Fuel return pipe; via filter or direct to tank

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pcatterall

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I am trying to improve the logic and ease of maintenance ( filter changing etc) on my 4108.
From previous threads and thanks to the usual good advice I have a better idea of what to do.
A question remains about the fuel return, I think it is best put via this seperate thread.

My fuel return currently goes from the top of the injectors ( 5/16ths fitting I think?)
to the top of the CAV filter then from there (3/8ths out) by a hotch potch of flexi and copper pipe ( 12 feet in all) to the top of the fuel tank.

This method means that bleeding the CAV filter has to be done using the return connector rather than a valve and really the grotty pipe work looks like a problem waiting to happen!!

2 forumites responding to my earlier thread suggested that the had 'done away with' the return via the filter and gone direct to the tank.

I plan to fit a replacement filter unit with screw on filters SSLdieselparts SFA11 £19.21 this has just the bleed screw on top so rather than use that for the return fuel I would also like to go direct back to the tank. Questions arising:-

1) What was the purpose of taking the return back via the filter ?
2) Is the 'direct back to the tank solution in any way a problem?
3) Can I use flexi hose to run direct from the injector head to the tank?
4) Does 5/16th sound like the right size for the take off from the injector head?
5) Is there much pressure in this return pipe?
6) Anything else I should know,watch etc?
7) And will I need to bleed the return pipe after fitting or will it just pump through?

Appologies for all these questions. I would like to get it all measured up here in France, order the bits and bring them back here for fitting on my next visit..

Many thanks as allways.
 
pcatterall said:
I am trying to improve the logic and ease of maintenance ( filter changing etc) on my 4108.
From previous threads and thanks to the usual good advice I have a better idea of what to do.
A question remains about the fuel return, I think it is best put via this seperate thread.

My fuel return currently goes from the top of the injectors ( 5/16ths fitting I think?)
to the top of the CAV filter then from there (3/8ths out) by a hotch potch of flexi and copper pipe ( 12 feet in all) to the top of the fuel tank.

This method means that bleeding the CAV filter has to be done using the return connector rather than a valve and really the grotty pipe work looks like a problem waiting to happen!!

2 forumites responding to my earlier thread suggested that the had 'done away with' the return via the filter and gone direct to the tank.

I plan to fit a replacement filter unit with screw on filters SSLdieselparts SFA11 £19.21 this has just the bleed screw on top so rather than use that for the return fuel I would also like to go direct back to the tank. Questions arising:-

1) What was the purpose of taking the return back via the filter ?
2) Is the 'direct back to the tank solution in any way a problem?
3) Can I use flexi hose to run direct from the injector head to the tank?
4) Does 5/16th sound like the right size for the take off from the injector head?
5) Is there much pressure in this return pipe?
6) Anything else I should know,watch etc?
7) And will I need to bleed the return pipe after fitting or will it just pump through?

Appologies for all these questions. I would like to get it all measured up here in France, order the bits and bring them back here for fitting on my next visit..

Many thanks as allways.
Click to expand...

The return fuel is already filtered, why do it again
sounds like a typical "previous owner" improvement :D
 
pcatterall said:
1) What was the purpose of taking the return back via the filter ?
2) Is the 'direct back to the tank solution in any way a problem?
3) Can I use flexi hose to run direct from the injector head to the tank?
4) Does 5/16th sound like the right size for the take off from the injector head?
5) Is there much pressure in this return pipe?
6) Anything else I should know,watch etc?
7) And will I need to bleed the return pipe after fitting or will it just pump through?
Click to expand...

1) No idea!
2) No problem at all; indeed in many ways it's the better solution.
3) You could use flexible hose, usually the leakoff lines are metal, followed by a flexible hose to the tank.
4) 5/16" is probably OK.
5) No real pressure as the end of the pipe is open to the tank.
6) Ideally, the fuel return to the tank should go through a tube reaching down to the bottom of the tank, this will reduce possible aeration.
7) No, the fuel will just flow through.
 
Some of these posts seem to assume that the return fuel will somehow be filtered and then sent on its way back to the tank. Of course, all that's actually happening is that the returned fuel is being mixed in with the fresh fuel. Each molecule of diesel doesn't somehow remember which inlet it came in through, and then choose the corresponding outlet! Depending on flows and pressures, there may not even be any returning, just less being drawn from the tank.

Can't see any possible reason for this setup. Run a new return line back to the tank.

Pete
 
If it goes straight to the tank the warm fuel will create condensation but you may have no option and it is standard now.

Our return goes straight to the fuel lift pump, but I guess you might have trouble finding the right size lift pump with a return fitting. Also they may not exist now, the engine is an Volvo MD17C made in 1970. Worth a look though.
 
KellysEye said:
If it goes straight to the tank the warm fuel will create condensation but you may have no option and it is standard now.
Click to expand...

