Fuel pressure on older B/C/D models

[ed]

I have a 1963 M20C (a D that was converted to a C just after leaving factory).  180 HP, carb so the Low Pressure mechanical fuel pump (4 PSI).

Mechanical pump holds 4 PSI  in level flight.  In full power climbs FP drops to near zero on the gauge but engine runs OK.  If I turn on the electric boost pump on climb, then pressure is higher, but not quite the full 4 PSI.   On ground electric pump holds a full 4 PSI as does the mechanical pump.  Also seems like it uses more fuel thru fuel totalizer than actual.  I put on new mechanical pump and did not change anything. 

I am wondering if fuel could be going back thru the electric pump into the tank?  See picture. This is the fuel system on B, C , SN 1701 thru 2622

and D  SN 101-200.   Its an odd design, in that the boost pump path bypasses the gas-colator.  Also I don't see any check valve to prevent fuel from going back thru the electric pump.  So the elect pump must have some internal flow back restriction method.   Item 11 is a short fuel hose.  You can see that the T on the firewall connects the electric pump path and the mechanical pump path.   I think it has always been this way on the pressure but was curious what others see.   Only way that fuel could go back into tank would be if whatever device is in the electric pump to prevent flow back is leaking. 

[hammdo]

My old B had a similar issue. DMax fixed it when he found a ck ball or some restriction (don't have the logs handy). Near 0 on climb out. Pressure was almost 6 when I had the boost pump on after fix... 

edit:

here you go:

http://accelerationtech.com/photos/Mooney/Don Maxwell/Maxwell-5.pdf

-Don

[Skates97]

2 hours ago, hammdo said:

My old B had a similar issue. DMax fixed it when he found a ck ball or some restriction (don't have the logs handy). Near 0 on climb out. Pressure was almost 6 when I had the boost pump on after fix... 

edit:

here you go:

http://accelerationtech.com/photos/Mooney/Don Maxwell/Maxwell-5.pdf

-Don

Thank you, I have had the same issue as long as I have had my plane. I've searched but just found a lot of posts saying that's what they do. My fuel flow doesn't change, just the pressure drops. This is the first time I've seen an actual solution. 

[47U]

12 hours ago, ed said:

I am wondering if fuel could be going back thru the electric pump into the tank?  See picture. This is the fuel system on B, C , SN 1701 thru 2622 and D  SN 101-200.

I’ve got one of those iconic fuel systems.  

I’d calibrate the fuel flow instrument.  How much discrepancy are you seeing at the pump?  Is the error consistent?  Less than a gallon?  More?

[Guest]

10 hours ago, hammdo said:

My old B had a similar issue. DMax fixed it when he found a ck ball or some restriction (don't have the logs handy). Near 0 on climb out. Pressure was almost 6 when I had the boost pump on after fix... 

edit:

here you go:

http://accelerationtech.com/photos/Mooney/Don Maxwell/Maxwell-5.pdf

-Don

You have to wonder how a restricted fitting found it’s way into a main fuel line.  Wow!  Restricted fittings that I’ve seen had red paint on them as an indicator.

Clarence

[ed]

Hammdo, I looked at the log pdf you posted. I know for certain that the two fuel pump fittings (in and out) of my mechanical pump (same PN as you have) do NOT have any restriction.  They are stock "fuel pump fittings" that you can see on ACspruce if you search for that in quotes.

Mine has a 45 degree into the pump and a 45 degree out of the pump.  I also installed two new teflon hoses from PHT in Tulsa.

I know restrictors are put on the fuel pressure fitting and oil pressure fittings as those don't flow any fluids, they just fill up and indicate pressure and if one of those hoses break then you don't want a lot of fuel or oil pumping overboard, so most have solder with a small hole drilled in them. But no reason to have that in the main fuel lines. 

I think the low pressure 4 PSI pump just results in a low reading at high power (max fuel consumption) and with the nose up high for climb it has to do more work to pull the full.   I need to find my POH (its in the plane) to double check, but it likely recommends boost pump on for climb.

I kind of avoid using it as it does bypass the gascolator (partially as some fuel likely going thru there and the mechanical pump and the extra thru the electric boost pump).     I was mainly looking to see if others see similar pressures in climb on these carburetor engines.

[hammdo]

M20Doc, I have no idea. The B had only 400 hrs since factory overhaul and no other record in the logs for any fuel work...

