Fuel pressure high at idle, normal range full power

[Jakes Simmons]

     Appreciate any help/advice on possible diagnosis. Ranger with O360. Mid time engine. Original combo manifold and fuel pressure gauge. Starting engine from cold, boost bump on brings pressure from zero to 6 on the gauge face (redline). Once engine started, always on 1 to 3 blades, boost pump off, fuel pressure reads 7 or slightly above, at least a full inch above posted redline. Boost pump on or off only tickles the needle regardless of throttle setting. At take off, full power, fuel pressure needle drops back to between 5 and 6 but stays below redline. Cycling boost on and off during flight only nudges the pressure needle letting me know it’s working, plus noise when on the ground. Bring power back to idle or very low, fuel pressure rises back to 7 or just over and stays there.  After being tucked back in its hangar, fuel pressure gauge back to zero as normal.

     So idle and low throttle settings, fuel pressure reads high and above red line, full and high throttle settings, fuel pressure reads below red line. No fuel drips after flight. Engine runs just as perfect as always. 
     Mechanic by phone suggests gauge is bad. No Mx on field where aircraft is based. Guesses? Safe for cross country to Mx? If weather ever breaks down south, next planned flight is Maxwells for the annual.

    Hope everyone is staying healthy and safe...and employed! Thanks ahead of time for consideration.

[Oscar Avalle]

There has been a similar thread somewhere a while ago. I have a similar problem, however FP hovers around 6 PSI. I makes me nervous, but I had the system inspected and the conclusion was that I should not worry too much about it, specially if it remains around 6 and it goes down when you open the throttle. 

 

Oscar

[yvesg]

Same here with my 1965 C. My mechanic checked the gauge pressure and very close to reality. That thread Oscar is talking about from memory concludes that this is not a big deal. I would worry if it would be double the pressure but it never goes high enough for concern.

Yves

[Prior owner]

Lycoming specifies 8psi max for O-360 (page 3-10 of the Lycoming Operator’s Manual). I do understand that Mooney POH says limitation is 6 psi.
 

My plane does exactly what yours does. Normal in cruise, and above 6psi at idle.

 

Edited March 17, 2020 by PilotCoyote

[Hank]

Mine will reach 6-1/2 psi running the pump before engine start, but it goes down when I pump the throttle with Mixture Full Rich, and down more after it cranks. Typically see ~2psi in flight, I don't recall idle-on-the-ground values.

[carusoam]

If able...

See if you can post a vid of the gauge in action...

This would help when people say they are seeing the same thing... there will be less errors if something got misunderstood.

If flying to the annual... share this detail prior to going... DMax is pretty good at phone advice.

Is this a new behavior, or it has always done this?   

PP thoughts only...

Best regards,

-a-

[Jakes Simmons]

Appreciate the input. I had done a thread search but was only seeing the fluctuating issue. My engine runs awfully fine, it was needle over red making me nervous. I wonder if the POH vs Lyc manuals was one more example of the manufacturer being conservative.

[0TreeLemur]

21 hours ago, Jakes Simmons said:

Appreciate the input. I had done a thread search but was only seeing the fluctuating issue. My engine runs awfully fine, it was needle over red making me nervous. I wonder if the POH vs Lyc manuals was one more example of the manufacturer being conservative.

You nailed it, I think.  It is a carburetor/engine spec., not an airframe spec.    Go with the Lycoming value.   I really wish Mooney would revise it so we could get the red line raised.   I regularly see my engine monitor flash the red warning on takeoff with the boost pump on for 6<P<8 psi, which causes me to clear the warning in case a real warning needs annunciation.

[Oscar Avalle]

Lycoming specifies 8psi max for O-360 (page 3-10 of the Lycoming Operator’s Manual). I do understand that Mooney POH says limitation is 6 psi.

 

My plane does exactly what yours does. Normal in cruise, and above 6psi at idle.

 

Thank you

 

 

Sent from my iPhone using Tapatalk Pro

[carusoam]

Interesting related thread...

with JPI graphs and discussion of Fuel pressure challenge...

With an invite for @J. mitchell to join this conversation...

-a-