unstable fuel pressure

jeff beckley · #1 07-30-2007, 07:15 AM

On my RV7 with an O-360 I have been noticing low fuel pressures on climb out.
With the electric fuel pump on I see 4 to 5 lbs which is fine but if I were to turn off the pump then I would see around 1 to 2 lbs until I leveled off then the mechanical pump will show 4 to 5.
For what it is worth I have been also noticing a slight roughness in the engine between 2300 and 2450 rpms with intermittent misses when I have leaned the mixture.
I get good idle and good climb power and it runs smooth at any setting above 2450rpm and under 2300rpm.
One last note. I have also noticed that there is a slight hesitation in engine acceleration when I push the throttle when the engine is running at 1200-1400rpm.
I might have two distinct problems here. Fuel pressure and carb problems.
Any ideas?

N520TX · #2 07-30-2007, 07:32 AM

My O-360 behaves the same way. It concerned me at first quite a bit, but i've since been informed by many out there that this is normal. The engine runs fine with as low as a pound of pressure.

The 'reason' for it, i've been told (no proof mind you) is that the pressure transducer was not designed for the rapid changes our engines experience during climb out. Whether that's true or not, I don't know. I do know that in 3 yrs and 400 hrs, it's never been an issue.

robertahegy · #3 07-30-2007, 05:24 PM

It's very typical to see fuel pressure variations during climb and altitude. As long as you have at least 0.5 psi you are OK with a carby engine, per Lycoming.

Your hesitation could be a slightly lean carb adjustment. Try resetting your idle "mixture" screw per the Lycoming instructions.

Roberta

billy d · #4 08-02-2007, 05:41 PM

I've checked the fuel pump PSI with a mechanical gauge and the Van's Electric, both at the same measuring point...Mechanical gauge=6.5 PSI, Van's Electric gauge= 3.5 PSI...is the transducer/Van's Gauge that far off...are other people finding this discrepancy...ground running the engine...runs fine...Carb is not flooding/leaning...almost ready to fly ...SOON!!!! O320 H2AD with Facet Electric pumps.
your comments appreciated....billy d

FrankK90989 · #5 08-03-2007, 07:17 AM

Billy.
Do a fuel flow test (gph) take the fuel line off at the carb, measure vs time.
You should see that the facet pump pumps plenty of fuel.
I have reviewed my Dynon in flight fuel pressures and they range from 2.1-5.7 psi. I don't think the actual pressure varies that much,and I don't worry about it
o-320 D2J

jeff beckley · #6 08-03-2007, 01:04 PM

I can tell you this. It is way spooky to look over at your fuel gauge on climb out and see it reporting 1 pound. If it is vans gauge/transducer then I think a phone call is in order.

Sam Buchanan · #7 08-03-2007, 01:30 PM

Quote:

I can tell you this. It is way spooky to look over at your fuel gauge on climb out and see it reporting 1 pound.

It's even spookier to see the fuel pressure display on your RMI uMonitor flashing "0" since I have the alarm threshold set at one psi. But the engine has been running happily for over 800 hrs and as has been stated many times before, the "low" reading is not at all unusual. Many pressure tranducers just can't resolve low pressure very well.

rvpilot · #8 08-03-2007, 01:37 PM

Anyone else ever noticed how the indicated fuel pressure follows the throttle setting??? WOT, no pressure, half throttle and it returns like magic!

Yukon · #9 08-03-2007, 01:40 PM

Is there any chance that that long length of hose up to the sender can get an air lock? Wonder if it would help to crack the upper fitting and bleed it?

robertahegy · #10 08-03-2007, 04:11 PM

One PSI is twice what Lyc. recommends at minimum. My neighbor's Tailwind with 0-320 runs fine with no fuel pump at all. The fuel tank is behind the firewall, leaving very little pressure head from gravity feed.

Roberta