Air in fuel system

[jesperht]

I am overhauling a Volvo Penta 2040 to my boat, and did a 1 hour test run. Before that i had to bleed the fuel system. After the test run, it stood still for half an hour, then it would not start. The fuel system needed bleeding again! I took the fuel pump out and took it apart. One of the 2 small membranes was broken, so thats it, i thought.
I am waiting for the part to arrive, but now i wonder, if this really is the problem? Because the air have to get in somewhere, and this has nothing to do with the membrane or? The leak has to be on the low pressure part, but i have checked all connections and hoses, cant find any leak.
The can with diesel is located 0,5 meters below the engine, could this cause the problem?
I have to solve this before i put it in the boat, it is much easier to fix in the workshop..

[barnakiel]

Replace the membrane. Run the test again.

I am with you on the leak being elsewhere.

Check out all hoses - older ones tend to become brittle and may let air in esp. round the clips.

b.

[jesperht]

Oh, i found the problem.. The return hose was above the diesel in the can, letting air in, when the engine was not running.

[Sailmonkey]

Originally Posted by jesperht

Oh, i found the problem.. The return hose was above the diesel in the can, letting air in, when the engine was not running.

99.9% of return lines are above the level of the fuel. If the fuel is draining all the way out the check valves in the fuel pump (low pressure) are probably leaking.

[SkiprJohn]

There are many ways air can get in the system but the most common are loose drain nuts and compressed washers. Fuel filter gaskets are sometimes a problem. There could be an unnoticed crack in a metal fuel line. I think one of the best ways to trace it is to install a hand or electric pump just after your tank (fuel can) and apply some pressure. Notice where in your system there appears a wet fuel spot and you've found the source of your leak.

I had a friend with an AD2 that had the very same problem you have and it was the nut on the bottom of his fuel filter that was letting in just a little bit of air and after not running for about an hour, needed to be bled again.

After your engine install. Don't use the starter to bleed your engine unless you close your cooling water intake first. You can flood your engine with water if it doesn't fire right away.

[Hoofsmit]

Just wanted to share this In case it may help someone.
I fitted a Kad 32 in my Sealine sports cruiser back in 2001 , all was great until I put her under load on the plain for a few minutes and then she would die and not restart, the pre filter had air in it bled everything then off we would. Go for 5 mins and dead again .... More air, after putting a direct feed of rubber hose to the pickup I found all it was was a wire wool type filter on the end of the pick up copper pipe, it was fizzing the fuel , I would presume all diesel has a percentage of water in it but was surprising how much air it would create ( this engine used 8gph , but the lift pump would put 40gph through the prefilter every hour)

[jesperht]

I have checked the valves, 2 small diaframes, they are leaking a bit. I find it unlikely that they are supposed to keep it absolutely tight, sometimes for months??

There are connection between the outlet hose and the hose going from feed pump to pressure pump, so if the outlet hose is placed above the diesel, the only thing to prevent air from getting in the system, are the 2 small diaframes. They consist of a small pvc disk, 0,15 mm thick, hold in place by a small rubber "nut". My guess is, they can never be 100% leaking proof.
Would it be an idea to connect the outlethose to the hose between the dieseltank and the "waterfilter", or maybe use one of the ekstra connection on the filter?
By waterfilter i mean the filter that filters the water, that might be in the tank, from the diesel. Dont know the english word.

[Sailmonkey]

Originally Posted by jesperht

I have checked the valves, 2 small diaframes, they are leaking a bit. I find it unlikely that they are supposed to keep it absolutely tight, sometimes for months?? .

My standby generator at the house that sometimes I don't start for months is on top of the fuel tank. It does not leak fuel down over this time and starts right up.

[DeepFrz]

The best bet would be to extend the return hose to near the bottom of the tank. It would reduce oxygenation of the diesel fuel.

[jesperht]

I just went down to the boat to check things out. I dont think it will be a problem, since the top of the fuel tank will be at a higher point than the pump, so the diesel will not run out, unless it runs all the way back? So maybe i WILL have problems if the level of diesel becoms lower than the pump...

I discovered that there is no return hose going to the tank right now, why not? My current engine is a MD 17. Though it is old, i guess the MD 17 is a better engine than the new, much more solid....