Fuel Pressure issue

[eison]

I have the following in my rv 10 fuel system. Left and Right tank pickups go to the Andair Duplex fuel valve. From there to EFII fuel filter and EFII Auxiliary pump. From there to the firewall, then a line to the mechanical pump, the mechanical pump outlet T's to a Garmin pressure transducer on the firewall while the main line goes to the input for the Airflow FM 150 throttle body. From the throttle body, the metered fuel goes between cylinder 3, 5 and enter the red cube immediately before going into the fuel spider. Engine is factory new with 10 or so hours TT. Fuel Lines are fire sleeved and from TS flightlines.

When I am flying with the aux pump, I have 30+ PSI and everything runs fine. If I shut the aux pump off PSI quickly drops below 15 with no sign of stopping. I have repeated these results with both tanks.

I have already replaced the mechanical fuel pump with a new one and have the same symptoms. I made sure to have the pin Raised and was able to put the pump in by hand without forcing it. I also took the magneto off and was able to put a scope down and am pretty sure that I saw the pin resting on top of the Fuel pump lever.

Prior to starting the engine with the new fuel pump, I capped the line going into the throttle body and pressurized it with the aux pump. Although it showed a slight leakdown of tenths of PSI per minute, 12 hours later the line still had enough pressure to squirt everywhere when I took the cap off of the line, leading me to believe that there is no air suck anywhere between the Aux pump and the Throttle body.

I am struggling to come up with my next steps to troubleshoot. Is it possible that the pin that drives the fuel pump lever is not actually moving? It seemed to move fine when I replaced the pump, but I didn't try and rotate the engine to see how it moved. I am hoping the answer isn't taking the pump off again....

All thoughts are appreciated.

Eric

 

[drinkwater]

I can't help directly, but I think the first question will be to confirm the model number of the mech pump to ensure you have the high pressure version, not low pressure for carb.

What happens when you run on the ground with just the mech pump?

Given this seems to not be an indication issue, I don't think you should be flying until it's sorted...

 

[Walt]

The mechanical pump traps the fuel pressure going forward, the inlet will have no trapped pressure.
Is this a new install or a new problem?

 

[eison]

The mechanical pump traps the fuel pressure going forward, the inlet will have no trapped pressure.
Is this a new install or a new problem?

It is a new install and a new problem. All new components. I did verify that the replacement and the original Mechanical pump were the high pressure variety.

Eric

 

[eison]

I can't help directly, but I think the first question will be to confirm the model number of the mech pump to ensure you have the high pressure version, not low pressure for carb.

What happens when you run on the ground with just the mech pump?

Given this seems to not be an indication issue, I don't think you should be flying until it's sorted...

I will do a better ground run test tomorrow at a higher power setting to see how it acts on the ground under a higher load.

 

[N890GF]

For what it’s worth, my mechanical fuel pump pressure at idle (750-800rpm) on my IO360 is 24-25psi and 34psi with the boost pump on. My fuel flow is at a similar location right before the spider.

 

[lr172]

I have the following in my rv 10 fuel system. Left and Right tank pickups go to the Andair Duplex fuel valve. From there to EFII fuel filter and EFII Auxiliary pump. From there to the firewall, then a line to the mechanical pump, the mechanical pump outlet T's to a Garmin pressure transducer on the firewall while the main line goes to the input for the Airflow FM 150 throttle body. From the throttle body, the metered fuel goes between cylinder 3, 5 and enter the red cube immediately before going into the fuel spider. Engine is factory new with 10 or so hours TT. Fuel Lines are fire sleeved and from TS flightlines.

When I am flying with the aux pump, I have 30+ PSI and everything runs fine. If I shut the aux pump off PSI quickly drops below 15 with no sign of stopping. I have repeated these results with both tanks.

I have already replaced the mechanical fuel pump with a new one and have the same symptoms. I made sure to have the pin Raised and was able to put the pump in by hand without forcing it. I also took the magneto off and was able to put a scope down and am pretty sure that I saw the pin resting on top of the Fuel pump lever.

Prior to starting the engine with the new fuel pump, I capped the line going into the throttle body and pressurized it with the aux pump. Although it showed a slight leakdown of tenths of PSI per minute, 12 hours later the line still had enough pressure to squirt everywhere when I took the cap off of the line, leading me to believe that there is no air suck anywhere between the Aux pump and the Throttle body.

