Air in fuel on BMC 1.5

[Cattleya]

I posted yesterday about issues changing my engine fuel filter. Today I took everything apart and fitted another new filter and rubber rings, this time I used a mirror to check the top seal was fitted correctly. I still have a LOT of air coming from somewhere.  Another boater came to help, he took off the top banjo bleed screw on the CAV filter and checked the filter was full of fuel, he also checked the filter was fitted correctly. There was still loads of bubbles coming from the top bleed point and we must have pumped at least a pint of diesel through!

So I am now thinking the air might be coming in somewhere else. The lift pump maybe? Or the CAV water trap filter, or maybe even a joint on the fuel line. 

Can air get in the lift pump? Or is it more likely to come from downstream of the lift pump? Would air even get in through a seal on the engine fuel filter? I'm thinking it is more likely for fuel to leak out than air to come in? 

[Jen-in-Wellies]

It can come in upstream of the lift pump too. Was bleeding the system on a friends boat where the compression olive in a connection between the lift pump and the fuel tank was letting air in. It didn't drip fuel, but when the pressure dropped inside with the lift pump being operated by hand you could hear a faint whistle of air being drawn in. Remaking the joint with a new olive solved it and after bleeding the engine fired up and ran fine.

[Tony Brooks]

Do you have a primary filter/water trap between the tank and lift pump? If so, I would start looking at that. Not all CAV filter as fitted to 1.5s use a small O ring to seal the centre bolt and some O rings supplied with new filters have been found to be too thin. If there is not a small taper at the top of the centre bolt hole in the filter head it is sealed with a soft washer, not at O ring. I would try to find a Dowty washer.

[Cattleya]

Hi Tony, yes I do have a water trap which is another CAV filter between the tank and lift pump. I will see if there is air present in that one, I presume it bleeds the same way as the engine filter by opening the top bleed screw and working the lift pump lever? 

[Tony Brooks]

2 minutes ago, Cattleya said:

Hi Tony, yes I do have a water trap which is another CAV filter between the tank and lift pump. I will see if there is air present in that one, I presume it bleeds the same way as the engine filter by opening the top bleed screw and working the lift pump lever? 

 

No, it bleeds via the engine filter unless you are lucky and can get gravity to bleed it from the top bleed screw. If you can get your lips to the top, you may be able to start it flowing by giving the bleed port a good suck. It will bleed from there if the fuel tank take off is at the bottom rather than the top.

 

If you have bug or suffered  alack of maintenance that first filer may be blocked and that will cause air in the fuel.  I would have another very careful look at all 3 or 4 of its seals and make sure the large top seal is not twisted in its groove.