Air still getting into fuel - please help tearing hair out

S

sj8070

Active Member
Joined
2 May 2017
Messages
49
Visit site
Hi all
We have a problem that we are 99% sure is air getting into fuel as whenever engines are bled they will then run for another 5 mins
have changed all filters, even had one of the fuel pump replaced - still getting the problem
We then assumed must be an issue with the fuel tank - thinking perhaps a layer of gunge (as boat had been stood for a year without use before we bought)
Today we checked the tank (big job as carpets had to be removed, sensor removed etc) and the fuel seemed absolutely fine - see pic below
Absorleyl lost as to what it could be now - both engines connect to the tank with diff hoses so unless both engines/fuel circuits have developed the same issues it must be the tank
Only possibility we can think of is that gunge has collected up by the intakes that we can't see ?
 

Attachments

  • IMG_2104.jpg
    IMG_2104.jpg
    72 KB · Views: 0
sj8070 said:
Hi all
We have a problem that we are 99% sure is air getting into fuel as whenever engines are bled they will then run for another 5 mins
have changed all filters, even had one of the fuel pump replaced - still getting the problem
We then assumed must be an issue with the fuel tank - thinking perhaps a layer of gunge (as boat had been stood for a year without use before we bought)
Today we checked the tank (big job as carpets had to be removed, sensor removed etc) and the fuel seemed absolutely fine - see pic below
Absorleyl lost as to what it could be now - both engines connect to the tank with diff hoses so unless both engines/fuel circuits have developed the same issues it must be the tank
Only possibility we can think of is that gunge has collected up by the intakes that we can't see ?
Click to expand...
I've lost count how many times I've pulled lengths of sikaflex out I'd Sealine fuel tank stack pipes .
Fit a vacuum gauge inline and read it. That will tell you if it's a supply problem.
 
take the whole fuel pipe system apart, somewhere in the mess of joints, valves and connectors etc is the culprit. could be air, could be some c@rp in the pipes that's been in/got into the tanks. Open up it all up and run a stiff wire back through it all to make sure its all clear. Once on a previous boat I ran the fuel from one tank into the other via a pump and big coarse filter (ebay) . All sorts of bits and pieces turned up in the big filter I had in line for doing it. I was looking for diesel bug but found a lot of sand, rust, bits of metal swarf, tissue paper, (the blue stuff mechanics use) a condom (don't ask) , £2-72 in loose change, but no water, no sikaflex (sorry Paul) and no diesel bug.

It takes as much effort to check one joint/valve as checking them all, as you will have to bleed the system so go through the whole line and check it all carefully.
 
Last edited:
A friend's boat had the same issue that he chased for months, would idle and rev unloaded no problem, but under load 5 mins at best... Turned out to be blocked fuel breathers, causing a vacuum and then the fuel filters would empty to the point the engine wouldn't run...

Steve
 
On a mates Princess diesel bug slime inline between tank and filters gave a blockage with the exact same symptoms. We thought the Racor filter seals were at fault because youd then see air in the bowl. Seems everone is saying the same thing. Check for fiel line blockages
 
If you have not yet been able to check the fuel pick up line you often get a bit of crud partly blocking it.

Blow through the line back to the tank with a dinghy pump or a shot of compressed air, do not pressurise the tank. It often clears the crud.
 
Where air will leak in under vacuum normally fuel will leak out under pressure. On my boat I have connections for filling my primary and secondary filters from my polishing rig and at the same time blowing back to the tank, thus this would clear any pick up blockage.

This also means the fuel pressure from the rig is only partial, enough to fill the filters and bleed the system, and show any leaks, but not enough to put any real pressure onto the system.

To discount the possibility of a blocked vent, remove the filler cap and stuff in a wad of tissue or rag to prevent any contamination entering and see if the problem goes away.
 
Thanks for the responses guys.
A few questions on some of the suggestions-

- If it's a blockage, then isn't at a bit unlucky that this has occurred simultaneously in the lines for both Port and starboard engines ?