I am struggling to come up with my next steps to troubleshoot. Is it possible that the pin that drives the fuel pump lever is not actually moving? It seemed to move fine when I replaced the pump, but I didn't try and rotate the engine to see how it moved. I am hoping the answer isn't taking the pump off again....

All thoughts are appreciated.

Eric

If the pushrod was not in alignment, you wouldn’t get any pressure. I would consider the remote possibility that the new pump is bad.

Has it always performed like this or did the mechanical pump used to work fine?

Another possibility is the check valve in the aux pump is too tight. That aux pump needs to free flow when off, otherwise it will make too much of a restriction and lower the performance of the mech pump. You sure it is an actual aux pump and not their efi version? The pumps ued in EFI systems don’t need a pass through ability, but those used as an aux pump with a mechanical pump do.

 

[TS Flightlines]

Eric---start at the tanks and check EVERY nut for torque/tightness. You shouldnt have a duplex valve in a mechanical system, because you dont need to return fuel to the appropriate tank. If all of that is good, get a known good mechanical pressure gauge, plumb it into discharge side of the mechanical pump, and verify the pressure is the same that you are reading on the EFIS. Go from there.

 

[dmattmul]

I have the following in my rv 10 fuel system. Left and Right tank pickups go to the Andair Duplex fuel valve. From there to EFII fuel filter and EFII Auxiliary pump. From there to the firewall, then a line to the mechanical pump, the mechanical pump outlet T's to a Garmin pressure transducer on the firewall while the main line goes to the input for the Airflow FM 150 throttle body. From the throttle body, the metered fuel goes between cylinder 3, 5 and enter the red cube immediately before going into the fuel spider. Engine is factory new with 10 or so hours TT. Fuel Lines are fire sleeved and from TS flightlines.

When I am flying with the aux pump, I have 30+ PSI and everything runs fine. If I shut the aux pump off PSI quickly drops below 15 with no sign of stopping. I have repeated these results with both tanks.

I have already replaced the mechanical fuel pump with a new one and have the same symptoms. I made sure to have the pin Raised and was able to put the pump in by hand without forcing it. I also took the magneto off and was able to put a scope down and am pretty sure that I saw the pin resting on top of the Fuel pump lever.

Prior to starting the engine with the new fuel pump, I capped the line going into the throttle body and pressurized it with the aux pump. Although it showed a slight leakdown of tenths of PSI per minute, 12 hours later the line still had enough pressure to squirt everywhere when I took the cap off of the line, leading me to believe that there is no air suck anywhere between the Aux pump and the Throttle body.

I am struggling to come up with my next steps to troubleshoot. Is it possible that the pin that drives the fuel pump lever is not actually moving? It seemed to move fine when I replaced the pump, but I didn't try and rotate the engine to see how it moved. I am hoping the answer isn't taking the pump off again....

All thoughts are appreciated.

Eric

Maybe I’m not understanding the plumbing but users with a duplex valve typically have a fuel pressure regulator to insure the fuel pressure stays constant. Do you have a return line to the tank ? I’m assuming you are using the EFII pump module ? Maybe include a simple schematic of your fuel piping ?

 

[Walt]

This is the Efii module for Bendix / AFP injection, is this what you have?
Why do have a duplex valve? Do you have a purge valve? (this would be the only reason for a duplex valve)

 

[eison]

Eric---start at the tanks and check EVERY nut for torque/tightness. You shouldnt have a duplex valve in a mechanical system, because you dont need to return fuel to the appropriate tank. If all of that is good, get a known good mechanical pressure gauge, plumb it into discharge side of the mechanical pump, and verify the pressure is the same that you are reading on the EFIS. Go from there.

I think I may have misspoke. I have the andair valve that is plumbed for left right center. 20x7T I believe it is. I said duplex assuming that meant two tank valve. There is no return fuel line.

Eric

 

[RV7 To Go]

You didn't mention the engine but assuming it is a Lycoming there are 3 different pumps, a low pressure, medium pressure and high pressure. The mid pressure is about 15 psi. Since you get 30+ with the boost pump on the transducer seems ok. I would be looking at the pump.