Perkins 4236 No Obvious Fuel Return and Twin Tanks

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HI,

I have twin fuel tanks at 2 x 60 gallons each and a Perkins 4236 Engine. I cant find the fuel return from the engine or any indication of a fuel inlet to the tanks: fuel outlet, diesel heater outlet, vents and filling inlets. The owner's manual for the engine and the workshop manual do not explain much about the fuel system.

I am planning on taking one tank out of service as I just don't need that amount of fuel onboard. The tanks are connected together and can be isolated, used singly or in parallel. Obviously if the fuel returns just to one tank then I need to keep that tank in service.

My question really is about excess fuel return, could this engine be fitted without a fuel return? It has what appears to be a standard diaphragm cam activated lift pump supplying the remote filters, final filter and injection pump. However, there is no return line to the tank for excess fuel. Could the lift pump just pump against a closed system or have some bypass feature, maybe the diaphragm just stretches when the fuel as no where to go?

The final fuel filter on the engine has 4 ports. On the injector pump side the one having an arrow pointing in, the other arrow pointing out are connected into the injector pump, the other side has the out port blocked off and the in port connected to the fuel filters down stream of the diaphragm pump.

Any advice or guidance would be appreciated. Should I look more closely for a fuel return line to the tank(s).

Thanks,

BlowingOldBoots
 
Given we have essentially the same boat I can answer for mine but it was re-engined (with a Nanni 4.150) before my ownership. I do have a return system which is obvious in the engine room, I can even select which tank the return goes to so I can draw from one and return to the other (it was surprising how quickly I could drain a tank doing that :o)

What surprises me a bit with your question is that my return tank fittings dont look new, the pipework could be any age. The returns go in at the top front, inboard corners iirc (as I'm cruising on her its a bit hard to check because of all the stuff stored around the accesses). btw, my weeping tank was fixed with POR15 sealant painted in through the tank access hatches (it was only the weld on the lowest corner that had gone) - the port tank would have been a b*tch to remove, engine out at least - so far the fix is working well and I plan to do the other tank this winter
 
BlowingOldBoots said:
The final fuel filter on the engine has 4 ports. On the injector pump side the one having an arrow pointing in, the other arrow pointing out are connected into the injector pump
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I don't understand the second sentence. But it sounds like only one of the four ports is blanked off. Normally there would be only one "in" and one "out". Could your third line be the return from the engine?

Pete
 
prv said:
I don't understand the second sentence. But it sounds like only one of the four ports is blanked off. Normally there would be only one "in" and one "out". Could your third line be the return from the engine?

Pete
Click to expand...

Thats a distinct possibility, I have a similar sounding primary filter and tried feeding the return back through the filter - didnt work, kept injecting air into the system
 
The CAV filter has 4 connections 2 input arrows point in and 2 outputs arrows point out.

It looks to me that there is 2 connections from the injector pump to the filter and one from the lift pump to the filter.

The lift pump connection should be to an import port arrow inward.

The feed to the injector would be from an out port on the filter and if the other connection from the injector pump goes to an iput port arrow pointing inward that would be the return. This should go back to the tank. This way saves a return feed valve to return the fuel to the correct tank. The problem with this arrangement is that the injector return line is under pressure from the lift pump. This line would also have a connection to the top of each injector for excess fuel bleed off. This could also effect the correct operation of the injectors.

This was how my engine was setup but changed it to feed back to my tanks through a return selector valve to return to the correct tank as I have 3 tanks.
 
prv said:
I don't understand the second sentence. But it sounds like only one of the four ports is blanked off. Normally there would be only one "in" and one "out". Could your third line be the return from the engine?

Pete
Click to expand...

Yes, 3 out of the 4 ports are in use, one is the inlet from the diaphragm pump side, with the adjacent port blanked off, the other two are plumbed into the injection pump. On the top of the filter housing the ports have arrows indicating the direction of flow. The two ports that are plumbed to the injection housing have a inlet and outlet arrow, respectively. I am assuming that the one with the inlet is the excess fuel return to the filter.

