air bubbles in fuel line

[binka]

Had this happen a few months ago and it wouldn't start but has been fine since. Happened again today. Air bubbles are only in return pipe from pump back to tank. Aa man says no air in pipe going to pump. He's puzzled as to where air getting in, says no sign of anything. Anyway it did start after a while but id like to get it sorted properly. Any ideas what to look.at. its diesel.

[Aidan]

either leak in leak off pipe return to pump, though I'd expect to see fuel leak out or throttle spindle seal at a guess, basically wear due to age and useage, but remember I don't do diesel myself, so someone more knowledgeable will be along shortly

[Oldiebut goodie]

That seems an impossible problem to solve - I have the same on my boat engine - very minor air leak into the fuel line on the tank to engine line which can lead to difficult starting sometimes if I don't use the manual pump to pump fuel to the inj. pump as the fuel drains back to the tank. I've replaced all fuel lines/filters , checked all joints and still haven't solved it.
If fuel were to be leaking it would be easy! If you use a temporary clear fuel line you could see if air is being drawn in at a certain point. The only way that I can think of checking further is to pressurize the fuel in the inlet side with a bicycle pump to see if a diesel leak develops.
Other than that good luck with solving it.

[Titus A Duxass]

I had this problem and it was down to the return lines being perished.
The return lines are the black cotton covered rubber tubes that go from injector to injector and then back to .....

You buy it by the metre and cut it to length.

I still have the odd small bubble in the return line but it's not a problem.

[binka]

Thanks everyone. Will try replacing the return lines first.