I'm not sure that warm fuel would create enough condensation to be an issue. However, remember that the leakoff fuel performs an important role in cooling the injector pump and the injectors; returning it direct to the engine via the lift pump or the primary filter will increase the temperature of the fuel in the engine, thus reducing power output. Additionally, returning the fuel to the tank helps in removing dirt/bugs from the fuel tank due to repeated filtering.
 
Photo shows the BMC 2.2 litre diesel engine in my motorsailer. The fuel return is the thin blue pipe in the foreground, it has a very small bore, only about 1/8 inch, and goes straight back to the tank. However, I think it feeds into the filter first, using a weir system to return only the overflow.

IMG_2370.jpg


How does warm fuel cause condensation? I thought atmospheric vapour condensed on surfaces below the dew point, so warming them up prevents it?
 
Last edited:
Thanks all,
So the concensus is get rid of the connection at the filter!
AS PVB says there is no filtering going on as the connector is just in at one side and out of the other, if for some reason the fuel in the filter was low then the returned fuel could possibly top it up but something would be decidedly wrong for that to occur.
My plan is to measure up accuratly an order one lengh of armoured flexihose with appropriate fittings on each end.
When I return I will fit the hose first job and replace the connector with a proper bleed screw (and just check that the filter is still bled)
I can then run the engine for a while and if all OK then remove the disconnected scrap pipes etc.
I can the move on and do the proposed change of the filter to a screw on CAV type.

Out of interest can I make up my own hose? this means I could bring some assorted fittings in case I have got any sizes wrong?

Thanks once again
 
pcatterall said:
Out of interest can I make up my own hose?
Click to expand...

My hoses were originally secured to their barbs using the kind of clips that start circular and are then crimped at each side to reduce the diameter and squeeze the hose (afraid I don't know the name). I replaced them with good quality stainless jubilee clips in the correct size, which seems just as good to me.

Pete
 
pcatterall said:
I am trying to improve the logic and ease of maintenance ( filter changing etc) on my 4108.
From previous threads and thanks to the usual good advice I have a better idea of what to do.
A question remains about the fuel return, I think it is best put via this seperate thread.

My fuel return currently goes from the top of the injectors ( 5/16ths fitting I think?)
to the top of the CAV filter then from there (3/8ths out) by a hotch potch of flexi and copper pipe ( 12 feet in all) to the top of the fuel tank.

This method means that bleeding the CAV filter has to be done using the return connector rather than a valve and really the grotty pipe work looks like a problem waiting to happen!!

2 forumites responding to my earlier thread suggested that the had 'done away with' the return via the filter and gone direct to the tank.

I plan to fit a replacement filter unit with screw on filters SSLdieselparts SFA11 £19.21 this has just the bleed screw on top so rather than use that for the return fuel I would also like to go direct back to the tank. Questions arising:-

1) What was the purpose of taking the return back via the filter ?
To pick up small amounts of air from the filter, it doesnt go through the filter. Also to provide a good strain free mount for the spill rail and return plumbing
2) Is the 'direct back to the tank solution in any way a problem?
How do you secure the pipe end from vibration, however in high power installations the spill rail is frequently taken directly to the tank, with its own tank connection to remove any back pressure from the pumps return line.
3) Can I use flexi hose to run direct from the injector head to the tank?
So now there is the weight of the return pipe stressing the small injector pipe
4) Does 5/16th sound like the right size for the take off from the injector head?
Has it worked for years with the pipe this size, is it the same size as other similar units?
5) Is there much pressure in this return pipe?
No only the head of diesel from that point to the tank top or the fuel level, if the return pipe is continued in the tank towards the bottom of the tank
6) Anything else I should know,watch etc?
If it works why alter it unless there has been some bodge alteration to put it in the present state. Be careful of introducing a new problem which did not exist before.
7) And will I need to bleed the return pipe after fitting or will it just pump through?
Just think from first principals, what does this pipe do, it is a return pipe, so will the engine run, yes, as there is fuel at the engine all ready, so it does not need bleeding, just run engine and check for leaks.

Appologies for all these questions. I would like to get it all measured up here in France, order the bits and bring them back here for fitting on my next visit..

Many thanks as allways.
Click to expand...

reply interlaced

Perhaps your best improvement could be to tidy up the plumbing too and from the engine, from your discription.
 
Last edited:
pcatterall said:
Thanks all,



Out of interest can I make up my own hose? this means I could bring some assorted fittings in case I have got any sizes wrong?

Thanks once again
Click to expand...


The best is to get a hydraulic hose co to fit swaged ends on the correct spec of fuel hose. so if there isnt one locally to where the boat is , see a co here , buy hose & fittings here take it out , cut to length ang mark the hose with the orientation of banjos etc & take back for the swaging, so you can get a neat strain free installation.
 
