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Fuel system bleed

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Created by JonE 1 month ago, 4 Nov 2025
JonE
VIC, 536 posts
4 Nov 2025 3:20PM
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Ok folks we're close!

i have one of those combo primary filter and separators with the big glass. It's virtually full apart from a small air bubble.

i understand how to bleed the secondary filter, but whats the deal with getting all the air out of the primary? Do i have to basically pump it through until the bubble goes out through the secondary?

cheers,

jon

woko
NSW, 1745 posts
4 Nov 2025 9:10PM
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I guess you're referring to the CAV type filter / water trap ? If so on the top casing is a bleed bolt. Unless you have gravity thanks you will need to run the lift pump while bleeding the CAV type filter

Trek
NSW, 1183 posts
5 Nov 2025 7:36AM
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As Woko said.

Also look carefully into the lower glass part and see if there is a layer of water in the bottom. The diesel will be a light golden colour and if there is water under it it looks clear. You might need to use a torch to light it up. The diesel floats on top of the water. IF there's been a lot of recent agitation the diesel will look cloudy instead because the water didn't settle out.

If there is water, while the bleed screw on top is open loosen the water bleed screw under neath and drain the water out. Its usually a finger adjustable wing nut or similar. When the water layer has gone tighten it back up then bleed air out with lift pump.

Jolene
WA, 1618 posts
5 Nov 2025 5:00AM
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The small air bubbles won't effect much , it's often common to find a bubble in the primary filters if the filter is operating downstream of the lift pump.

whiteout
QLD, 269 posts
5 Nov 2025 7:55AM
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JonE
VIC, 536 posts
5 Nov 2025 9:12AM
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Select to expand quote
whiteout said..




Ja, that's the one I have.
For context, I've pulled all the old diesel out, cleaned both filters, replaced both cartridges and put new diesel in. The challenge is if I get a little bubble sitting in the top of that unit the only sure way to remove it is to disconnect the mounting and try and bring it lower than the top of the fuel level in the tank then bleed it out from the top.

My boat spends a fair bit of time on it's ear so I really would like to know I've shaken all the bubbles out of the system...

JonE
VIC, 536 posts
5 Nov 2025 9:13AM
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Ramona I like this. Have you been running this system for long? I would want it between the tank and the water separator yeah? So I would then be pushing fuel into the separator which would then bleed out the top?

garymalmgren
1343 posts
5 Nov 2025 7:32AM
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RE: The challenge is if I get a little bubble sitting in the top of that unit the only sure way to remove it is to disconnect the mounting and try and bring it lower than the top of the fuel level in the tank then bleed it out from the top.

One option is to mount the filter housing in a lower position permanently.
That will eliminate the pesky bubble for good.

Ramona
NSW, 7722 posts
5 Nov 2025 5:48PM
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Select to expand quote
JonE said..




Ramona I like this. Have you been running this system for long? I would want it between the tank and the water separator yeah? So I would then be pushing fuel into the separator which would then bleed out the top?


Exactly. Mount it as low as possible. I have had it for about 3 years but it's probably run for a total of five minutes.

MorningBird
NSW, 2697 posts
5 Nov 2025 5:57PM
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Select to expand quote
Ramona said..

JonE said..






Ramona I like this. Have you been running this system for long? I would want it between the tank and the water separator yeah? So I would then be pushing fuel into the separator which would then bleed out the top?



Exactly. Mount it as low as possible. I have had it for about 3 years but it's probably run for a total of five minutes.


I did the same on Morning Bird.

bullrout
QLD, 58 posts
6 Nov 2025 7:14AM
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Absolutely the best idea, My boat already had one fitted and makes bleeding a breeze

Jolene
WA, 1618 posts
6 Nov 2025 8:07PM
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But is it the best idea??

bullrout
QLD, 58 posts
7 Nov 2025 11:18AM
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Select to expand quote
Jolene said..
But is it the best idea??


So are you waiting for someone else to put a better idea forward or do you in fact, have a better idea?

r13
NSW, 1712 posts
7 Nov 2025 1:36PM
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I put in a Matsumo fuel hand primer pump between tank and separator - works perfectly. Three pumps before first start of the day and that's it.

www.onlineautoparts.com.au/product/zpn-13029-fuel-primer-pump-assembly-matsumo



JonE
VIC, 536 posts
7 Nov 2025 4:15PM
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OK great thread folks. I like the 12 volt option. I think I will really like it when I'm offshore, bouncing up and down, turning green below decks trying to get the motor started.

Ramona
NSW, 7722 posts
7 Nov 2025 5:31PM
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Select to expand quote
r13 said..
I put in a Matsumo fuel hand primer pump between tank and separator - works perfectly. Three pumps before first start of the day and that's it.

www.onlineautoparts.com.au/product/zpn-13029-fuel-primer-pump-assembly-matsumo




Should not have to prime. Just press the button and a Beta will start on first rotation. Check whether you have an air leak on top of the fuel filter.

Jolene
WA, 1618 posts
7 Nov 2025 8:55PM
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Select to expand quote
bullrout said..
Jolene said..
But is it the best idea??


So are you waiting for someone else to put a better idea forward or do you in fact, have a better idea?


I just like the idea of priming the fueling system with the engine/hand operated lift pump. That way you know your pump is operating correctly and you have eliminated down stream problems that may not be exposed if you choose to force feed the system.
If you cant draw a bit of fuel a few yards through a 1/4 inch pipe without wanting an electric pump to that for you , you're just lazy and a poor mechanic.
It's your right to be lazy and a poor mechanic if you choose,,, I won't hold that against you. I just won't call it a good idea.

Ramona
NSW, 7722 posts
8 Nov 2025 8:23AM
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Select to expand quote
Jolene said..

bullrout said..

Jolene said..
But is it the best idea??



So are you waiting for someone else to put a better idea forward or do you in fact, have a better idea?



I just like the idea of priming the fueling system with the engine/hand operated lift pump. That way you know your pump is operating correctly and you have eliminated down stream problems that may not be exposed if you choose to force feed the system.
If you cant draw a bit of fuel a few yards through a 1/4 inch pipe without wanting an electric pump to that for you , you're just lazy and a poor mechanic.
It's your right to be lazy and a poor mechanic if you choose,,, I won't hold that against you. I just won't call it a good idea.


If you were a good mechanic you would never have to prime a diesel. Just press the starter button. If you need to prime then there is a fault somewhere. Normally, an air leak that is not easily visible like a diesel leak. CAV filters and Racor are regularly problematic with the tops suffering air leaks. The 12v pump just saves time and eliminates the need to turn the motor over to get the lift pump on the cam. $15 spent once can save a lot of agro.

Jolene
WA, 1618 posts
8 Nov 2025 8:21AM
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, just for the record I never said I do prime the system as a pre start before normal operation.
If the operating manual of any engine stated a pre start prime was required ,
I would do it.

woko
NSW, 1745 posts
8 Nov 2025 9:40PM
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For the record no matter what type of fuel tank to pump , electric, gravity or manual operation, if you have the cav type filter, it is imperative to change out ALL the seals including the one in the housing, which can be difficult to get to.



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"Fuel system bleed" started by JonE