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Air in fuel line


diesel66
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Hi all, I'm in need of assistance as all efforts so far have been fruitless!

Engine is a 2.5 NA diesel. Symptoms began as being hard to start the first drive of the day, subsequently fine until left overnight and then same repeated cranking required. Diagnosis air getting in and fuel draining back to tank (?) So found leak from top of filter banjo fitting, banjo cracked so replaced and clear pipe fitted to aid further diagnosis. No difference. Found further marginal leak in lift pump to filter pipe, so removed, olive cracked so replaced whole pipe as only £3. No difference. Suspected lift pump so swapped out with new - no change! Symptoms remain the same. Hard to start first time, then fine,even if cold - runs well no power loss. Able to bleed system fine and will start beautifully after change in filter, pump, pipe or whatever. But when stopped air bubbles start to emerge along clear pipe from top of filter to fuel return main pipe that T's into spill rail.

Does this point to main fuel line from tank being suspect? Any advice appreciated. Many thanks. Diesel66.

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try replacing the leak off pipe that runs between the injectors, these perish over time letting air in. you can usually see if one of these is causing a problem as it will be damp where the diesel has soaked in. should be a couple of quid from a factors for a metre length

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An update; the bubbles are no longer coming from filter side (too timid in tightening the lift pump to filter pipe so as to avoid cracked olive), however the air seems now to be definitely coming up from the spill rail join. Yet the spill rail is copper or brass tube with no apparent leaks. Thanks again.

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I forgot to mention that when running, no air is visible from spill rail to filter direction only the rare and barely noticeable bubble from filter toward spill rail/return join. However when engine stopped, air bubbles from the spill rail / fuel return start migrating toward the filter. Clear as mud!

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Might you have a leaking front shaft seal? I have seen that problem on Bosch VE pumps but it might be possible on that pump too.

Some people mask the problem with an electric fuel pump that operates for the duration of the glow plugs' glowing.

Not needed on the tdi's

I have heard too that if you fill the IP with transmission fluid, leave it for about 24 hours,it will swell seals ever so slightly and fix leaks very temporarily. If that works then returns, it might give something to go on.

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