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Low Pressure Fuel Pump


Guest danny_samb

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Guest danny_samb

Hi all I'm running a facet cube low pressure fuel pump to feed my swirl pot. But my high pressure pump is emptying the swirl pot quicker than it being filled so I'm getting fuel starvation. I have a inlone fuel filter before the low pressure pump could this be restricting the flow. I also think the pump could be slightly to far away from the tank. Any other suggestions or suggestions on different pump to fill the swirl pot.

Thanks

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Guest peter2b2002

Do you know the output of the low pressure pump it should match the out put of hi power pump, most fuel filters have or xhould have little effect on the flow

Peter2b

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Wrongly plumbed. The low pressure fuel pump needs to replace fuel used by the engine. Unused fuel from the fuelrail should return to the middle of the pot. LP pump should enter mid pot also but below fuelrail return. Air or excess fuel delivered by the LP pump should return to the tank from the top of the pot. HP pump draws from the bottom of the pot. Drawing below.

I run the cheapest facet copy to fill the pot. Has about one third the flow of the HP pump.

 

Nigel

post-21-0-02260600-1429915376_thumb.jpg

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Guest danny_samb

I'm sure that's how I have mine plumbed. I know I definitely have the high pressure pump at the bottom and the fuel rail return at the top. I'll check the feed to the pot and the return from the rail. How high is your low pressure pump above the outlet on the fuel tank

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I know I definitely have the high pressure pump at the bottom and the fuel rail return at the top.

From bottom to top of the pot you must have four connectors. Bottom is fuel out to HP pump. Next up is LP fuel in. Next is fuelrail return. At the top it's tank return/air bleed out. This last is critical. It ensures no air stays in the pot. The pot can only be filled up to the level of the tank return. The pot above this point will always just contain only air. If you have plumbed it correctly then the LP pump failing, blocked filter or blocked pickup in the tank are the only options I can think of.

My facet like pump is 8" above the top of the tank.

 

Nigel

Edited by Longboarder
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Are you using the Facet-supplied inline filter that screws into the low pressure pump? It's a bronze-looking tapered cylinder, screw fitting for the pump end and a push-on hose fitting at the other.

 

If so, DITCH IT. Trust me ;). Their internals block/collapse; replace with a simple see-through plastic replaceable filter somewhere else in the line.

 

edit: not saying that this is necessarily your problem, as they tend to be fine new, so do investigate the other issues first. But just thought I'd chip in with this advice - from experience, I assure you.

Edited by brumster
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I should have stated the obvious. Clamp the LP line between the swirl pot and LP pump. Pull the line off the pump. Add a bit of new tube to the pump outlet and lead it into a bottle. Switch on ignition and observe the pump output. You can even time and measure the output but basically if it's a good steady flow it's OK.

 

Nigel

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Guest danny_samb

I will unplug it out the swirl pot and put the pipe into a jam jar to see the flow. My facet is located about 9" above the outlet of the tank as the outlet as at the bottom of my tank. I'll go check my hoses are correct when I get home

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Guest danny_samb

Just checked my hoses. Bottom of the swirl pot is high pressure pump next 1 up is coming back from the fuel rail. Next 1 up is feed into the pot from the low pressure pump and on the top is return to the tank. On the top of the fuel tank is a t piece when the swirl pot overflow goes to and the other side of the t piece is a breather. Could this be causing me problems too

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Guest danny_samb

That's a pic of my setup. From what you have said my middle 2 hoses are the wrong way around. My high pressure pump is slightly angled so the outlet is a tad higher than the inlet.

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Guest 2b cruising

I will unplug it out the swirl pot and put the pipe into a jam jar to see the flow. My facet is located about 9" above the outlet of the tank as the outlet as at the bottom of my tank. I'll go check my hoses are correct when I get home

 

When fitting a system to an earlier car, the instructions stated low pressure pump ideally below tank.

This was on a carb set up but should make no difference. Suction head takes a lot of efficiency out of small pumps.

Edited by 2b cruising
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Your set-up looks fine to me. Swapping the fuel-rail return to be above the LP feed is not going to make a difference. (The only reason is to deposit warmer returning fuel higher in the swirlpot than colder fuel from the tank so it tends to be returned to the tank rather than sent back to the rail.) So my money is now on something causing a major loss of efficiency on the low pressure feed.

Have you tried a fuel pressure gauge just before the rail to check pressure and confirm drop when running. I did have this suggested once during a rolling road session but it turned out to be the cossie injectors I was using malfunctioning over 4500rpm. Swapping the injectors cured the fuel starvation!

 

Nigel

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Guest danny_samb

It's deffinatley something to do with the low pressure pump feeding the pot. If I turn the ignition on and off a few times it fills the swirl pit. Start the car up and the high pressure pump runs cold but as that fuel gets used up the high pressure pump warms up this is where I'm guessing getting the fuel starvation. Could my high pressure pump be too powerful I'm running a bosh 044. Or does that not matter as the unused fuel out the rail is returning to the swirl pot

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Exactly your last sentence - you have to remember that nowhere near all the fuel pumped by the high pressure pump is used - it's just in a loop back to the swirl pot, with the pressure regulator holding a steady 4bar (or whatever) at the fuel rail. The injectors open on cue and squirt a relatively small amount of fuel out of the circuit into the cylinders. So it's not your high pressure pump being too powerful - everything comes back to the swirl pot anyway (minus a very small percentage of fuel that's actually used by the engine). A fuel injected engine shouldn't really be using any more fuel than a carburettor-equipped one, give or take.

 

edit: I should just say, some high pressure pumps don't like being run dry for long periods of time, so during your testing avoid this if at all possible.

 

So short of a chronic fuel leak I don't think it's a problem with your swirl pot being emptied too quick - more a case of it's not getting filled.

 

Is the low pressure fuel pump on some sort of prime circuit where it only pumps on engine prime but once started it's not running?

No big puddles of fuel under your car :D ?

Do Longboarder's suggestion and check the actual fuel delivery from the low pressure pump over time.

Edited by brumster
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