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DPA pump Back Leakage connection ---To filter or tank?


JohnB

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I have just relocated the final filter fuel on our Perkins 3152 engine (to make for easier access).

 

Whilst re piping it looks as though it would be possible to send the Back Leak from the pump directly to the fuel tank, along with the spill return, but still keeping the air vent on the filter into spill return pipe.

I notice the pipe work on the engine, like most, doesn't follow what would be the ideal route (always rise to an outlet) however the installation is working fine; except ---- sometimes a filter change takes more than just bleeding at the filter head and the pump also needs attention....

 

I think an easier route for air to clear the fuel system, particularly when doing a filter change, could be arranged but I can find no references as to the ideal arrangement for the back leakage pipe.

 

Does anybody have any ideas or experiences please?

 

John

Edited by JohnB
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As long as you retain the small air/fuel bleed from the top of the filter head back to the tank it does not really matter what you do.

 

However I am trying to work out what the back leak from the injector pump is. There are normally two low pressure fuel pipes on DPA pumps. One is the main feed from the filter/lift pump and the other is a return for any excess fuel delivered from the transfer pump inside the injector pump. This is a high volume flow and normally returns to the filter via a non-return valve. I would be very reluctant to modify this. If you do the lift pump will probably never idle and the high volume return to the tank may aerate/foam the fuel.

 

I do not know what Loddon is talking about because I have yet to see a Beta with a DPA pump, but who knows.

 

DPA pumps are definitely not designed to be self bleeding, unlike more modern designs so do need care with the bleeding of the filter. (and injector pump). I do not think that you will alter this by modifying the pipe that returns to the filter. Make sure any banjo bolt in the filter head has its 0.5mm bleed hole clear. make sure the NRV in the filer head for the return is not blocked and working.

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Thanks Tony, that all rather confirms what I was thinking and is very helpful.<creep>

I think there is no NRV present on this setup I will see what I can I can do. I do not anticipate this ever being self bleeding.

 

A bit of background, for you; and to help others who may pick up on this thread.

 

This engine was marinised from an ex-ministry unit by a professional vintage engine restorer and there were a few things to sort out.

Anyhow it was a good learning experience for me so I won't complain but I guess you can see how I am approaching this. Once sorted the whole setup has proved so ideal I would be tempted to declare it the standard :-)

 

Much of my understanding of all this is based on some lectures by yourself and a couple of relevant CAV instruction books.

Interestingly the priming instructions in the instruction book suggest only the filter head should need bleeding and it also shows two arrangements for what it refers to as the back leak connection of the fuel system.

 

I originally moved the filter because the vent was *&%!! difficult to get at and the centre bolt was impossible to remove so the "O"ring could not be replaced at filter change time!

This work had the added bonus that the fuel pump itself did not then need bleeding after a filter change. Just like the book says; result!

 

John

 

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The NRV (or possibly a PRV) is located in a hexagon screw in fitting in the side of the filter head where an arrow points into the filter. The pipe union screws into this and unless you know what it is it is easy to mistake for a simple pipe union. The other unions in the filter head have thin hexagons. this one has along hexagon and the the thin pipe union hexagon.

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Thanks, I know what you mean.

There is in fact such a valve its in the out feed to the thermostart device (bonfire plug) though in fact that seems to be non-functional and fortunately not needed :-)

 

John

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It sounds as if someone may have re-piped the filter. The thermostart should, I would have thought been connected to a plain union adjacent to an out arrow on the filter head. The "normal" NRV or PRV is likely to stop it working as no fuel could reach it when the bimetallic valve opens on heat.

 

I would change it to beside an IN arrow and connect the return from the DPA to it.

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I think that is very probable.

The pipework indeed seemed rather odd. The connection to the pump from the final filter swept round "in an air trapping loop" it could easilly have had a better route from the other outlet on the filter head.

I guess you can see why I am doing this work on the engine!

 

In fact this filter head was the agglomerator when we got the engine, made redundant because I had fitted chassis mount agglomerator and sedimentor before the engine. ( Now where did I get that idea from ? :-)

I also understand that these cyclone type devices don't work as they should when vibrated mounted on engines....

 

I shall test your suggestions next.

Thanks

John

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Further investigations reveal the NRV is in the right direction for flow out of the filter head. I have no sense of what was intended....

(as a side point, only relevant to access issues though, I am starting to suspect the exhaust manifold has been fitted the wrong way round!)

 

Anyhow I think I can re-pipe this lot sensibly and indeed so that the back leak doesn't drain back.

 

The problem now is getting fittings with the right thread " You always used to get them at the Lucas depot!"

 

But I have a plan and access to a lathe ----- I'll report on my efforts.

 

Best regards

John

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So----- Having at, last!, got fittings made and installed, discovered the back leak on pump outlet was a NRV . I piped every thing up to the theory. to avoid air traps.

 

Here goes; turned fuel on ----Opened vents on agglomerator and filter pumped the primer and closed the vents as the fuel flowed out.

What next! all the pipes had been changed and empty, should I, I wondered do some bleeding at the pump?

 

Give it a try! wound speed wheel open, pressed start and away it ran as sweet as could be RESULT.

Never had it work like that before .....

 

 

Thank for advice and encouragment

John

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