Fuel pressure issues

[J. mitchell]

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. 

[Floyd]

Sounds like the fuel pump is barely able maintain the fuel flow at high power setting.  I would have the fuel pump checked out.

[RLCarter]

Does the electric boost pump change anything?

[J. mitchell]

Yes, the electric boost pump does push the pressure up. It’s interesting that in cruise the pressure is fine.  On descent and approach I get the opposite reaction, the fuel pressure is high and I am throttled back.  I will be taking my a Mooney to annual next month, so the fuel pump will be checked out.  

[0TreeLemur]

Good catch.  Isn't that EDM 900 awesome!   Something is not right.  On our C, the takeoff f.p. with booster pump on causes the JPI to give a P>6.0 psi warning, usually about 7 psi.   When I switch it off, the pressure drops back to the normal range after a minute or so,  even in a climb with wot and full rich mixture.

[J. mitchell]

Thanks for those comments. Yes, the EDM 900 is awesome.  In the climb with the boost on our fuel pressure is right up there too.  I am becoming more convinced there is an issue with the mechanical fuel pump.  I did see there was a mandatory service bulletin for a number of fuel pumps on certain 0-360 lycoming engines, but it didn’t include the engine in the M20C.  

[markgrue]

I would guess the mechanical fuel pump is on the way out.   Might want to give it a look before you find out that it is not working at all about 100 ft off the ground just after take off.

 

Mark

[HIghpockets]

I had a similar problem. Replacing the fuel pump fixed it.

[J. mitchell]

I appreciate all the thoughts on this issue.  When the issue is resolved I will post a note.  We thought we had addressed the issue 2 years ago with a faulty instrument, but it is looking more and more like a weak mechanical fuel pump.  

[carusoam]

The amount of fuel pressure feeding the carb is a pretty small number...

If it was only an old mechanical FP pressure gauge... the gauge would be suspect...

Sometimes the pressure changes depending on the angle of the attitude...

A number near zero is not ever good...

FF doesn’t happen without pressure...

It may be possible that A fuel jet has fallen off inside the carburetor... or some other restriction to flow has been removed...

Are you seeing anything related to poorer fuel distribution than before?

That is a gami spread question for the O360... a tough measurement...

Any signs of fuel leakage anywhere?

Any changes in mixture control settings?

Have you ever changed out the line going to the pressure sensor?  Sometimes they require having a certain amount of air at the end... sometimes the air gets let out causing pressure reading issues... there should be an install manual for the instrument that covers this...

Keep both eyes open... 

PP thoughts only, not a mechanic...

Best regards,

-a-

[J. mitchell]

I really appreciate those thoughts. I spoke with my mechanic this afternoon and the issue of air in the line and perhaps in the mechanical fuel pump might be causing the problem.  The engine does not appear to be suffering fuel starvation and now that I can see EGT AND CHT numbers, they are all where they should be.  My fuel flow also reads where it should be on climb out and cruise when we lean.  The fuel pressure returns to normal in cruise.  We have flown about 10 hours since the EDM 900 has been installed.  All the other numbers look fine.  

[carusoam]

J,

Often, people will upload their JPI data to savvy.com and click the share data option...

Then post the link here...

You may get a few people to look at the various data points to see if anything stands out...

Some people are really good at recognizing things that they are familiar with...

The odd power of MS...

Best regards,

-a-

[J. mitchell]

At 5,500 MSL in cruise.  Still adjusting the K factor on fuel.  Fuel pressure is fine in straight and level flight.  

[carusoam]

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-

[J. mitchell]

A look at our flight back to AVL.  

[carusoam]

Sorry to be a pain, J...

See if you can copy the link to that page here....

It will require pushing a share button to make it work...

one step at a time...

That collection of EGT/CHT data looks really stable.

Best regards,

-a-

[J. mitchell]

Let’s see if this works.  I am on a learning curve here and did not know just how much information is available.  I did check fuel pressure and it is up and down on the climb.  
 

https://apps.savvyaviation.com/my-flights/22650/4380a0b8-5503-4b65-aa52-1e555a7d1556

[carusoam]

Take a look at how the FP is behaving over time...

Then compare to previous flights...

Then ask @DXB /Dev if he see anything similar on his C’s JPI?

My plane doesn’t even get a FP gauge... just FF. 

Best regards,

-a-

[J. mitchell]

That’s what we are seeing. The first few minutes is with the electric fuel boost on, turned off and the fuel pressure drops. In this case I climbed to 4,500 MSL and at that point it was mechanical pump only.  Yet all the rest of the numbers I believe look OK.  There doesn’t appear to be fuel starvation.  

[carusoam]

What happened to EGT#4 here?

This was the oldest file on the record...

To become more familiar with all the data you have available...

Click on the data window that is currently showing CHT and EGT... there are four windows that can be changed...

Cool data to display...

-a-

[J. mitchell]

-a- That was one of our first flights with the new engine analyzer.  #4 EGT had a loose connection. Pretty quickly tracked down.  
John

[carusoam]

Another way to use all four variables...

From the second flight down the list...

it would help if I selected better ranges to separate out the CHT and EGt data a bit...

It looks like the FP data is falling off before getting collected... possibly another loose wire like the EGT #4(?)

Send this graph to your instrument guy... and ask if he can check on that for you...

-a-

[J. mitchell]

Thanks for the thoughts.  I will be doing that. 
John

[carusoam]

Fuel pressure is caused by fuel flow, running into a restriction...

Since the FF never goes to zero...

The restriction would have to disappear...

The restriction to FF is the fuel jet in the carb... a spray nozzle...

The EGTs would be really erratic if the spray nozzle disappeared...and reappeared... (not physically possible either...)

Expect a loose wire...

Keep in mind I’m only a private pilot. Not a mechanic or instrument guy...

Best regards,

-a-

[PT20J]

Float carburetors are relatively insensitive to fuel pressure so long there is enough flow to run the engine at the power set. The float bowl acts as a head-type pressure regulator to set the pressure at the inlet side of the main jet -- the pressure on the other side being equal to the venturi pressure -- by controlling the fuel level (head) in the float bowl. Since the float bowl is vented to atmospheric pressure, the fuel pressure across the float needle valve drops from whatever the pump puts out to atmospheric. As long as there is enough flow, it really doesn't matter what this pressure drop is, so the indication of a failing fuel pump is generally not enough fuel flow rather than reduced pressure.

When the pressure is low but you have fuel flow, the first (and most dangerous) possibility is a fuel leak between the pump and the carburetor.

Whenever there is a questionable gauge reading, the first thing to try (after ruling out leaks) is to T in a calibrated gauge to eliminate instrumentation. With any electric transducer, wiring and grounding are also likely culprits.

Skip

Edit: I don't think this was as clear as it might be, so let me see if I can summarize the main points:

1. All fuel metering systems (float carbs, pressure carbs, fuel injection) incorporate some sort of pressure regulation so that fluctuations in fuel pressure are isolated from the metering system. In a float carb, pressure is regulated by maintaining a constant fuel level in the float bowl. This means that fluctuating pressure (within reason) should not greatly affect fuel flow.

2. The fuel pump puts out flow against the restriction of the float needle valve and the pressure in that line is related to the flow rate. Generally, higher flows mean less restriction and lower pressure.

3. There must be pressure in line if fuel is flowing. A low pressure reading is most likely a measurement problem or a leak.

4. The only time I had a fuel pump fail, both flow and pressure dropped to zero.

Skip