Davepar

Davepar

Well Known Member
I'm getting really close to firing up the (fuel injected) engine for the first time. The last step was to check the fuel flow. I put some fuel in the tank, disconnected the fuel line from the fuel servo, and turned on the pump. It's humming, but nothing is coming out the hose. I tried all positions on the fuel selector. I put fuel in both tanks (about 2-3 gallons each). I compared the flow directions on the filter and check valve with the plans. All looks correct. I checked to make sure I had +12v hooked up to the red wire. I even tried switching the wires.

Any ideas of what to try next? I'm hoping this is something really simple and that my AFP pump isn't bad.
 
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Picture worth a 1000.........

Dave,
Does this help.......... to check the plumbing?
Pete.
fuelpumpflowfr7.jpg
 
You might have a slight air(fuel leak when the pump gets fuel) leak somewhere on the intake side of the pump. When had a couple. My building partner did the flares on the lines from the valve to the pump and he over flared the tubes. He basically tightened the **** out of the flaring tool and it crushed the ends. We fixed these and we had fuel shooting out.
 
Thanks Pete. That's how I have it hooked up. I double checked the connections from the drawing.

After thinking about it some more last night, I think the air leak Todd mentioned is a likely explanation. I'm going to disconnect the fitting at the pump inlet and check for suction. If it's there, I'll keep checking fittings upstream until I loose the suction.
 
Got it working. I took the whole assembly out of the plane and rigged a test stand of sorts.
IMG_2428.jpg


With fuel plumbed directly into the filter, it still didn't work. I moved the input directly into the pump and that's when it finally kicked in. I re-assembled everything and it all seems fine now. It was maybe an air bubble in the pump?

New question: the fuel pressure reading on my EMS says 40psi. Isn't that kind of high? I thought the relief valve was supposed to open up at 25psi??
 
Still Not sure

Dave,
I think you have it plumbed incorrectly.

If you look at my Previous picture you will see I have indicated there should be TWO(2) inputs from the fuel selector which you can see in the new pic below. (Forget all the other lines on the selector. I have tip tanks)

It looks to me as if you are running it backwards through the filter which has two arrows on it for the direction of flow. Marked in BLUE IN THE PIC.

In the picture below, I have marked the TWO Inputs from the fuel selector, one in RED and one in Orange.

The OUTPUT is actually at the aft end -as it is mounted in the aircraft- and you do not seem to have anything connected to it.

I know it is a puzzle and I did sit down and work out the flows. There is two bypasses. One for a blocked Fliter and one for the pump.

I shall see if I can find the drawing I did.

Pete.
PS. I think that is why you are seeing more than 25psi.

fuelpumpflowxd6.jpg
 
Filter Bypass?

Which AFP Fuel System Schematic are you guys using? My manual is about 8 years old, and it shows 14 different configurations. Schematic 1 is preferred for RV's, unless the manual has been changed.

Pete, is your intention, when the boost pump is not running, that the fuel does not go through the filter? Do you have another filter downstream?

Forgive me if my memory is wrong: The blue cylinder is a check valve. It allows the engine pump to pull directly from the fuel selector (almost straight through, just a little jog in the black manifold). The gold cylinder is a pressure relief valve, which sets the pump's output pressure. I believe the red arrow you have drawn on the relief valve (gold cylinder) is going the wrong direction. Fuel volume pumped by the constant volume pump, which is not used by the engine, is going from right to left through the relief valve (gold fitting) in your picture, ending up back in the pump inlet. It takes about 25 psi to push fuel through the relief valve, hence setting the output pressure.

I will copy your picture, Pete, later today and try to draw some more arrows.
 
Thanks for all the guidance Pete. I actually have mine hooked up just like you show in your picture. Looking at the picture I posted above, there's a bit of an optical illusion. I didn't hook up the pressure relief line (blue cylinder) in my temporary setup. The line going into the gas can is hooked up to the output of the manifold.

My understanding of the assembly is that there is no filter bypass. There is a pump bypass. When the electric pump is off and the mechanical pump is running, the fuel will be drawn through the check valve, around the pump. But it still goes through the filter

And I think fuel runs forward through the blue pressure relief valve. Once the pressure hits 25 psi, that valve is supposed to open and recycle fuel back to the input of the assembly (or more ideally back into the tank).

I thought the regular set up was complicated. Looks like you did a lot of tube bending for the tip tank set up!
 
fuel flow

Pete,

I think that the fuel flow arrows in your diagram may be off in one respect. As Dave mentioned, it is my understanding that the blue cylinder is an overflow valve that sends unused fuel back out of the pump to a place where it can be re-introduced into the fuel flow back into the pump.

The plumbing depicted in your diagram is correct I believe, just the fuel flow and function may be different. I don't think that I would want the fuel to bypass the filter when the pump was not running.

Just my thoughts and understanding, I do not claim to be an expert.

Tony
 
I satand Corrected

Alex, Dave and Tony,
Glad for your input here because I am making a lot of assumptions simply from the way it is plumbed.
My assumption that there was a filter bypass is obviously wrong.

You all seem to agree that the Gold and Blue Valves are a Check and a Relief valve, although you have different favorite colours.

The following seems the best deduction from your inputs.
The Fuel ALWAYS flows through the Filter.
The Blue Valve is Pressure Relief Valve.
The Gold Valve is a Check Valve that,;
When the Pump is OFF, allows fuel (drawn by the Engine Pump) to flow as indicated by the RED arrow.
When the Pump is ON is held closed by high Pressure from the pump
in the outlet manifold.

Pete.


pumpconsensusfi8.jpg
 
Confirmation

Roy,
Thanks.
That confirms what we arrived at after a little discussion.
Pete.
 
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