Fuel pressure issues

[Bob_Belville]

10 hours ago, carusoam said:

The advantage of downloading the data...

The graphical output over time makes things like a loose wire or interactions of various variables more readily obvious...

There is a piece of hardware that gets installed with many pressure sensors called a (calibrated air hole) (Insert name here) It’s purpose is to level out the bouncy data caused by the real pressure variations...

Lots of natural variations caused by cycling fuel pumps can be expected... and messy for digital data...

I always ask @Bob_Belville what that device is called... because his memory is at least 10X what mine is... 

Best regards,

-a-

JPI called the in line thingy a "snubber" when they gave us one to dampen MP display.

[DXB]

On 3/17/2020 at 1:36 PM, J. mitchell said:

I recently put a JPI EDM 900 in my M20C Mooney. Everything seems to be working accurately with the exception of our fuel pressure readings. On the climb out the fuel pressure essentially drops to zero after the boost pump is turned off yet the fuel flow indicates a normal fuel flow for the climb.  Once in cruise the pressure is back in the green. On descent the opposite occurs with a high fuel pressure warning light.  It is interesting that with the old gauges i also experienced a drop in fuel pressure on the climb yet I experienced no issues with the engine performance.  All EGT and CHT numbers are good.  Any thoughts would be appreciated. 

FWIW my FP gauge occasionally behaves exactly like yours does in climb on my C, as do the gauges of MANY other C and G model owners. It certainly does not happen every time. It's been that way for the 700hrs and >5 years that I've owned the plane. The behavior was there on my analog gauge before an EDM900 was installed and remains there on the digital gauge, so it's not the JPI.  I have never seen fuel flow drop when it happens, and I have looked closely.   I am convinced it's an artifact of the fuel pressure line design - perhaps an intermittent vapor lock that develops in it at full power, high deck angle, and lower pressures in the main line that are still in the acceptable range (>0.5psi).  I don't even bother turn back on the boost pump when it happens now but instead just keep a close eye on fuel flow. The carb'd birds have a rather different, much lower pressure fuel system than the injected ones, so guidance above from injected bird owners does not apply.  

BTW it's also very common for C/G owners to see fuel pressure go just above the high limit of the "normal" range upon turning the boost pump on before takeoff or during GUMPS - I wouldn't worry about this.  I've never seen it simply upon reducing power for descent though.  

[carusoam]

Yes! The snubber is the calibrated hole near the pressure sensor that uses an air bubble to balance out the micro peaks and valleys caused by the cyclic nature of the fuel pump...
 

Another thing to consider... about when and why Fuel pressure changes up stream...

There is a float valve that is responsible for opening and closing the valve that maintains The fuel level in the carb’s bowl...

This is most likely to be the restriction to flow that actually generates the fuel pressure.

what is drawing fuel from the carburetor is the vacuum generated by the pistons on their intake stroke...

The fuel pumps maintain fuel flow to the float valve... 

The pressure sensor will be sitting between the mechanical pump and the float valve...

each time the float rises, the valve opens more... the more open the valve the lower the pressure...

FF used will be related to MP and rpm... drawing fuel into the engine...

FF generated will be related to rpm... the speed the pump operates at...

The system is designed to be able to over produce FF to the engine...

Things that cause high pressure... the pump continues to run.... and the carb bowl is full... the float valve will close... the pump can only generate so much pressure before the fuel doesn’t go anywhere...

Things that cause low pressure... the pump is running at low rpm... and carb bowl is low... the float valve will be full open... and the fuel pump can only deliver its max FF...

Sort of explains why the actual FP peaks and valleys appear to be at random times related to the flight regime.

The FF gauge is critical data for flight... without proper fuel pressure... FF won’t continue for very long...

FF is easier for a pilot to use to know how well the fuel system is working... in flight.

FP is a direct piece of information to know what is going on in the fuel system... great for starting, and problem solving...

Keep in mind FF sensors are incredibly new compared to the days when M20Cs were born...

Ideas off the top of the head of a private pilot only... not a mechanic...

Thanks to Bob and Dev for your awesome support! 
Best regards,

-a-

[J. mitchell]

Guys, I really appreciate your thoughts here.  I especially appreciate DXB’s comments that many C models experience what I have experienced with my Mooney. Both your explanations make sense.  Thanks so much for your help with this issue.  
John

[tmo]

Is it possible that the boost pump "overpowers" the float (the thing that closes the carb fuel "input") and when it gets turned off, the intake is indeed closed for a moment, till the engine sucks out the "excess" fuel from the carb bowl and then things return to normal?

