Air entering fuel line when engine stopped.

[ColinR]

I had starting issues with my new Nanni D21 last year which eventually seemed to due to the fuel return incorrectly terminating at the top of the tank, not being submerged in the fuel. Took the tank out over the winter ( difficult!) and had the return fitted properly plus a new pickup pipe and everything checked out. It started and ran every time when checked in the boatyard over the winter, and when launched a couple of weeks ago. I motored for a couple of hours, all seemed fine. It started again this weekend and ran for 15 mins then died. The engineer came out and found no trace of poor connections, all were dry or fuel leaks. He bled it to the injectors and again it ran fine for half an hour. Next morning it started first time and only ran for a few minutes before dying. I started bleeding it at the secondary filter and there was clearly air in it again. This seemed to have entered overnight when the engine was stopped, not when it was running. As I said the tank has been out and overhauled, all fuel lines checked repeatedly and the lift pump replaced just in case. It's driving me to distraction as I am preparing a trip to the Azores in about a month so I have to get it sorted. Excuse the long post, any thoughts welcome. Colin

 

[pvb]

I'm afraid it's a case of checking everything meticulously. It may be a tiny leak in the lift pump. It may be a tiny leak on the primary or secondary filter, or their bleed screws. But, somewhere, there's a leak.

When this happened to me some years ago, I had almost given up hope of finding the air leak when, on check-tightening the connections, a banjo bolt disintegrated. It had cracked, relieving the compression on its washers, and allowing air in. Highly unlikely to happen, but that was the culprit. Good luck!

 

[PetiteFleur]

I had exactly the same problems several yrs ago after fitting a Beta 25 - air getting in somewhere. After speaking to the engineer in uk(I was in Holland) - it was air getting in through a union somewhere, no evidence though, tightened all unions and problem cured. Hopes this helps.

 

[savageseadog]

It might be debris, diesel bug in the tank or filter. Best approach might be to strip everything, fit new seals, washers and lines, clean the tank out.

 

[duncan99210]

I had a short lived problem like this - came down to a not quite correctly assembled primary fuel filter. Recheck all the unions, check the assembly of the filters, check the integrity of the pipe work. It'll be there somewhere..

 

[Cloven]

I had a short lived problem like this - came down to a not quite correctly assembled primary fuel filter. Recheck all the unions, check the assembly of the filters, check the integrity of the pipe work. It'll be there somewhere..

+1 Many of us had this problem. Mine occurred after an engineer fitted a new fuel filter. Turned out he had cross threaded one of the joints. On the basis that this was the only change, I bypassed the filter on a temporary basis and cured the problem. As others have said, there will be an air leak somewhere but may well be no evidence of a corresponding diesel leak.

Good luck with finding the problem.

 

[KellysEye]

Put kitchen roll around all the fuel line joints, the bleed screw, fuel lift pump and around where the filter(s) screws on and the bleed screw on the bottom of the filter if it has one. Diesel will go through a microscopic hole and draw air in, run the engine for at least an hour or until it stops. The leak(s) should be obvious.

 

[Chris_Robb]

Put kitchen roll around all the fuel line joints, the bleed screw, fuel lift pump and around where the filter(s) screws on and the bleed screw on the bottom of the filter if it has one. Diesel will go through a microscopic hole and draw air in, run the engine for at least an hour or until it stops. The leak(s) should be obvious.

But air entering the system does not show as a leak. Diesel is not escaping - air is getting in.

Is the tank below the level of the lift pump? This set up is much more likely to have air leaks as fuel is being sucked up.

If so, investigate every join from the lift pump to the tank. You are not looking for a diesel leak out as air, whilst in operation, is being sucked in, so you wont see anything!

A leak of diesel after the lift pump normally shows as diesel weeping out and unlikely to be an air leak in - but should still be cured.

Talcum powder sprinkled over the joints and surrounding area are a good way of finding leaks.

Very frustrating - good luck!

 

[vyv_cox]

But air entering the system does not show as a leak. Diesel is not escaping - air is getting in.

Is the tank below the level of the lift pump? This set up is much more likely to have air leaks as fuel is being sucked up.

If so, investigate every join from the lift pump to the tank. You are not looking for a diesel leak out as air, whilst in operation, is being sucked in, so you wont see anything!

