Low fuel Pressure in Climb ( Again)

[RFazio]

I know this has come up a number of times. I get low fuel pressure in a steep climb like many RVs. I have an O-360 A1A carbed engine, factory new from vans. I never paid much attention to the low fuel pressure. I have 380 hours on my 6 and it has never missed a beat. On climb out from take off I always have my electric boost pump on and pressure is 4 or 5 pounds. The few times I took off forgeting the boost pump I still have a pound or two and like I said it's never missed a beat. The other day I was fooling around doing a little mild aerobatics. I was in a steep climb and my girl on the dynon skyview came on and said "fuel pressure". I get it a lot, when fooling around in a steep climb. I've seen zero pressure. I hit the boost pump and I heard "fuel flow". I did think anything of it. Then later on a different flight it did the same thing fuel pressure" then after hitting the boost pump I got "fuel flow" I looked down at the screen and saw my fuel flow at 18 or 19 GPH in the red. It never goes above 15 on full power climb outs. Again I didn't think anything of it, But after thinking about it later, I think I know what is happening. The fuel level in the bowl is getting low, and when I hit the boost pump it refilled the bowl with extra high flow rates which prompted the "fuel flow". Something to think about. I don't know how long I would have to climb that steep for it to die but I think it would if I kept going. At the least I'm getting a lean mixture to the engine under full power which is not a good thing. So doing aerobatics from now on gets the boost pump on.

[Russ McCutcheon]

I have seen air in the ling cause this same high fuel flow symptom.

[jdiehl]

I'm only 8 hrs into Phase I, but I've also noticed my fuel pressure drop when I've made some steep climbs and/or steep turns. I don't yet have enough hrs on the plane to make any concrete correlations to cause and effect, but I'll be keeping an eye on the FP gauge.
Otherwise, all is going well.

Jim Diehl #1976
7A
Lock Haven, Pa

[Kevin Horton]

I believe that many of these reports of low fuel pressure indications in a climb are due to the use of fuel pressure senders designed for automotive applications. The fuel pressure sender has pressurized fuel on one side of a diaphragm, and the other side of the diaphragm is vented to the atmosphere. The sender measures the difference between those two pressures.

I believe that many pressure senders designed for automotive use may have diaphragms that are vented to atmospheric pressure through very small orfices. This is OK in a car, which doesn't see rapid changes in altitude. The small orfice size helps keep contaminates out of the sender.

If you are in a climb, this may cause the pressure on the vented side of the diaphragm to respond slowly to the change in atmospheric pressure. The pressure on the vented side of the diaphram would be higher than the atmospheric pressure and the fuel pressure indication would be lower than the actual fuel pressure.

[Sam Buchanan]

Originally Posted by Kevin Horton

I believe that many of these reports of low fuel pressure indications in a climb are due to the use of fuel pressure senders designed for automotive applications. The fuel pressure sender has pressurized fuel on one side of a diaphragm, and the other side of the diaphragm is vented to the atmosphere. The sender measures the difference between those two pressures.

I believe that many pressure senders designed for automotive use may have diaphragms that are vented to atmospheric pressure through very small orfices. This is OK in a car, which doesn't see rapid changes in altitude. The small orfice size helps keep contaminates out of the sender.

If you are in a climb, this may cause the pressure on the vented side of the diaphragm to respond slowly to the change in atmospheric pressure. The pressure on the vented side of the diaphram would be higher than the atmospheric pressure and the fuel pressure indication would be lower than the actual fuel pressure.

This is precisely what I have observed many times over the past 13 years in my carbed RV-6. But if the pressure stays low all the time or varies with no consistent pattern, this could be an indication of a fuel pump failing....