- if it's air getting in to a connector, then shouldn't diesel be seeping out somewhere? Haven't seen any residue anywhere as far as I can tell

- interested in the tank breather possibility ... how can I check this not sure there even is one / where it is?

- what is a polishing rig???!!

Some photos of the tank i took yesterday, never know may help
https://dropfile.to/SPrM8mH
https://dropfile.to/LUzYHUC
 
This is my homemade polishing rig - a Baldwin pre filter with a Holley 12v pump that draws the fuel through. The connections are male instant connectors with matching female connectors on the fuel pick up line, tank low point, and spill return on each tank.

With it I can polish from the low point, pump from one tank to the other, prime the filters and blow back the pick up pipes at the same time.



To rule out the tank vent open the filler cap, and stuff in a wad of rag to prevent contamination entering, obviously do not do this if there are goffers landing on the deck.

This shows the pick up and spill connections to one engine.

 
Thanks that's very helpful
I do doubt it's a vacuum in the tank as it's a 30yr old boat and I don't think the system is totally airtight, including the deck filler
Will try and run it with it open though to be sure

One thing, though, is that I don't see bubbles in the pre filter bowl... or would you not be able to?
 
No mention on here yet as to what we are looking at? i.e. what engine, what fuel pump, what filters. When we know that maybe we can help, ours set up is about the same age as yours
 
Sorry not up on Vilvoes, but we had much the same "happenings" found after lots of swearing the CAV filter drain plug was worn out, and the feed pipe from the tank to the lift pump had many holes in it caused by vibration against the bilge floor. Hope you find the problem,
 
sj8070 said:
Thanks for the responses guys.
A few questions on some of the suggestions-

- If it's a blockage, then isn't at a bit unlucky that this has occurred simultaneously in the lines for both Port and starboard engines ?

- if it's air getting in to a connector, then shouldn't diesel be seeping out somewhere? Haven't seen any residue anywhere as far as I can tell

- interested in the tank breather possibility ... how can I check this not sure there even is one / where it is?

- what is a polishing rig???!!

Some photos of the tank i took yesterday, never know may help
https://dropfile.to/SPrM8mH
https://dropfile.to/LUzYHUC
Click to expand...

Does it have separate pick ups for port and starboard engines? Pic shows 3 hoses. Could be one pick up, one return and one breather. If so, pick up could be blocked. Are there any other hoses on the tank anywhere, apart from the fat filler hose?

Air getting in and diesel seeping out do not always both happen.

Breather. You must have one, otherwise you would create a vacuum in the tank, unless the filler is leaking.
 
Last edited:
When you say "had all the filters replaced "
Did you check ( or the person doing it ? ) that the old o rings /gaskets were removed before refitting the new ?
Otherwise a double o ring ,although can be tight never quite gets air tight .
What type of primary and 2nd filter set up are we talking about here ?
 
Portofino said:
When you say "had all the filters replaced "
Did you check ( or the person doing it ? ) that the old o rings /gaskets were removed before refitting the new ?
Otherwise a double o ring ,although can be tight never quite gets air tight .
What type of primary and 2nd filter set up are we talking about here ?
Click to expand...
Having thought about the OPs problem further, yes the filters are good candidates for this problem. I would strip them remove the seals, check for no doubling, then thoroughly grease the seals, seal recesses and bowl threads if there are any. Air leaking in at the top of the bowl will not be seen in the bowl.
 
Thanks again for all the responses

I should have said we've just had all filters changed by a specialist Volvo penta servicing company (couldn't do ourselves as we don't live in Barcelona) - it feels unlikely that they did it incorrectly ? Surely they would have replaced and re-greased the seals?
Also, this fault would have needed to have occurred simultaneously on both engines..

CLB thanks for the idea that one of those pipes might be the breather - hadn't thought of that
If indeed there is only 1 pickup, it feels like that might be the most logical problem?
 
But a counter to the tank pickup possibility is that we don't see air in the bowl (though they are hard to observe as in an awkward spot...)

How much air would be visible?

When I pump the manual lift pump to bleed it it takes a long while for fuel to come - does this not indicate there's a fair bit of air in the system ?
 
You must log in or register to reply here.

Share this page

Top