel filter between your fuel tank and the fuel pump?
Paul

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[LadyInBed]

An alternative to -

"A quick way to check on the tank breather system is to release the fller cap after the engine has stopped itself. If there is suction - air whooshing in as the cap is removed - then the pump has been unable to draw any more fuel against the suction for the engine."

would be to run the engine with the fuel filler cap OFF. If it runs for longer than the 1/2 hour, then it will point towards the breather. If you then put the filler cap back on tight and it stops, thats conclusive proof.



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[Heckler]

you have answered your questio

the suction in the tank is a vacuum caused by a blockage in your tank breather, the lift pump sucks it to a vacuum. check all your tank breathers.
stu

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