High fuel pressure

[N6758N]

Temperatures seems to have no effect Anthony. Here is a video I took last night performing my ADS-B validation flight. 2,500 ft MSL, 22" MP and 2400RPM. 

 

IMG_8338.MOV

[Reflex]

A little repetitiveness here, the link below gives an excellent explanation as to how these pumps work.  While it's for laymen, it expands on N201MKTURBO's diagram.

https://www.youtube.com/watch?v=PYKoLUsTJU4

One of the things that has me baffled is why I would have normal fuel pressure with everything off and the pump on; and high pressure with the engine running at idle with the pump off (7.5psi).  When the pump is turned pressure runs up to 8.5 psi.

'68 M20C

[carusoam]

Welcome aboard, Reflex.

That Tempest video is pretty good.

What I think I got from it....

1) It shows the fuel flow is generated by the diaphragm pump.

2) fuel pressure is directly related to the restrictions to flow down stream.  More restriction, more pressure.

3) Restriction to flow comes in the form of the fuel nozzles in the carburetor, or any bends or deposits in the line.

4) A pressure regulator In the system would control pressure.  But there doesn't seem to be a regulator in the M20C's fuel system.

5) The most likely cause of increased fuel pressure seems to come from increased restrictions in the line after the pump...

Follow up thought...

Is there a way to make sure the fuel lines and nozzles are clear and working properly?

Private Pilot thoughts only, not a mechanic...

Best regards,

-a-

[Reflex]

A little update, but not much new data.  We've had a short stretch of warmer weather here so I decided to take a run out to the airport and mess with the plane.  The low the night before was 38 degrees and the OAT at the time of the following test was 63 degrees.  The low temp for the previous 48 hours was 38 degrees.  Removed the cowling to look for obvious issues and/or blue stains.  After an exhaustive search, I found no issues or places where the fuel was leaking.  However, I could not see the top of the fuel line or the spot where the pressure gauge T's off.  I reinstalled the cowl, pulled the airplane out of the hanger and when through the starting procedure. 

Here's what I found:

1) Engine off, pump on, throttle closed, mixture lean = 5 psi (which is 1 psi under the maximum allowed pressure and is in the green arc.)

2) Started the engine without the electric pump running, 1100 rpm fuel pressure reading = 6psi exactly (right on the red mark)

3) Turned on fuel pump and reading increased roughly .2psi to 6.2 psi (.2 over maximum)

4) Fuel pump turned off fuel pressure returned to 6 psi

5) Allowed engine to warm up and come to operating temps

6) Increased throttle setting to 1700 rpm fuel pressure dropped to top of green arc or about 5.8 psi

7) While at 1700 rpm, turned pump on fuel pressure increased to 6 psi

8) While at 1700 rpm, turned pump off fuel pressure went back to 5.8 psi while at 1700 rpm

While the problem didn't exactly repeat, the fuel pressure is still high for this particular aircraft.  To recap the issue, fuel pressures on the last flight climbed as high as 8.5 psi.  Normal fuel pressure for this plane is 4-5 psi.  While it's possible the OAT has an effect on the problem, my "guess" is that the warmer temps could have allowed the fuel line to expand just enough to lower the pressure.  It's my belief I still have a problem.

Any chance anybody out there has a fuel system diagram for a M20C with a O-360?

Thanks,

Reflex

[N6758N]

4 minutes ago, Reflex said:

A little update, but not much new data.  We've had a short stretch of warmer weather here so I decided to take a run out to the airport and mess with the plane.  The low the night before was 38 degrees and the OAT at the time of the following test was 63 degrees.  The low temp for the previous 48 hours was 38 degrees.  Removed the cowling to look for obvious issues and/or blue stains.  After an exhaustive search, I found no issues or places where the fuel was leaking.  However, I could not see the top of the fuel line or the spot where the pressure gauge T's off.  I reinstalled the cowl, pulled the airplane out of the hanger and when through the starting procedure. 

Here's what I found:

1) Engine off, pump on, throttle closed, mixture lean = 5 psi (which is 1 psi under the maximum allowed pressure and is in the green arc.)

2) Started the engine without the electric pump running, 1100 rpm fuel pressure reading = 6psi exactly (right on the red mark)

3) Turned on fuel pump and reading increased roughly .2psi to 6.2 psi (.2 over maximum)

4) Fuel pump turned off fuel pressure returned to 6 psi

5) Allowed engine to warm up and come to operating temps

6) Increased throttle setting to 1700 rpm fuel pressure dropped to top of green arc or about 5.8 psi

7) While at 1700 rpm, turned pump on fuel pressure increased to 6 psi

8) While at 1700 rpm, turned pump off fuel pressure went back to 5.8 psi while at 1700 rpm

While the problem didn't exactly repeat, the fuel pressure is still high for this particular aircraft.  To recap the issue, fuel pressures on the last flight climbed as high as 8.5 psi.  Normal fuel pressure for this plane is 4-5 psi.  While it's possible the OAT has an effect on the problem, my "guess" is that the warmer temps could have allowed the fuel line to expand just enough to lower the pressure.  It's my belief I still have a problem.

Any chance anybody out there has a fuel system diagram for a M20C with a O-360?

