So, I have a similar problem with my Grumman Traveler (O-320) where the fuel pressure without electric pump is around 3-4 psi and with electric pump on it increases rather quickly to 5 and then slowly to maximum of 6 psi. From most planes I flew before I remember that electric pump on or off did not make a difference, so fuel pressure remained stable (and in the middle). I think its clear that I want to have it looked at but at what point should I leave the plane on the ground rather than fly with the lower fuel pressure, even though knowing that the electric fuel pump can make up for it? The plane is quite new to me, so I do not have the full history if it just dropped recently or whether it was always a bit on the lower side.
My Grumman gives similar values…..and has done for many years.
If it is in the green then all is well…imho!
Your POH will give the limitations, my Tiger is 0.5 – 8.0 psi
As a maintenance engineer and a Grumman Tiger owner, I’d second PeteD: no reason to worry.
It’s an electric transmitter/gauge system? If so, have a look at the location of the fuel pressure transmitter. If it’s a crimped closed zinc plated steel “can”, screwed into a hard line of the fuel system, that may be a source of inaccurate readings over time. My O-360 did this. When the transmitter is screwed directly into a T in a hard line, it’ll vibrate with the engine. Depending upon the orientation (and you cannot tell by looking at the outside of the can), this can cause the wiper inside the sender to vibrate and wear. The electric power will take the shortest path, which will be through the worn out edge of the wiper, so no longer the center of the wiper arm where its been calibrated. This was my problem, and a new transmitter solved it. When I cut apart the old transmitter, what should have been a nice pressed dimple on the wiper, to make a single point contact on the wire wound rheostat, instead was worn through, so as to be a hole, with two contact edges touching the rheostat. When I replaced the transmitter, I also remoted it with a flex line, so it was no longer subject to engine vibration. Problem solved, nothing to do with pumps.
2 – 5 psi (depends on the day) on a O-360-A3A. Mechanical pump is 15 years old…
This is after switching either elec pump off (after TO to before LD), which usually maintains 5 – 6 psi.
Ok, this definitely lifts my trust in my airplane. Thanks everyone for sharing!
I will also try to find the transmitter next time I open the cowling, let’s see how that goes 
I had a long saga over this with my -161 starting when I noticed low fuel pressure half way across the Baltic. Replacing (one at a time) both engine and electric pumps, and cleaning all the fuel lines, etc. etc., made little difference. It was only after further replacing both pumps simultaneously that normal service was restored, at some cost. Of course all the replaced pumps were working normally.
I concluded that the fuel system design is inherently marginal and that any small discrepancies in either pump can lead to a big drop in fuel pressure. Maybe one pump leaks back past the other? Ten years later, it hasn’t happened again (yet).
As others have said or implied, it’s normal for the two pumps to pressurize the carb supply line to a slightly different pressure. It is also inconsequential because the supply pressure (unless it’s really very low) will have no effect on how the engine runs. High wing planes use gravity flow only to do the same job, with the supply line almost unpressurized except for the head of fuel from tank to carb.
The fuel system is very simple and unfussy by design.
When I fitted a jpi fuel flow sensor to my Grumman, the STC required a fuel flow check.
I forget the exact figures but the fuel flow achieved and measured was 2 to 3 times max fuel required by the engine.
My understanding is that a carburettor requires flow, not pressure.
In checking a fuel pressure gauge in flight, I am looking for a deviation from what I usually see (as a proxy for a possible problem with flow) rather than any specific value.
These engines run fine in gravity-fed applications, and indeed when testing a modification to any gravity-fed system it is a check of flow that is performed. I believe the usual metric required is a flow of 150% of the engine’s maximum rate of consumption.
NickL wrote:
Turning over the engine on the starter with the fuel pipe disconnected at the carb produces a modest fuel flow – but beyond that I have no idea of how to quantify it.
You could probably measure this. With gravity systems one normally measures flow for one minute and multiplies by 60, but you probably don’t want to crank the starter over for one minute. 15 seconds should be sufficient – collect the volume of fuel it pumps during this time, multiply by 240 and compare to the specified maximum consumption per hour. When running it will actually flow a lot more than this, since it pumps X volume per rpm and obviously running rpm is considerably higher than cranking rpm. You could probably multiply on an additional factor to account for the difference in speeds.
Pressure is only really required for racing automotive applications where you need a bit of pressure in the line to keep the float bowl full when you suddenly boot the throttle. Weber & Dellorto carburettors specify 1.5 – 2.5 psi (not a great deal) and most mechanical pumps can easily supply this.