Or do airplane carbs work that much differently from what I am used to in old cars and motorcycles?

[Prior owner]

14 minutes ago, tmo said:

Is it possible that the boost pump "overpowers" the float (the thing that closes the carb fuel "input") and when it gets turned off, the intake is indeed closed for a moment, till the engine sucks out the "excess" fuel from the carb bowl and then things return to normal?

Or do airplane carbs work that much differently from what I am used to in old cars and motorcycles?

This might be possible... another factor to consider when estimating the force required to push a float valve open would be the buoyancy of the float- does an old brass float (with no leaks) have the same shape/displacement/weight and the same buoyancy as the modern composite float? I’m curious about that. 
 

My O-360 has the old brass float in the carb.
 

As far as car vs airplane (not pressure carbs) carburetors go, the float bowl and float valve systems are essentially identical. Too much fuel pressure will push the valve open. I have no idea how much press it takes to do that on a Marvel Schebler carb....when rebuilding the carbs, they are tested at a fairly low head pressure, if I remember correctly.

Edited March 20, 2020 by PilotCoyote

[Hank]

My cars had downdraft carbs, mounted atop the engine. My Mooney has an updraft carb mounted below the engine . . . .

[Lionudakis]

I glad I came across this thread ! I've been chasing this similar issue down for a few years now. I installed my JPI900 in 2014, and never really took the bird on a trip, or over 3000ft for that matter until 2016 when I noticed low and decreasing FP around 5k ft, low enough to trigger the warning light for the 900. Down low the pressure is normal/acceptable. The higher I go the lower the pressure, The pressure increases on descent, regardless of power. It is more reactive to altitude than anything.  I replaced the mechanical and electric pump and thought i had it, but it still does it, and its not consistent. 

I've vacuumed leak checked and pressure checked the lines from the carb to tanks as a whole, looking for air leaks. nothing. I'm stumped other than the mechanical pump. The electric does bring the pressure back up some

I'm throwing on a new, not overhauled engine driven pump on this week while working on other things in hopes I got a defective overhauled one. 

[J. mitchell]

It appears that a number of M20C owners have experienced the same issue I have been experiencing.  I would be glad to know if replacing the mechanical pump resolves the issue. Once again, thanks for all the thoughts shared. 
John

[PT20J]

If the pressure is high enough to overpower the float needle valve, it will raise the float level and flood the engine. I had this happen in a Beaver seaplane when the fuel pump failed and I had to use the manual wobble pump. I got a bit too enthusiastic pumping (I was over a rocky coastline in Alaska at 500' with no safe place to set down) and caused the engine to stop. It started again when I stopped pumping and the fuel level went back down to the operating range.

Output pressure varies with flow for lever type fuel pumps but it should stay within reasonable bounds. The fuel pressure is not what's important -- it's the fuel flow that counts. If the engine runs properly at full power, the pump is putting out enough flow to run the engine and low pressure indication might not caused by the pump. Here's a good description of the operation of these pumps:

 

[Tugla]

On 3/21/2020 at 8:20 AM, Lionudakis said:

I glad I came across this thread ! I've been chasing this similar issue down for a few years now. I installed my JPI900 in 2014, and never really took the bird on a trip, or over 3000ft for that matter until 2016 when I noticed low and decreasing FP around 5k ft, low enough to trigger the warning light for the 900. Down low the pressure is normal/acceptable. The higher I go the lower the pressure, The pressure increases on descent, regardless of power. It is more reactive to altitude than anything.  I replaced the mechanical and electric pump and thought i had it, but it still does it, and its not consistent. 

I've vacuumed leak checked and pressure checked the lines from the carb to tanks as a whole, looking for air leaks. nothing. I'm stumped other than the mechanical pump. The electric does bring the pressure back up some

I'm throwing on a new, not overhauled engine driven pump on this week while working on other things in hopes I got a defective overhauled one. 

Did your new pump solve the issue. Or anyone find a smoking gun as to the issue, carb, pump, lines?

Thanks

 

[Lionudakis]

9 minutes ago, Tugla said:

Did your new pump solve the issue. Or anyone find a smoking gun as to the issue, carb, pump, lines?

Thanks

 

Did not fix it. What did fix it, is I installed a Dynon HDX system with engine monitoring. Fuel pressure is rock solid into the low teens. I suspected the JPI sensor early on, and had even had them test it. I'm guessing it was the wrong sensor becuase of the way it reacted to altitude