High Fuel Pressure After Startup

[BigTex]

Hi All,

Over the weekend I went out to do some flying in my M20C.  My typical startup procedure is to turn on the boost pump until it reaches the 6 PSI red line.  Then I pump the throttle twice.  Each time I pump the throttle, it drops 1 PSI per pump.  So normally I'm around 4 PSI when I start the engine.  Well this time, the fuel pressure stayed at 6 PSI after the two pumps.  The engine fired up and ran normally but the fuel pressure jumped up to 7 PSI at idle.  I tried all kinds of different power settings and nothing seemed to make much of a difference.  If I turn on the boost pump while the engine is running, the PSI jumps up to 8 PSI. 

The engine from all accounts seem to be running just fine.  This is the first time in 10 years of flying this plane, I've seen this occurrence so any ideas would be appreciated!

Thanks,

Gary

[carusoam]

Is it unusually cold out where you are?

I used to see similar behavior with my M20C in 20°F kind of weather…

The pressure has to drop if the accelerator pump is delivering fuel…

Hard to get the engine started without the primer working…

 

Could be something affecting the FP sensor… (?)

 

See if @Hank is around for insight… (M20C priming issue)

 

PP thoughts only,

-a-

[BigTex]

Thanks... 70 degree day so temperature isn't the issue.  Really strange.  The plane started just like it always had... less than one revolution of the prop.  All seems normal but high fuel pressure.  The MP/FP gauge was overhauled last year so I'm pretty sure it's not the gauge.  The fact that fuel pressure doesn't drop when I pump the throttle prior to starting the engine is a definite clue... just don't know what it's telling me!

[Hank]

The fuel pressure in my C has always dropped ~1" on every throttle pump, except when I don't run the pump (hot starts). Since 2007 . . . .

I've had other issues, but not this.

Does your fuel pressure read normal during cruise? Mine is 1/2--2", varying from time to time. 

[BigTex]

Hi Hank,

I found this issue over the weekend and don't plan on flying it until whatever this issue is gets resolved.  But prior to this weekend, I've had zero issues with fuel pressure at any part of my flights..  Why it's not dropping that 1 PSI per throttle pump has be stumped. 

[Hank]

I seem to get larger drops in colder weather.

[Browncbr1]

@BigTex  I'm not sure if this applies to your carbureted engine, but it does on the fuel injected...  check that your sniffle valve is draining properly.  Mine got gummed up once and fuel would pool up in the intake plenum.  This caused the servo to react in a strange way to increase fuel flow and fuel pressure on start up and ground ops until after run up, which would create enough velocity inside the intake to suck the pooled fuel out, then it ran fine and FP and FF went back to normal...    the throttle pumping you're doing may be worsening the problem.  but, the curious thing about your problem is that your FP doesn't drop when pushing in throttle when the engine isn't running.    

Edited February 1, 2022 by Browncbr1

[BigTex]

Thanks for the idea but I think sniffle valves are only on fuel-injected engines. 

[hammdo]

Humm, my 66 C is ~7 PSI cold start recently boost pump on - seems fine in cruise. Slightly over 6 on mechanical pump.

-Don

[MBDiagMan]

Kind of me reminds of the patient telling the doctor that “ it hurts when I move nip my arm like this.”  To which the doctor replies “ then don’t move your arm like that.”

[carusoam]

Carbs don’t have sniffle valves…

But, they sure do have the equivalent….

If you pump the throttle 10 times prior to start… the excess fuel drains all over the front tire…

A product of really cold starts in NJ…  not recommended.

-a-

[hoot777]

M20 G here. My experience is I need to prime with electric pump only when plane has not flown for over a week or so and on cold starts. Most of the time I pump throttle twice and she starts right up. I have had some strange FPS in flight. I have overhauled the gauge and had everything thing checked by a great mechanic  . It’s a mystery   Most it time it reads normal.  Fly safely.

 

[BigTex]

You know how it is... When you live with a plane for over 10 years, you learn all of it's quirks and mannerisms.  Now she's behaving differently, so I'm trying not to ignore what she's trying to tell me... Just don't know what it is.   

[0TreeLemur]

On 1/30/2022 at 7:48 PM, BigTex said:

Hi All,

Over the weekend I went out to do some flying in my M20C.  My typical startup procedure is to turn on the boost pump until it reaches the 6 PSI red line.  Then I pump the throttle twice.  Each time I pump the throttle, it drops 1 PSI per pump.  So normally I'm around 4 PSI when I start the engine.  Well this time, the fuel pressure stayed at 6 PSI after the two pumps.  The engine fired up and ran normally but the fuel pressure jumped up to 7 PSI at idle.  I tried all kinds of different power settings and nothing seemed to make much of a difference.  If I turn on the boost pump while the engine is running, the PSI jumps up to 8 PSI. 

The engine from all accounts seem to be running just fine.  This is the first time in 10 years of flying this plane, I've seen this occurrence so any ideas would be appreciated!

Thanks,

Gary

Lycoming specifications for the O-360-A1D give 8 psi for the red line fuel pressure.   The Mooney POH states 6 psi.  I frequently got high fuel pressure alerts from my engine monitor (JPI EDM900) on takeoff, with 6.1 or 6.3 psi.   A red-line alert on takeoff is a major distraction.  I saw this one as meaningless and very annoying. 

A few months ago I had to return the engine monitor to the factory for an update.  I sent a copy of the Lycoming specification and asked them to reprogram the FP red-line to 8.0, and they changed it in the engine monitor.  No more FP alerts on takeoff.

I applaud your effort to try and understand the cause of the change.  Changes are how our aircraft talk to us about changes in their condition.  Hard to imagine what would cause the behavior you are seeing short of some change in the operating condition of the fuel pump or a gauge issue.

[Old Chub]

I can only speak for my C model (1968) and the way the fuel pressure is picked up off of the carb inlet cavity.   I'm sure someone will soon tell me how wrong I am but here goes.  My C model has no external priming lines or way to prime the cylinders other than the carb accelerator pump (spraying fuel into venturi).  

The fuel pressure is built up with the boost pump against the float valve.   Once the carb is full the float valve closes and pressure is built up in the lines and provides a guage reading (as there is no place for the fuel to go).   Once the throttle is pumped the carb accelerator pump pushes fuel into the venturi throat and removes fuel from the float chamber and thus reduces pressure as float valve opens slightly.  If you are holding pressure solid (boost pump off) while pumping the throttle and no pressure is dropping it means your accelerator pump is not moving any fuel from the carb bowl and into carb venturi throat.     There is some elasticity in the system so you could be moving some fuel but maybe not as much as you once did and therefore have a change from what you normally see.