The reason I ask, is that every thread that I read on fuel flow mentions returns to tanks. I guess the question is quite simple, does a diesel engine, of this ilk have to return excess fuel to a tank, or can it just spill back excess fuel to a filter housing which acts like a low tech accumulator.
 
Rogershaw said:
The CAV filter has 4 connections 2 input arrows point in and 2 outputs arrows point out. .....
Click to expand...

Okay, I will need to check housing again but I am sure that the ports which are side by side have opposing arrows. I am now doubting myself.
 
Yes, you can indeed take the return to the filter inlet or indeed cloe the circuit downstream of the filter. My little 14hp is plumbed in that way with the return going straight back to the injector pump. I would guess this system was chosen for your installation to avoid running one tank dry whilst the other overflows from the return flow. I've heard mutterings about this closed circuit being "a bad thing", but no-one has so far explained why - so I'm looking forward to hearing the arguments...

Rob.
 
rob2 said:
Yes, you can indeed take the return to the filter inlet or indeed cloe the circuit downstream of the filter. My little 14hp is plumbed in that way with the return going straight back to the injector pump. I would guess this system was chosen for your installation to avoid running one tank dry whilst the other overflows from the return flow. I've heard mutterings about this closed circuit being "a bad thing", but no-one has so far explained why - so I'm looking forward to hearing the arguments...

Rob.
Click to expand...

As I posted it caused a slight pressure in the return line that can effect the operation of the injectors an injector pump. If the return is feed into the inlet of the lift pump that problem would not occur as that side would be under a slight vacuum if the tank was below the pump level.

BOB " ports which are side by side have opposing arrows "

I think that is correct sorry to confuse you. Its the arrow direction that important not the position on the filter
 
BlowingOldBoots said:
Okay, I will need to check housing again but I am sure that the ports which are side by side have opposing arrows. I am now doubting myself.
Click to expand...

http://www.ybw.com/forums/showthread.php?196748-CAV-filter-connections-fuel-bleeding-problem Post No 25 in the linked thread has a picture of an identical filter top to mine. It is as I described (badly).

rob2 said:
Yes, you can indeed take the return to the filter inlet or indeed cloe the circuit downstream of the filter. My little 14hp is plumbed in that way with the return going straight back to the injector pump. I would guess this system was chosen for your installation to avoid running one tank dry whilst the other overflows from the return flow. I've heard mutterings about this closed circuit being "a bad thing", but no-one has so far explained why - so I'm looking forward to hearing the arguments...

Rob.
Click to expand...

I sort of assumed this Rob because I have never found a return line, despite looking. There are old diesel heating lines glassed in which in my early years of owning this boat I thought might be part of a return system but they have been decommissioned and closed off. Thanks for your comments.
 
Rogershaw said:
As I posted it caused a slight pressure in the return line that can effect the operation of the injectors an injector pump. If the return is feed into the inlet of the lift pump that problem would not occur as that side would be under a slight vacuum if the tank was below the pump level.
Click to expand...

Actually the tank is above my lift pump and I believe from memory that the return line is fed into a tee by the secondary filter. It may well be that it is a less than optimum configuration, but it seems to work. I would guess that this was preferred to returning to the primary (CAV) filter as that could be problemattical if the return flow were greater than the engine's consumption - it would be trying to force fuel back into the tank.

Rob.
 
One function of the excess fuel to the injectors is cooling, if the return fuel isnt cooled or returned to the tank then the closed system downstream of the filter may get hotter than is good for the engine.
Another benefit of excess fuel to the injector pump is that the return is situated such that a small amount of air can be removed from the supply at that point, might be what RobbieW experienced here http://www.ybw.com/forums/showthread.php?436225-Perkins-4236-No-Obvious-Fuel-Return-and-Twin-Tanks&p=5380630#post5380630 Not so much injecting air as not removing the small amount of air that was already present.


https://www.dieselnet.com/tech/diesel_fi_comp.php
Fuel pumps are commonly sized to deliver more fuel than is consumed by the engine at any particular operating system. This extra fuel flow can serve a number of important functions including providing extra fuel to help to cool injectors,
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if you close the system downstream of the pump, you negate any of those "important" functions.
 
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