Low Fuel Pressure When Tank Low With High Power Setting

[rpigeek]

We’ve been trying to diagnose and fix a low fuel pressure that we see as the fuel tank gets low (i.e 5-6 gallons). As we get that low if we are operating at 5,500 RPMs we’ll drops in the fuel pressure every couple minutes to less than 2PSI and sometimes even 0.9 PSI range. If we throttle back (i.e. 5000 RPM) the fuel pressure will increase to 2-3 PSI. When we add more fuel to the tank it we don’t have the problem except on steep climbs. I’ve replaced the fuel pressure sensor (just in case) and also replaced to the latest mechanical fuel pump. This is an older RV12 with a 912 ULS. Turning on the electric fuel pump helps, but doesn’t remove the drop we’re seeing.

Any recommendations where to look next? I was going to start to go through the fuel system to look for blockages.

 

[sailvi767]

Sounds like a slight blockage somewhere in the system. When the tank is full you have more head pressure helping with flow.

 

[Avanza]

What happens is that when the fuel level in the tanks gets lower, the static pressure from the fuel decreases.
This causes bubbles in the fuel line at high fuel flow.
Make sure fuel filter is clean before doing anything else.
To correct this problem you need to take a look at the fuel line from the tank to the pump.
Remove all 90 deg fittings in the fuel line (if any) as the pressure drop are high in them.
Replace with SS braided hose or tubing that is less restrictive in terms of pressure drop, larger radius / inner diameter.
Start with the "worst" restriction that you find. Then test fly and move on if problems is still there.


Good luck

 

[TS Flightlines]

What happens is that when the fuel level in the tanks gets lower, the static pressure from the fuel decreases.
This causes bubbles in the fuel line at high fuel flow.
Make sure fuel filter is clean before doing anything else.
To correct this problem you need to take a look at the fuel line from the tank to the pump.
Remove all 90 deg fittings in the fuel line (if any) as the pressure drop are high in them.
Replace with SS braided hose or tubing that is less restrictive in terms of pressure drop, larger radius / inner diameter.
Start with the "worst" restriction that you find. Then test fly and move on if problems is still there.


Good luck

First thing I would do is verify the low pressure with a known good fuel guage. Duplicate the issue and see if the readings match. Most probably will. So do you get power loss and engine stumble, or just low readings? The RV12 has alot of bent tubing in the fuel system, but if that were the issue, you would thinnk this problem would have been there from the beginning. Now, you do have the boost pump and the flow transducer in line, and if there was some debris, the passages, 'could' have become restricted. I somewhat doubt that. So I'd check all the connections for tightness, especially on the suction side. We are assuming that this issue has just started and everything was operating fairly normal until this point. That eliminates alot of stuff. So were there any changes made just prior to the anomaly?
Tom

 

[glongpilot]

I would suggest this test listed in section G4 of the Performance Acceptance Procedure available here https://www.vansaircraft.com/wp-con...V-12-Production-Acceptance-Procedures-R14.pdf. An excerpt here:

Gascolator testing:
❑ Add 4 gallons of fuel to the tank.
❑ Move fuel valve to ON position.
❑ Turn master switch "ON". Verify no fuel leaks. Turn master switch "OFF".
❑ Disconnect fuel line at gascolator outlet.
❑ Slide a piece of 5/16 inch (8mm) inside diameter rubber hose over the gascolator outlet fitting
and feed into a bucket at waist height.
❑ Charge battery to full. Voltage should read at least 12.7v.
❑ Turn master switch "ON", reduce load on the electrical system by turning off strobes and nav
lights, dimming screens, etc. Turn fuel valve "ON".
❑ Time from master switch on until 1 gallon has emptied into bucket: ________________ (Max
Time: 180 seconds)
❑ Re-connect fuel line at gascolator outlet.
❑ Filter fuel and put back into the fuel tank.

This test verifies adequate fuel flow that the engine needs using only the electric pump and low fuel quantity. If this doesn't pass, then start by cleaning the gascolator page 13-7 in the maintenance manual available here https://www.vansaircraft.com/wp-content/uploads/2022/10/RV-12-MM-R10.pdf. Other items in the maintenance manual may be helpful.

Good luck

 

[Piper J3]

Check to make sure vent hole in fuel cap is unrestricted. Pull electric fuel pump fuse to see if pressure drops. Electric fuel pump should be running 100% of time. I believe there is an upgrade for electric fuel pump for increase pressure.

 

[rpigeek]

Thanks for the ideas and plan to diagnose this weekend

 

[DaveWelch]

Thanks for the ideas and plan to diagnose this weekend

You have received some good suggestions.
Not knowing your background, skills or experience I'll offer some points to consider (you may know these already)...
First, is your RV-12 an ELSA or SLSA? If it's an SLSA, you are restricted as to what can be changed/modified and by whom without Van's-approved design options/changes.
Second, the older -12's had early issues with intermittent fuel pressure "excursions", and if I recall correctly there were some different fuel pumps, sensors and flow meters approved for your plane. If cleaning doesn't help, review all of Van's applicable change documents for possible causes and remedies.
Third, if the original RV-12ULS design was/is causing consistently low fuel pressure across the fleet, one would think a lot of folks would be talking about it and Van's would have issued design modifications long ago to address flow restrictions. Therefore, I'd be cautious about eliminating or changing fittings, etc.
Let us know what you find!
All the best,
Dave