A leak of diesel after the lift pump normally shows as diesel weeping out and unlikely to be an air leak in - but should still be cured.

Talcum powder sprinkled over the joints and surrounding area are a good way of finding leaks.

Very frustrating - good luck!

Exactly right. Air entering the fuel can only be in the suction side, i.e. between the tank and the lift pump. After that the pressure is always positive (at least a little) and air leaking in is almost impossible. Leaking primary filter is a regular one, with the glass bowl type it is sometimes possible to see the air getting in.

 

[charles_reed]

I had a series of very similar incidents - finally run to earth on the fuel tap, leaking air - only a problem below 1/2 tank when the level was below that of the fuel pump.
Quickly resolved by re-grinding the tap taper, though it took nearly 3 years to run the problem to earth. Check all the joins between tank and pump - sloppy filter-bowl replacement is the most common, followed by leaking banjo-joints.

 

[RichardS]

Exactly right. Air entering the fuel can only be in the suction side, i.e. between the tank and the lift pump. After that the pressure is always positive (at least a little) and air leaking in is almost impossible. Leaking primary filter is a regular one, with the glass bowl type it is sometimes possible to see the air getting in.

I had a problem with one of my glass bowl Racors in that when the engine was not running the fuel level in the bowl would drop a few cm and be replaced by air. There was no leakage of fuel anywhere and when the engine was started it would pull the air through the engine or return and run fine and the air pocket would vanish. 3YMs are supposed to be self-priming. However, the air pocket would return once the engine was off.

After a couple of years of living with this, I took out the Racor and stripped it down. The rubber washer in non-return valve in the middle of the unit was assembled upside down and was not sealing properly against the ball bearing so the valve was letting fuel flow in both directions.

I can only assume that the level of fuel in the filter and the level in the fuel tank was such that somehow fuel managed to syphon back to the tank because of the dodgy valve. Once I had bought a refurb gasket kit and reassembled it all correctly, the problem vanished.

Richard

 

[ColinR]

thanks for all these helpful suggestions. The fuel tank it fairly high, the bottom of it perhaps a bit below the lift pump. Clearly the whole fuel line from tank to high pressure pump needs to be gone through meticulously. I have to assume that the tank is okay as it was removed, cleaned and all the connections etc renewed over the winter.
Colin

 

[Chris_Robb]

thanks for all these helpful suggestions. The fuel tank it fairly high, the bottom of it perhaps a bit below the lift pump. Clearly the whole fuel line from tank to high pressure pump needs to be gone through meticulously. I have to assume that the tank is okay as it was removed, cleaned and all the connections etc renewed over the winter.
Colin

Colin - not an assumption you should make! You have altered something - this is one of the things you have altered, therefore check every joint again - especially the dip tube because as this tank is feeding from above especially if full, this is the only place where there will be negative pressure.

 

[KellysEye]

>But air entering the system does not show as a leak. Diesel is not escaping - air is getting in.

Diesel does definitely escape as air gets in I had air leak problems with copper fuel lines and that's how I found the leaks. I got so fed with leaks I changed the lot to fuel hose. Diesel has been used to check welds.

 

[ColinR]

Colin - not an assumption you should make! You have altered something - this is one of the things you have altered, therefore check every joint again - especially the dip tube because as this tank is feeding from above especially if full, this is the only place where there will be negative pressure.

Fair enough but I'd prefer to start by checking the things that haven't been checked not those that already have, especially as the rest of the fuel system is fairly accessible whereas getting the tank out is a major project. Colin

 

[ColinR]

Colin - not an assumption you should make! You have altered something - this is one of the things you have altered, therefore check every joint again - especially the dip tube because as this tank is feeding from above especially if full, this is the only place where there will be negative pressure.

Chris, looks like you were right. The first thing that was checked was the connection to the tank and the valve seems to be leaking. Removed that and so far so good. Colin

 

[Chris_Robb]

Chris, looks like you were right. The first thing that was checked was the connection to the tank and the valve seems to be leaking. Removed that and so far so good. Colin

hope you have found it!

 

[Contest1]

Hope you found it! On my pals boat his son, a marine engineer, had failed to remove the primary filters orriginal sealing ring while fitting a new one, the air leak was intermittent and hard to pinpoint!