Fuel Lift Pump problems

[BlueSkyNick]

We have a Thornycroft T80 engine which has been playing up a bit recently.

In an attempt to resolve what I thought was a problem with either dirty fuel or an air leak somewhere, I am now unable to bleed the system and hence start the engine. There is a manual primer on the fuel lift pump which is intended to bleed air through to the screw on the injector pump. However the lift pump is dry, in that it is not drawing fuel from the tank (via the water seperator).

I removed the pipe which comes from the tank and tested the pump by operating the manual primer, but it does seem to be creating any suction.

Is there anything inside it, such as a diaphragm or whatever which could go wrong over time? Maybe a replacement pump is required? Or does the pump itself need to be primed in some way? How do I create a vacuum in the supply to the lift pump? I tried sucking on it a few times to get the syphon going, but all that has done is spoil my appetite for the evening !!

All help gratefully received.

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[bob_tyler]

I have just had a brand new engine fitted. In testing it the engine would run for about 5-10 minutes then fade and eventually stop. After much time spent cleaning the already clean fuel filter, checking all the joints of the new fuel system etc. we checked the pump by removing it from the engine.

It was brand new but there was very little movement on the cam. Got a new pump, totally different movement and works perfectly.

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[DavidGrieves]

Forgive me if you have already tried this:-

Try rotating the engine by hand via the fly wheel or starting handle, and then pump the prime lever again, the cam that drives the fuel pump might be at its highest point, so when you try to operate it there is no movement left. Also push the priming lever down as well as lifting it up.


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[Rustyknight]

I have a completely different engine, but the principle might be the same.....

The fuel lift pump is driven by a cam...... and when the engine is not running, the posistion of the cam will affect the manual operation of the pump. Try turning the engine a little, either on the starter motor or by a spanner on the flywheel, and then try the hand pump again. If it feels stiff, it's probably working.....

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[Althorne]

If you are sure that fuel is getting to the lift pump but not passed it you will find it either has a blockage or the diaphram is damaged. Failing that the spring that actuates the the diapham is weak or broken. Either way take it off, dismantle it and see whats wrong. A servic kit is available.

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[oldharry]

Could also be one of the valves in the pump is sticking open

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[Heckler]

cam drives arm which is attached to a diaphragm which has 2 valves one lets fuel in and one lets it out. arm must be on lowest part of cam so that you can move the diaphragm manually. turn engine over until when you push the primer you can feel it moving the diaphragm. this is th most common cause of not priming. second the diaphrams do rip, also the little flap valves do get kaka on the seats and stop it working. undo the 2 nuts holding it on to the engine as well s the pipes. mark al the flanges where the to pseparates from the bottom and then undo the ring of screws around it and disaasemble, you will soon see if there are any glaring probs.
stu

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[Davy_S]

I know I have a different engine than you, but had similar problem, mine was a small split in the diaphram (you could hardly see it) but it was enough. After replacing it I fitted a universal electric pump in line £35. I run the engine as normal, pump switched off, but if it ever fails again Isimply switch on leccy pump and it gets me home. It is also very handy for bleeding system, switch on and fuel is pumped straight through. Dont know how true it is, but I got told the pump should squeek when it is pumped by the lever, if not its nackered.

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