MD11C Air in fuel pump

[StefanSG]

Hi I have an MD11C which I reconditioned about 2 years ago with new rings, gaskets, deglazed liners, serviced injectors valve seals etc. The engine starts readily from cold. It has developed an issue where after running for about 45 minutes the revs start hunting, for a few minutes, and then the engine stalls and won’t restart. New fuel
tank, fresh inspected fuel, 2 brand new filters. If I bleed the fuel pump with the manual lift pump, I get bubbles and air for 20 seconds, after that the engine will instantly restart, and will run fine at constant 700 rpm but then stall again after 15 minutes. Where should I start looking for the problem or leak ? All of the fuel hoses were replaced during the refit and they are proper swaged expensive ones. The oil filter has a pressure equaliser fitted on the top which I notice many MD11c’s only have a bolt. Most washers and seals are new and tight. I have bought a new set of seals for the vacuum side of the supply and will redo the seals on the CAV primary fuel filter (glass bowl type) and the bleed screw seal on the secondary filter housing. Manual lift pump appears to work fine.
Hoping someone can advise. Running out of season and very frustrated. (One thing that wasn’t reconditioned during refit was the high pressure pump so if that’s a likely culprit I can get that serviced locally)
Many thanks in advance.

 

[Metalicmike]

I just changed the hose from my primary filter with a clear re enforced hose (has a light green tint) that is suitable for diesel. With this hose I now know that no air is being drawn prior to the lift pump and am currently waiting for a replacement. I found this hose in Leroy Merlin Tenerife so it may be available in places like B&Q. It is not Garden Hose. You probably also have a faulty lift pump but can confirm that by replacing this hose.

 

[B27]

As well as the lift pump, it could be filters nearly blocked, tank breather, possibly a few other things.
You could try using an electric lift pump to check the filters are not starving the system and instead of the mech lift pump.
You could put a pressure gauge between the lift pump and the injector pump, this should show positive pressure.
Once you know the lift pump is delivering pressure, then air can only be leaking in between tank and lift pump, so you just have to check all those connections until the problem goes away.
If you use an electric lift pump for testing, you can do it without the engine running quickly see bubbles if you pump fuel into a glass jar or similar.

We once had an engine which would idle for several minutes then die, it turned out to be a blockage in the tank pick-up.
Might be worth back-flushing the pipe work and checking the tank is clean?

Another idea is to run the engine from a gravity tank to eliminate part of the system?

 

[Metalicmike]

As well as the lift pump, it could be filters nearly blocked, tank breather, possibly a few other things.
You could try using an electric lift pump to check the filters are not starving the system and instead of the mech lift pump.
You could put a pressure gauge between the lift pump and the injector pump, this should show positive pressure.
Once you know the lift pump is delivering pressure, then air can only be leaking in between tank and lift pump, so you just have to check all those connections until the problem goes away.
If you use an electric lift pump for testing, you can do it without the engine running quickly see bubbles if you pump fuel into a glass jar or similar.

We once had an engine which would idle for several minutes then die, it turned out to be a blockage in the tank pick-up.
Might be worth back-flushing the pipe work and checking the tank is clean?

Another idea is to run the engine from a gravity tank to eliminate part of the system?

I'm going to replace all the fuel hose so if I'm pulling air, I can see the bubbles.

 

[B27]

I'm going to replace all the fuel hose so if I'm pulling air, I can see the bubbles.

Not sure that there's clear hose available that is fit for this purpose?
PVC is sold as fuel resistant, but is only good for 60degC, so not ideal for an engine bay on a boat?
Also I think the 'proper' rubber stuff seals better on the fittings, I recall issues with leaking clear hoses on motorbikes in the bad old days.

I do have a clear plastic inline filter feeding my chinaspacher, it not only shows if any air is present, but traps it.
Again probably not suitable for the heat of the engine bay?
But could be temporarily fitted for diagnostic purposes?

 

[StefanSG]

I just changed the hose from my primary filter with a clear re enforced hose (has a light green tint) that is suitable for diesel. With this hose I now know that no air is being drawn prior to the lift pump and am currently waiting for a replacement. I found this hose in Leroy Merlin Tenerife so it may be available in places like B&Q. It is not Garden Hose. You probably also have a faulty lift pump but can confirm that by replacing this hose.

Thanks. I went through the vacuum part of the fuel supply and replaced every seal, reseated the CAV filter (will be buying a Racor type filter to replace it) and a new lift pump which was £50 or so from Parts4Engines. Ran for 1-1/2 hours without missing a beat. Thanks all for your suggestions.
SSG

 

[Bilgediver]

Well done sounds like the offending part is either sitting more comfortably or it has been replaced