Engine doesn't stay primed

[Hydra]

I am having trouble with my engine (Nanni 2.50HE): the fuel line doesn't stay primed. When I pump fuel manually to fill the filter with the bleed screw removed, I see the level rising in the filter holder. But as soon as I stop pumping, the level goes down by at least 1cm.

I manage to bleed the lines from the injection pump to the injectors by cranking the engine with the starter. It eventually starts, at higher RPM than usual, then cuts immediately. I have to crank it 2 or 3 more times, then it runs OK. But if stop the engine and I check the fuel level in the filter holder, it is already down.

All this happened after running the engine with the tank vent clogged.

The tank is lower than the engine. I have checked the fuel lines from the tank to the lift pump, from this pump to the filter and from the filter to the injection pum: there is no leak. The lift pump is OK too, no leak there. There is obviously no leak on the HP lines from the injection pump to the injectors: the engine would not start.

Then, I suppose there is a leak in the injection pump. But where does the fuel go? I see 2 possibilities: in the engine oil sump or back to the tank, through the return line. There is no obvious rise in the oil level. I don't dare to remove the injection pump from the engine.

What could I do to find a lead to the solution?

Alain

[undercutter]

Suggest that you try to run the engine directly from a jerry can. Disconnect the flexible fuel line ahead of the lift pump and place the hose directly into the jerry can. Prime the engine with the lift pump hand actuator and start the engine. The return fuel will still flow back to the fuel tanks but a 5 gallon can should have sufficient volume to run for quite a while. Let engine run for 5 minutes or so and then shut down. Let sit for a couple minutes and then restart engine. If the engine starts then your problem is ahead of the lift pump. If it does not, then your problem is from the lift pump onward. ie. lift pump, engine mounted filtres, engine fuel lines or injector pump.

The diaphragm on a lift pump can be damaged due to a blocked vent on the fuel tank and may not necessarily result in fuel leaking into the sump.

[Snowpetrel]

I had a similar problem with my nanni. It slowly drained back over a few days and got air in the fuel line. I solved it by putting a valve in the return line from the injector pump.

I suspect it was slowly draining back though this line somehow and letting air into the system.

[undercutter]

Originally Posted by Snowpetrel

I had a similar problem with my nanni. It slowly drained back over a few days and got air in the fuel line. I solved it by putting a valve in the return line from the injector pump.

I suspect it was slowly draining back though this line somehow and letting air into the system.

Then you have a problem with the check ball at the return fitting on the injector pump. Usually just a bit of debris which is preventing the ball from seating properly however it could be that the seat is damaged slightly. Easy enough to clean or replace the check valve itself. It is a good idea to investigate further as you may find metal filings which would indicate internal damage to the injector pump. There is also usually a screen on the pressure side fitting from the fuel filtres which can become plugged.

In either case the last thing you need is the injector pump to fail when it is critical.

[twinboat]

You say there is no leak in the line from the tank.

If you don't see fuel leaking, there still could be an air leak.

The air will leak IN and the fuel will drain back into the tank.

With a plugged vent, you put the fuel line under vacuum. That could cause a suction leak as I described above.

Re-check every connection from the tank to the pump.

If it worked fine before the blocked vent issue, you don't need to add check valves or anything else to a system that worked. You just need to find the problem.

[undercutter]

Originally Posted by twinboat

You say there is no leak in the line from the tank.

If you don't see fuel leaking, there still could be an air leak.

The air will leak IN and the fuel will drain back into the tank.

With a plugged vent, you put the fuel line under vacuum. That could cause a suction leak as I described above.

Re-check every connection from the tank to the pump.

If it worked fine before the blocked vent issue, you don't need to add check valves or anything else to a system that worked. You just need to find the problem.

I concur. You just need to isolate the problem to determine if it is at the engine or in the supply lines, pre filtre, etc.

Is it the prefiltre that is draining down or the primary/secondary filtres on the engine itself?

[Snowpetrel]

Originally Posted by undercutter

Then you have a problem with the check ball at the return fitting on the injector pump. Usually just a bit of debris which is preventing the ball from seating properly however it could be that the seat is damaged slightly. Easy enough to clean or replace the check valve itself. It is a good idea to investigate further as you may find metal filings which would indicate internal damage to the injector pump. There is also usually a screen on the pressure side fitting from the fuel filtres which can become plugged.

In either case the last thing you need is the injector pump to fail when it is critical.

Yes, thanks, I suspect the check ball is playing up. It doesnt have the normal pressure sounding squirt when it releases as I manually pump the fuel pump. Its on the low presure side of the injector pump, tee'd of the fuel supply line not off the main body of the injector pump, so it shouldn't effect it in any way. The base engine is a kubota and the manual for it shows only a bleed screw at this location, with a conventional injector return line to the tank.

So I am thinking the return line there is not really even needed except that nanni run the injector return line back into the fuel filter not the tank, so this might be there way of bleeding small amounts of air out of the system?

Certainly the valve has now isolated the problem for me, and now i've just got to strip it down and see what I find.

