Air leak in fuel line

[Keith-i]

I have an annoying air leak somewhere in my fuel line. It doesn't seem to have any impact on the engine running but each time I go to start the motor I can see a bubble of air has accumulated in the clear section of line on top of the engine (highest point in circuit). This is after all the filters but before the HPFP. As the fuel tank is lower than the filters and pump everything is under suction when running. There is no sign of any diesel leaks at any joints so it must be a very small leak.

My plan is to disconnect the pipe from the top of the tank, top it up with diesel and somehow pressurise the line to see if I can force a diesel leak from the suspect part. My question is what to use and how do I go about pressurising the line?

 

[fisherman]

We used to blow in the air vent.

 

[bristoljim]

Rig up a small plastic bottle / tank with a pipe to your fuel line - tie it up say a metre above the engine - dust all connections with talc powder - overnight you should be able to see any leaks
Jim

 

[Keith-i]

Thanks Jim. That idea had crossed my mind but I wondered whether it would create sufficient pressure. I suppose if I can adapt a plastic bottle to fit I could then give it a good squeeze.

 

[Graham_Wright]

I had/have the same problem and I have a permanently fitted electric pump in the line. There is absolutely no sign of a leak when the line is pressurised but a sufficient leak on suction to stop the engine. My guess is it is a one way leak; closes under pressure but opens under suction.

 

[jdc]

Are you sure it has to be a leak? I ask because air does dissolve in diesel to a small extent, and the greater the pressure the more dissolves so conversely reduced pressure will cause some outgassing. Hence you will always get a bubble at the top of a fuel pipe. Let's try to estimate whether your bubble could be due to this:

Bubble dimension I guess as 3 x 5 x .7 mm = 10.5 mm3 (ie obviously a decent sized bubble but not a huge one)
Volume of diesel standing is maybe pipe dia = 8mm, 1.5m long = 7.5e4 mm3
Volume of diesel in filter is maybe 100mm long x 60mm dia = 2.8e5 mm3, so
total volume of diesel under slightly reduced pressure = 3.6e5 mm3

What is the solubility of air in diesel? Difficult to be exact as it depends on loads of things such as amount of aromatics, and goodness knows whether FAME (aka biodiesel) makes a difference, but the Ostwald coefficient (volumetric ratio of dissolved gas to liquid) is around 0.1 at stp.

Pressure difference between atmospheric (1e5 Pascals) and in the vacuum side one can estimate as a head of 1m say.
delta P = rho * g * h where rho - density and g = acc due to gravity. h = 1 as guessed above, so delta P = 0.85e3 x 9.81 x 1 = 8338 Pa. That's proportionally 8338 / 1e5 ~= 8.3e-2.

Apply this to the volume of diesel x the Ostwald coefficient = 8.3e-3 x 3.6e5 x 0.1 = 300.

So dissolved gas alone could cause a bubble some 30x the size of that you see. Could be that there is nowt to worry about!

 

[Keith-i]

Interesting thought jdc. The amount of air that accumulates as a bubble does vary so I am inclined to think it is not outgassing as I imagine that would be fairly consistent. I also know that I did have a slight leak on one of the fuel line connections which only appeared when I brimmed the fuel tank which put the joint (via syphon effect) under positive pressure. The pipe from tank outlet up to and including this joint has now been replaced as I just couldn't get the original 3/8" BSP fitting to seal properly. Since then the bubble is smaller, but I feel I still have a miniscule leak somewhere.

 

[Dull Spark]

You say it must be an air leak, but could it be that when running on a near empty fuel tank, you have entrained some air, but not enough to stop the engine?

 

[lw395]

To pressurise the line, you could just get a long length of fuel hose filled with fuel and raise the open end.
If you have aleak, you'll see the level go down.

I think the OP needs to change the clear hose for opaque, end of problem?

 

[fisherman]

In extremis, you could fit a tee where the bubble is, then a pressure bulb between there and the tank, bleed the air off. Might be useful anyway at filter change time.

 

[black mercury]

If engine is running fine and starting ok I would leave alone. Have a look at a diesel car engine's clear fuel line and now and again you will see tiny bubbles in the fuel. The system can deal with the tiny amount involved. You may be pursuing the near impossible, especially after reading jdc's post.

 

[KellysEye]

Cover wery joint and the fuel fiiter(s) with kitchen roll and you wil find any diesel leak, an air leak lets diesel out and the engine stops, that is not happening but it is worth checking anyway.