I fitted a new Yanmar to replace a Stuart Turner in a 26ft boat. It would have been very easy to direct the fuel return into a spare inlet on the filter rather than to the tank.
When I asked the supplier they threw their hands in the air and said I mustn't do that, all sorts of nasty thing would happen.
So I had to remove the old tank [not easy] and get someone to braze on a new connection [even harder]
A while later I was looking at an identical boat and engine combination which was installed by the same people. Of course, they had connected the return to the filter, because it was easy.
That engine ran fine.
 
pcatterall said:
I am trying to improve the logic and ease of maintenance ( filter changing etc) on my 4108.
From previous threads and thanks to the usual good advice I have a better idea of what to do.
A question remains about the fuel return, I think it is best put via this seperate thread.

My fuel return currently goes from the top of the injectors ( 5/16ths fitting I think?)
to the top of the CAV filter then from there (3/8ths out) by a hotch potch of flexi and copper pipe ( 12 feet in all) to the top of the fuel tank.

This method means that bleeding the CAV filter has to be done using the return connector rather than a valve and really the grotty pipe work looks like a problem waiting to happen!!

2 forumites responding to my earlier thread suggested that the had 'done away with' the return via the filter and gone direct to the tank.

I plan to fit a replacement filter unit with screw on filters SSLdieselparts SFA11 £19.21 this has just the bleed screw on top so rather than use that for the return fuel I would also like to go direct back to the tank. Questions arising:-

1) What was the purpose of taking the return back via the filter ?
2) Is the 'direct back to the tank solution in any way a problem?
3) Can I use flexi hose to run direct from the injector head to the tank?
4) Does 5/16th sound like the right size for the take off from the injector head?
5) Is there much pressure in this return pipe?
6) Anything else I should know,watch etc?
7) And will I need to bleed the return pipe after fitting or will it just pump through?

Appologies for all these questions. I would like to get it all measured up here in France, order the bits and bring them back here for fitting on my next visit..

Many thanks as allways.
Click to expand...
Peter
normal situation for the return to go straight back to the tank, its already filtered on the way in. The pressure is lo as long as it is not restricted, the lift pump provides a lo pressure feed to the injector pump, the high pressure to the injector from the pump is lost as soon as the injector "breaks" and the surplus bleeds out of the top.
Apart from the elf and safety wallers on here lifting their hands in horror, proper flexible fuel line is ok. My Bene has 5/16" flexi back to the tank, as std
Stu
 
Fuel returns from last filter.

I have never seen this situation on this forum before, I think all the posts so far are wrong, what makes people think that fuel coming through the return pipe at almost no pressure will enter the filter which is being pressurised by the lift pump.

This is how it works, some times the final fuel filter is designed to be connected to the tank through the returns pipe, before or after the injectors, it does not matter. This type of filter has the return connection at the top of the filter with the fuel passing through a small jet. Its purpose is to create a small continuous bleed off, so that any small amounts of air that get into the system will be passed back to the tank, together with the injector returns. Because of the possibility of returns from the injectors or from the fuel filter containing air the returns pipe must go back to the main tank.

If you know that I am wrong then please explain why. Regards to all, George.
 
GeorgeLlewellin said:
I have never seen this situation on this forum before, I think all the posts so far are wrong, what makes people think that fuel coming through the return pipe at almost no pressure will enter the filter which is being pressurised by the lift pump.

This is how it works, some times the final fuel filter is designed to be connected to the tank through the returns pipe, before or after the injectors, it does not matter. This type of filter has the return connection at the top of the filter with the fuel passing through a small jet. Its purpose is to create a small continuous bleed off, so that any small amounts of air that get into the system will be passed back to the tank, together with the injector returns. Because of the possibility of returns from the injectors or from the fuel filter containing air the returns pipe must go back to the main tank.

If you know that I am wrong then please explain why. Regards to all, George.
Click to expand...
I covered that on his other thread.
 
Thanks George I'm confused again but at least I can see why the return fuel is going via the filter.
My problem was that my filter looks to be a standard CAV filter but with the bleed valve hole being taken by the return connector, I found bleeding via the connector difficult.
I can see that the opportunity to use the returns flow to carry small amounts of air which have somehow 'made it' to the filter could be useful and I guess that this could be the answer to my question (1) ?
I supose that the question now is " balancing the potentiall benefit of gathering small amounts of air from the filter against the benefits of a simpler system and having a 'proper' bleed valve"
I rest my case! ...... er no: I dont have a case but would like to listen to some more advocacy!!!
 
pcatterall said:
My problem was that my filter looks to be a standard CAV filter but with the bleed valve hole being taken by the return connector, I found bleeding via the connector difficult.
Click to expand...

Is this CAV filter the final filter on the engine? CAV filters are usually primary filters. Isn't there a fine filter?
 
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