Air in fuel

[KMM]

I am getting air in the fuel feed somewhere between tank and engine, causing the revs to drop and pick up occasionally, and sometimes causing the engine to cut out. Bleeding will enable restarting and a few hours of clean running. I presume I need to check all the joints in the train but how do I know if any union is airtight? It has also been suggested to me that the cast housing of primary filters can become porous. Can anyone confirm this or suggest a method of testing/sealing the pipeline?

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[Evadne]

I think you need to apply Occams razor, i.e. look for the simplest source of trouble first (always assuming everything was working fine until it went wrong).
You can usually find the first bleed point after any air leak, because there is air coming out of it when you bleed the system, starting at the tank and working your way down.

The two most common sources of air are a loose joint or a hole in the pipe. It is difficult to detect air getting in but if it is, fuel will get out over time so a piece of blue paper towel tied around the pipe downhill of a suspected leak will show up any diesel running down the outside of the pipe. (Blue shows up fluid leaks better than white, for some reason.)
Loose joints are somewhat self explanatory, holes in the pipe occur due to corrosion or wear, where a pipe passes through a bulkhead. Spotting even quite big holes by eye is almost impossible, in my experience. Good luck, you may need it!


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[andyball]

I've known engine castings become porous enough to weep oil over time....wd think if it could admit a significant amount of air you'd easily see fuel coming out when not running.

Had loose joints on a friend's yacht recently....in this case it was easy to inspect each joint & find one was wet with diesel & needed several turns to tighten it up fully...after which-no prob.

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[Paul_H]

Got a similar problem myself. Put a new Delphi filter/water trap in fuel line and ended up with fuel starvation. Fuel has to be pumped up over 1-1.5m from my bilge mounted tank so the smallest leak is a big problem. Did a temporary swap of rubber fuel hose using clear reinforced placci to see neat fuel going into the filter and bubbly fuel coming out. Also when engine stopped can see fuel receding back into tank as air leaks in. Bypassing filter with a bit of copper pipe and its neat fuel all the way and the fuel stays in the pipe with the engine stopped. Ive tried rubber and fiber washers for the hose tails on the filter and Ive lost count how many times Ive checked the cartridge seals. Im starting to conclude that the casing is porous as posted in another reply to this thread.

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[omega2]

Not much to add but I have had experience of ill fitting or badly fitted rubbers on filter housings usually when done in cold tempratures. Also a mate of mine chased your problem for ages even to extent of rebuilding the minimec pump and new injectors before he found the filter housing was porous. It's a rocky road and one that is not easily traversed, I wish you good fortune.

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[pvb]

Has this happened suddenly?

If so, did it follow any disturbance of the fuel system (eg fitting a new primary filter element, etc). If that's the case, it may help you to identify what's wrong.

It's fairly easy to undo pipe unions and remake the joint using a liquid pipe sealant around the olives. I use some stuff called Hermetite, but Calortite would probably be as good.

If your primary filter is a CAV type, you could try replacing ALL of the rubber seals. Most filter elements are supplied with the right seals. Remember the tiny O-ring which goes on the clamp bolt, under the washer. If you have a glass bowl water trap, you'll need to buy an additional rubber ring, because it's not provided with the filter element.

I had exactly the same problem as you a few years ago, and it was a loose joint which, tightened and with sealant, has given no more problems.

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[ccscott49]

Ifthe fuel pickup goesin the top of the tank, it could be a corrodedc hole in the pickup pipe, a difficult one to spot, but clear plastic inserted between pipe and tank, should show it up.

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[NigeCh]

Check your bleed valve washers

If they are copper then check what their work hardening torque is. It was Andrew B who pointed me in the right direction when he suggested fibre washers. They were duly changed and no problems since.

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[sundancer]

KMM see my question not far under yours "is your pipe in the right place"
just got my boat back to normal today after a long long time with exactly the same fault.
i`m not saying its the answer to you particular problem but it might help.
best of luck.
james

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[mtb]

Have you got copper fuel lines if so take the lot out and aneal it right through. I'm not in agreement with porous filter heads the reasone being with carburetters the very early masac ones can become brittle and flake the condesation can then become a problem doing freezing weather and blow the casting appart. This is not a problem in my veiw from what I've seen from the 1960's on because the methods used in manufacture are better .
I do know and have had situations where the odd pratt has not renewed the diesel filter before winter and just occasionaly there's a build up of water in the filter from tanks left half empty which can both block the filter and if it's cold enough freeze expanding the filter which can also distort the light alloy head.
Mind you it is easy to either over tighten the filter or misaligne it while tightening again causing distortion.

Cheers
Mick

<hr width=100% size=1>Danbrit is for sale I'm spending all my time working on her with no play best offer's please !!

 

[longmoor]

I've just had to replace the secondary filter housing on a Yanmar 3 GM 30 because the bleed screw thread had stripped both it's thread and the thread in the housing. I've found stripped threads on bleed screws to be quite a common occurance.

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[MLBURGE]

had it before and checked all the joints and were ok it turned out to be a faulty stopcock letting in air.

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[gus]

Rather than searching for air entering the fuel line because most of the time the engine is trying to draw fuel (negative pressure). Fit a header tank to gravity feed the engine with a positive pressure and any breaks in the system will be obviously apparent as a leak of detectable fuel. If you thus develop an 'air in fuel' problem, you can switch to the header tank until you reach a safe haven to solve the problem.

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