[Snowpetrel]

Originally Posted by twinboat

You say there is no leak in the line from the tank.

If you don't see fuel leaking, there still could be an air leak.

The air will leak IN and the fuel will drain back into the tank.

What happens if you close the main fuel valve? Does it still get air in the lines. If not, then you know its draining back into the tank as described above. If air still leaks in with the valve closed then its something else.

[unclemack]

If your tank vent is still blocked loosening/removing the tank filler cap will prove that point. What blocked the vent anyway - and how long is the vent hose and are there any tight turns/kinks?

Obviously check every connector for tightness, they don't always leak fuel when they leak air - but they don't usually drain down as quickly as you describe, usually it's overnight.

Don't forget to check the fuel filter housing seals.

If you have flexible fuel hose temporarily replacing it with transparent hose can let you see any bubbles.
Occasionally an inner layer of a hose can detach from the outer layer. It can block under suction while the hose still looks good from the outside.

If it's draining down as fast as you say you could clamp a hose with mole grips - or add a ball valve with hose tails - to see if it still happens and narrow down the location of the problem that way.

Lift pump itself could be the issue - if it's old I might just replace it anyway. Mechanical or electric, they're both fairly cheap. Some people add a Facet type electric pump to the system as backup.

Watch your oil level like a hawk.

[Hydra]

Many thanks to all for the replies.

@undercutter & unclemack
The lift pump works OK. I have made tests with a borrowed pump of exactly the same model and I have had exactly the same problem.

@snowpetrel
Yes, I checked: even with the main fuel valve closed, the fuel drains fast (a few seconds) from the engine-mounted filter.

@twinboat
Of course, the vent is now cleared. I replaced the old line between the "coarse" filter and the lift pump. The rubber hoses between the lift pump and the injection pump are fairly recent, in good condition, and the connections are tight. The intake in the injection pump is lower than the exit on the engine-mounted filter. The hose between them is always full of fuel, so there cannot be an air leak there.

Yes, the engine worked fine before the blocked vent issue. The present problem obviously results from having run the engine with the vent blocked. I don't want to add a hand-operated valve because it would then take too long to start the engine.

I suspect the leak is on the return lines to the tank: there is a line from one injector to the injection pump, another from the second injector to the tank and a cross-connection between the injectors. So, it would be possible that the fuel drains from the injection pump, if a valve has failed there.

I am not on the boat now but this gives me an idea for an experiment tomorrow: I will disconnect the return line from the tank and hold its end higher than the engine. If the fuel stays in the filter, this means that the leak is on this side.

Alain

[twinboat]

Even if the fuel drains out thru the injector return lines, the air has to be entering from somewhere, to let it flow, yes ?

The only place air can enter a closed fuel system is at the end of the return line. Everwhere else should be air tight. Plug the return line at the tank and see if the fuel goes away.

[undercutter]

Originally Posted by twinboat

Even if the fuel drains out thru the injector return lines, the air has to be entering from somewhere, to let it flow, yes ?

The only place air can enter a closed fuel system is at the end of the return line. Everwhere else should be air tight. Plug the return line at the tank and see if the fuel goes away.

If the tank is reasonably full then the end of the return line should be below the fuel level in the tank anyway. In such a circumstance no air would be sucked into the system.

I think Hydra stated that his tank was below the engine level so gravity alone would cause any fuel in the return lines above the tank level to flow back to the tank even without the engine running. This should not be a problem.

Seems to me that the issue here is that the injector pump is losing its prime. the only way this can happen is if the check balls on the inlet or outlet sides of the injection pump are not seating properly. Even if the lift pump loses its prime and the filtres are empty my engine will start as long as the injector pump lines are not cracked. It won't run for long and it is not healthy to turn the injector pump without fuel supply as the fuel is also the lubrication for the pump.

Check the check valves at the injector pump. On the pressure side there should also be a screen (not always but 99%) which should be inspected and cleaned on a regular basis (annually) as this is the last line of defense for the pump. If you are losing the prime st the injector pump then this is the only location that can be causing the problem!!!!!!!!!!!!!!!!!!

[Hydra]

I did as I intended: I disconnected the return line from the tank and held it higher than the filter on the engine. As expected, the fuel stayed in the filter.

This means that a valve is defective on the injection pump. But I have not been able to find this valve. It should be apparent on the connection to the return line from the injectors.

Alain

[barnakiel]

Be very careful opening the pump. Many times the body is alloy and easy to nick. I did have to fix ours with epoxy after I fumbled too much trying to open the top cap in darkness.

A repair is very easy. The pump can often stay on the block.

b.

[undercutter]

Originally Posted by barnakiel

Be very careful opening the pump. Many times the body is alloy and easy to nick. I did have to fix ours with epoxy after I fumbled too much trying to open the top cap in darkness.

A repair is very easy. The pump can often stay on the block.

b.

The check valves should be in the supply and return ports of the injector pump. They may look like adaptors. The screen should be in the seat of the supply port below the check valve. You should never have to dismount the injector pump to service these parts.

Never dismount this pump without putting the engine in the timing position first.