Ural 4320 Fuel Supply problem

[Mark Preston]

I have noticed that if I leave the truck sitting for a couple of weeks, in order to get it started I have to use the hand pump to pump fuel back up to the engine. That seems to suggest to me that there is an air leak somewhere, air is getting in allowing the fuel to run back to the tank. I'm a bit stumped as to how to find the leak - however I suspect that it may be the hand pump itself as after a bit of vigorous pumping some fuel has leaked out from around the pump plunger. Fuel out when pumping, air in when not?

Anybody got any experience of this, ever taken the pump apart or got any ideas for finding a leak like this?

Cheers

Mark

[Abrs]

Is it a PAL priming pump? These are prone to leaking air around the plunger seal when priming, however when the plunger is locked down the piston at the end of the plunger should block off any fuel passageways to the priming barrel.

I suggest checking all unions along the fuel line first, if all seem ok then test the priming pump, this has valves in that should prevent fuel running back unless there's an airleak prior to the pump itself, but if this is the case it should also stop the injection pump body from emptying, actually the Pal injection pumps also normally have a small one way valve on the fuel return outlet on the pump's body, this should stop the injection pump from being siphoned, are there any leaks visible around the injection pump?

[Mark Preston]

I'm not sure of the make of the pump. I'm going up to the truck this weekend so will have a look then.

If I try and start without priming, then the engine fires pretty much immediately, but after a very very short time stops again. I'm not sure if this suggests that the fuel is running back out of the injector pump too. I must admit that I find the whole thing a bit confusing as right next to the primimg pump are the two big fuel filters and I can't see how they could become drained as both the 'in' and 'out' are at the top, so the low pressure pump has only got to pull the fuel about 6".

I have replaced all the rubber pipes in the system as they were badly perished and there are no obvious leaks.

Cheers

Mark

[Abrs]

Sounds like injection pump is staying primed then, look further back.

[Abrs]

I would say anywhere from the lift pump back to the tank, possibly the filter housings or at the lift pump itself.

[freckles]

Don't forget that filter canisters have o-rings at the canister mounting face. I KNOW for a fact that one needn't have a Russkie truck to have a broken o-ring, and they can be hard to find if one forgets they're there.

[Mark Preston]

I had a good look around and can see nothing obvious, all unions are tight with no sign of any thing leaking OUT, so I'll get a replacement priming pump and see if that improves things.

Thanks for your time in posting back to me.

Mark

[Rob Reznik]

Is there a non return valve on the fuel line?

[Mark Preston]

Looking at the drawings, there appears to be one in the primer pump - but it's not 100% clear.

[Abrs]

There should also be one on the outlet port(fuel return) of the injection pump, it's usually hidden inside the banjo union.

Do you have a schematic for the fuel system on the Ural that could be posted here?

[Mark Preston]

I do have a schematic however it doesn't mention non return valves! That makes me think that the fuel is kept 'up' by vaccumn, again pointing in the direction of a floppy priming pump... I also can't upload the relevant page as it says that the picture is too big! I'll see about hosting it myself and post a link....

[Mark Preston]

Try

http://ols10.com/jarkspeak/fuel.jpg

Thanks

Mark

[Abrs]

The non return valve on the injection pump should be located in the banjo union close to where No.8 is pointing to on the diagram, it should also act to stop fuel being siphoned off down the fuel return line, it's basically a spring loaded poppet/ball valve on most pumps. However from what you describe it sounds like this is working ok anyway, the injection pump is staying primed(bled) hence the engine runs ok for a while.

At a guess I would go for lift pump as the culprit, you can test it easily enough by disconnecting the pipe that goes to the filters (No.13), then try blowing back down the pipe, if the NR valves/piston in the lift pump are working you shouldn't be able to at all.

If the pump appears ok then you need to start looking further back at the other pipework/fittings.

I actually wouldn't suspect the main filter housings now, you'll notice from the diagram that these are after the lift pump(normal configuration on most diesels) and hence will be under pressure when the engine is operating, a leak here would be pretty obvious as fuel would be pumped out through it.

[Mark Preston]

I'll try blowing down the pipe - but as you say, that is probably the culprit. (Think I'll wait for warmer weather before wrapping my mouth around any part of the truck though).

Agree about the fine filters not being part of the problem. I was confused in an earlier post and thought the fine filters were before the LP pump.

Thanks for your time.

Mark