What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Low fuel pressures

Flyingnorth

Flyingnorth

Member
Greetings:
Any words of wisdom on the following appreciated. My RV 6 has a 0-360. On a past flight, the fuel pressure dropped to .5 from 4 during climb to 7500. I turned on the boost pump, changed tanks. Pressure returned 4-5. I turned boost off, pressure slowly dropped to .5 again, engine has purred along all this time. I leaned the engine and fuel pressure went to 2. It stayed there till 3000, then returned to 4. The engine fuel pump was changed recently due to leak out the overflow line indicating failure of diaphragm. However the low pressure had occurred once before. I had wrapped the lines and red box, after changing fuel pump.
Not showing any obvious fuel leaks.
Looking at all the low pressure threads, I’m still scratching my head.
Comments appreciated.
 
Assuming a carburetor and a fuel redline pressure of ~6 psi?

Configuration details?

What were old FP numbers at similar conditions with the old pump (when healthy)?

The more details you provide, the quicker you'll get meaningful responses.
 
Last edited:
Fuel pressure

As I understand it you have a carburated engine.
The pump AF15472 is rated 4-6 psi.
It means that the pump can increase the pressure between 2-6 psi depending
on the fuel flow. Low flow will produce higher pressure. When flow increases
the pump can´t keep up the full pressure.
The pump pulls fuel from the tank causing a negative pressure in the suction line typically -2 psi and deliver a positive pressure of about 2 psi.
Difference is 4 psi.
The fuel pressure instrument measure the pressure in relation to surrounding air pressure. Indicated pressure will then be 2 psi.
As long as engine runs fine, any reading between 0,5-5 psi is acceptable.

Good luck
 
Last edited:
There are numerous posts about this phenomenon with carbureted engines on our RV’s. They all sound similar. I have experienced the same indications on 5 of my 6 RV’s, all 5 of them carbureted, including my current carbureted O-360 powered RV6. Many of the posters have replaced parts - engine driven fuel pump, electric fuel pump, pressure transducer, fuel lines, etc - all with no remedy. My current RV6 will show a fuel pressure drop to zero during a climb, with no unusual engine indications. I can turn on my Aux (electric) fuel pump and pressure will return to normal, with no change in any engine operating indications, other than the fuel pressure indications, but nothing else changes. If I don’t turn on the boost pump, the pressure drops to zero during a climb, but it makes no difference in engine operating indications and the engine continues to operate normally, with all engine indications staying the same (including EGT’s). I think the previous post explained why.

I turn on my boost pump for takeoff, and for landing as a back up. Other than that, it’s not needed unless the engine stumbles or quits, which has never happened for me, even when the fuel pressure indicates zero. In level flight, I’ve never seen zero.
 
Climb out with a carburated engine, without flicking on the boost pump, will get your attention more often than not.

Just my experience
 
caryr said:
Climb out with a carburated engine, without flicking on the boost pump, will get your attention more often than not.

Just my experience
Click to expand...

Yep, my experience too. I’ll usually flick my pump on during a climb just to keep from getting the “fuel pressure” warning from my EFIS.
 
I put in a new fuel pump 1 year ago on my 0-360. I get low FP on taxi and the engine stumbles. Obviously, I am near idle 700-900 rpm. It idles fine when sitting before take off without pump.

At higher RPMs this has not been a problem.

My AFS EFIS gives me a warning at around 2.5 or 3.0 psi about time it sputters. I have never seen 1.0 or .5 PSI.

The sputtering on the ground at low RPM worries me a bit. I was considering it might be need to be replaced ? It is only a year old and no in flight trouble.
I use my aux pump at take off and landing. I worry it is masking a problem.

Thanks
 
Had this exact problem today, low fuel pressure warning on climb out with no apparent impact on the engine, pulling about 2200 RPM and 14.8 GPH (Catto fixed pitch). OAT around 85 degrees. Also occurred at the other end of the spectrum during AoA calibration of the AV-30 I just installed, 6000ft, idle RPM (approx 1000), clean configuration, about 78 MPH and high AOA (close to full stall onset).
Fuel pressure dropped to 0.8 psi and went to 3 with the boost pump. Normal range without the boost pump when settled down is 2.5 to 3.

Normally I only see this in the winter on first flight after a cold soak all night.

Trying to understand the physics noted in the prior posts to see if I need to be watching the pump :confused:
Figs
 
Do the reverse- check your fuel pressure at altitude, then descend at 1000fpm with the boost pump off. If you see an increase in pressure, consider what might cause both symptoms…. An automotive based fuel sender that isn’t vented well enough to keep up with atmospheric pressure and gives the sensor an erroneous relative ( atmospheric) pressure baseline.
 
Scott Hersha said:
There are numerous posts about this phenomenon with carbureted engines on our RV’s. They all sound similar. I have experienced the same indications on 5 of my 6 RV’s, all 5 of them carbureted, including my current carbureted O-360 powered RV6. Many of the posters have replaced parts - engine driven fuel pump, electric fuel pump, pressure transducer, fuel lines, etc - all with no remedy. My current RV6 will show a fuel pressure drop to zero during a climb, with no unusual engine indications. I can turn on my Aux (electric) fuel pump and pressure will return to normal, with no change in any engine operating indications, other than the fuel pressure indications, but nothing else changes. If I don’t turn on the boost pump, the pressure drops to zero during a climb, but it makes no difference in engine operating indications and the engine continues to operate normally, with all engine indications staying the same (including EGT’s). I think the previous post explained why.

I turn on my boost pump for takeoff, and for landing as a back up. Other than that, it’s not needed unless the engine stumbles or quits, which has never happened for me, even when the fuel pressure indicates zero. In level flight, I’ve never seen zero.
Click to expand...
Thanks, boost pumps on for take off and landing through out 20+K hours😃
 
As an update I had the old sensor (2007) connected to the D-120, and replaced with the newer Kavlico sensor and problem solved. maybe the old bellows type sensors age out.
Figs
 
You must log in or register to reply here.
Back
Top