Air in fuel

[steved]

My boat has 2 x 200 litre fuel tanks that both feed into one prefilter.
The intake and return lines can all be individually isolated at the filter, and I normally run the engine using the feed and return from one tank only, and then swap tanks every so often.

Took the boat out yesterday for a run, and when throttle was increased the revs built up and then power dropped off, initially I was thinking that the filters could be clogged up. However, the glass filter bowl was showing bubbles being drawn in from somewhere. Switched over to the other tank and the bubbles disappeared and the engine ran fine right up to 3500 rpm.

How do you think the air is getting into fuel lines? My initiall thought is to check and tighten all the joints, but if air is getting in I would have thought that there would be some leakage / diesel smell.

Any thoughts?

Steve

 

[burgundyben]

Ireckon you have a partial blockage upstream of the bubbles, the lift pump is sucking like crazy, partial blockage is causing a vacuum, hence air being drawn in, may well not leak at atmostpheric pressure while not running.

 

[spannerman]

I would check the fitting on the tank, if its a 90 degree bend with threaded unions I have experienced leaks here. Also had a 90 degree bend with the pickup pipe brazed into it which had vibrated loose so it pulled in air right at the top where it goes through the inspection cover, you need to work from the filter to the tank as its only affecting one system.

 

[propnut]

Good advice Spanerman,I've come across that more than once

 

[theoldsalt]

Also bear in mind that if a union is leaking then tightening may actually make things worse due to olive distortion/crushed pipe. So undo and inspect first.

 

[LittleShip]

but if air is getting in I would have thought that there would be some leakage / diesel smell.

Steve

It is possible that you won't see or smell diesel but when under suction it is possible to have air being pulled into the line but it won't leak diesel out under atmospheric pressure.

Personally I think you will have a "loose" fitting or at least one that needs pinching up.

Tom.

 

[dingdongs]

all rubber pipes should be removed and checked as they fail internally including any return pipes too.

 

[steved]

Thanks for the replies and thoughts.

Think I'll start at the tank end as this is the easiest to get to then work my towards the filter. I suppose the pick up pipe in the tank could be partially blocked causing a vacuum as bergundyben suggests.

 

[steved]

All sorted.

90 degree bend at the top of the feed pipe on the tank was indeed partially blocked, not sure what it was but the fuel gauge sender failed in the same tank last year and could have been some bits of that.

 

[rafiki_]

Good result Steve.

 

[duncan]

All sorted.

90 degree bend at the top of the feed pipe on the tank was indeed partially blocked, not sure what it was but the fuel gauge sender failed in the same tank last year and could have been some bits of that.

late to the party but glad you found it.

I would, as BB, have gone straight to this having spent 2 months trying to tighten things up back in 2001 with exactly these symptoms - although in my case it would eventually cut out if run at over 2500 revs for any length of time as the engine literally sucked the filter bowl dry!

The boat was new, the culprit was established when I was able to remove a wad of paper from the pick up pipe and unfold it sufficiently to find the instructions for making the tank (in Spanish). The fabricators had left them inside the tank on construction!

The most interesting lesson was that a large diesel engine will sunk in air regardless of how well sealed the connection are made - but it only normally bothers when there's a blockage somewhere !

 

[burgundyben]

late to the party but glad you found it.

I would, as BB, have gone straight to this having spent 2 months trying to tighten things up back in 2001 with exactly these symptoms - although in my case it would eventually cut out if run at over 2500 revs for any length of time as the engine literally sucked the filter bowl dry!

The boat was new, the culprit was established when I was able to remove a wad of paper from the pick up pipe and unfold it sufficiently to find the instructions for making the tank (in Spanish). The fabricators had left them inside the tank on construction!

The most interesting lesson was that a large diesel engine will sunk in air regardless of how well sealed the connection are made - but it only normally bothers when there's a blockage somewhere !

Given the number of fuel supply issues we see on this forum that last sentence of yours should be on a banner across the top in bloody great flashing text.