Thanks,

Reflex

POH should have a fuel system diagram in it. Pretty simple on the C. Two Tanks, two vents, both feeding into the fuel selector. From there goes through the firewall and to the fuel pump. Pump lines goes to the carburetor. Fuel pressure reading is tapped off of the carburetor. 

[Reflex]

Thought I'd give a quick follow-up to the problem I was having.  To briefly recap, my M20C was registering high fuel pressure at low throttle settings.  I inspected multiple items without finding anything that would contribute to to the issue.  These inspections were cursory as I don't believe I'm qualified to do an in depth check.  In the end, I considered the aircraft not to be airworthy  until I found a solution.

After making multiple phone calls to various mechanics all who had an opinion on potential problems/solutions, I found that each believed the aircraft to be safe for flight as long as the fuel pressure was under 6 psi when at cruise settings.  With this in mind, I made an appointment and took a short flight to a qualified mechanic.

The aircraft was pulled into the shop and the cowling was removed.  The mechanic I was working with spent a considerable amount of time checking everything from the electronic fuel pump to the carb.  In addition the screen was removed from the carb and flushed.  Hoses were pulled, fittings were checked, the hose to the fuel pressure gauge was checked and bled, crankcase pressure checked, etc.  I was quite impressed with the amount of work that was done.  No clogs, no anomalies, no traces of any issues were found.

In the end, it was decided that the gauge is going bad.  A new gauge was ordered.  When I got back into the airplane and headed home, the gauge read "normal"  while taxiing.  Pressures previously ranged from 6 - 7.5 psi.  Readings were 4.5 psi at idle and in cruise and just about 5 psi with the boost pump on.  A few minutes later the gauge read 2.5 psi.  I'd never seen a reading that low before.  I flipped on the boost pump and got a reading of 6 psi.  A few minutes later we were back to 4.5 psi.  All of this took place with no change in prop setting, additional leaning, or a change in manifold pressure.  The engine ran smoothly and never missed a beat.

During landing, when the electronic pump was turned on, I saw no change in pressure.  After landing with the pump off, no change in pressure.  While taxiing to the hanger, the fuel pressure rose to 7 psi with no change in sound, rpms, engine temps, or egt.

Although no "smoking gun was found", given the extensive search for a problem, the fact that the entire system between the electric pump and the carb were checked, I've decided that there can be no other answer but a bad/intermittent gauge.

Reflex

 

 

 

Edited February 3, 2017 by Reflex

[Oscar Avalle]

Thank you for the detailed report! I am observing (as many others) a similar reading. We also looked at all the options and everything looked just fine.


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[AlexLev]

Funny you mentioned this. I noticed my fuel pressure on the ground the other day right at the end of the green. It normalized after take off. I didn't give it too much more thought after that. (M20G)

[Hank]

1 hour ago, AlexLev said:

Funny you mentioned this. I noticed my fuel pressure on the ground the other day right at the end of the green. It normalized after take off. I didn't give it too much more thought after that. (M20G)

That's been typical for my C for years. Taxi at the top end, takeoff and climb around 5 psi, cruise in the wide green stripe (2.0-2.5 psi?). Is it supposed to do different?

[Oscar Avalle]

That's been typical for my C for years. Taxi at the top end, takeoff and climb around 5 psi, cruise in the wide green stripe (2.0-2.5 psi?). Is it supposed to do different?

Hank you are right... I think the issue is with a JPI you can see the figures, (there is even an alarm) while the needle may not be so obvious.


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[carusoam]

So many variables...

from two fuel pumps generating pressure, to two fuel jets dissipating pressure, and include the fuel float in the carb opening and closing a valve, more or less as needed...

The angle of the plane during the climb and the generally low pressure that is being used.

It is good to have a new gauge and clean lines...

Great follow-up on the issue.

Best regards,

-a-

Edited February 4, 2017 by carusoam

[M20F-1968]

For what it is worth...I have a JPI 930 in my F.  I have been seeing fluctuations of about 2-4 psi (although always staying within the green arc).  I spoke with Joe (owner of JPI) who is sending me a snubber to attenuate the fuel fluctuations from the mechanical pump.  I am waiting to receive it.   I do think my problem is an indication with the JPI.  Plane flies fine despite this JPI indication.

John Breda

[Lionudakis]

A little late here.. Did the snubber resolve this issue??  I'm still chasing my intermittent high / low pressure anomaly. I'm at a loss 

[PT20J]

A few thoughts:

1. A good video on fuel pumps: https://www.youtube.com/watch?v=PYKoLUsTJU4

2. Whenever replacing a part, verify you install the correct part number (not just the same part number as the one that you're replacing - in case someone previously installed the wrong part).

3. Air in the fuel line should cause variation in pressure readings and/or low pressure,  and it should most likely show up at high power settings.

4. Whenever a reading on a gauge is suspect, tee with a calibrated mechanical gauge to verify the accuracy.

5. High fuel pressure isn't a problem unless it gets very high in which case it can overpower the carburetor float and flood the engine (had a fuel pump fail in a Beaver and had to use the wobble pump and found out that if I pumped it more than a couple of psi over redline at cruise power that the engine would quit.)

Good luck,

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