-Don

[hammdo]

Ed, interesting. DMax said he actually measured the fittings restriction size and that is how he found it. Said the fitting looked fine until he checked the internal size.

Amazing how he found it.

-Don

[ed]

47U,   I realize now that, with the fuel transducer in fuel line #29 in the picture I posted, that it ONLY measures fuel going into the carb.

So even if some fuel was back flowing thru the electric pump (with it off) that would NOT affect the JPI FS450 fuel flow.

As an aside, my fuel flow over the last few years was always showing high by about 9% when I did the re-calibration math here at annual.

So I have adjusted the K factor. I just need to do some longer flights to see if it is more accurate now.   It use to be off about 1 GPH so maybe read 3 gallons high after a 3 to 3.5 hour flight.  I had left it that way on purpose as it was "conservative" as there was always more fuel left in the tank then than what the FS450 would show.   It should be closer now with the adjusted K factor.  I also found in the manual the FS450 can adjust the K factor at each fill up automatically, but you have to go through the right sequence of steps on the power up of it to do that. 

[ed]

Well, the other place where fittings could restrict flow would be the T at 28 and the inlet to the carb at #30 in the drawing.

I am quite sure I saw no restriction at the T,  I would have to pull the hose off the carb at 30 to check that.

[ed]

I dug out the POH , it shows to only use electric boost pump for takeoff so the factory apparently saw no need for it on extended climbs.

I will show 4 PSI on ground.   Near 0 on full power climb.   At lower power curse, at 4500 or 5500 (forget when this pic was taken),

fuel pressure shows about 2.5 psi, at 22 inches MP and 2400 RPM.  So that is in the normal green range.   So all that seems normal. 

JPI fuel usage showed 11.0 gal, and it took 10.4 gal, all from left tank.  I did the self adjust method to tweak the K factor, as it is still reading a little high on 1.3 tach hours. 

not sure why the picture gets rotated

[47U]

2 hours ago, ed said:

not sure why the picture gets rotated

Picture rotation is actually -a-‘s specialty… I try to not infringe (sorry, -a-).

Normal pressure (by the green range) is all the way down to 0.5 psi.  I’ve experienced much the same indication.  Keep tweaking your JPI, and relax.  

[carusoam]

Thanks for the gravity assist 47U!

 

+1 for showing 0psi for FuelP on a carb’d O360… not being right.

+1 for fuel pump off, and flaps 0° at 1k’ agl… (people have forgotten either at least once)

The whole range is pretty small, and it is only a handful of PSI…

FuelP will be sensitive to attitude… and a few other things…

If it is operating in the green arc… this is acceptable according to the design…

But something isn’t right if it is reading zero…  most likely a gauge issue, or the engine would stop running…

By-passing the fuel / water separator… is a method of skipping the screen in the event the screen gets filled up…

Interesting that we don’t see more discussion on this by-pass… as it is a possible source of dirt getting past the screen… if by-passing solves the engine problem… it won’t be for long…. (Odd plumbing solution?)

Inviting @Hank to the discussion (M20C fuel system issues)

Best regards,

-a-

[47U]

46 minutes ago, carusoam said:

Interesting that we don’t see more discussion on this by-pass… as it is a possible source of dirt getting past the screen… if by-passing solves the engine problem… it won’t be for long…. (Odd plumbing solution?)

Both paths of the parallel fuel system have screens.  The Bendix/Facet electric boost pump has a screen incorporated in the pump body.  The other fuel path to the engine driven fuel pump goes through the gascolator, which is located up in the nose wheel well (for convenience purposes, obviously). 

[Hank]

I don't recall fuel pressure in the climb, but typical cruise at 7-10K, my C will vary anywhere from 2.5 down to 0.5 psi. The lower pressure is in the narrow portion of the green stripe. Does anyone know why the green stripe is different widths at different pressures?

Watch yours closely the next time. There's a difference between "0.5" and "near zero," but it does keep the engine running.

Unlike our injected brethren, I've not found watching the Fuel Pressure to be a good way of telling when a fuel tank is about to run dry. Sometimes the engine will surge like my lawn mower, then I switch the tank; Sometimes there will be no change in feel or sound, it just smoothly and suddenly goes quiet, so I rapidly switch tanks.

The joys of "classic" equipment! Hope you enjoy your new monitor and get it dialed in soon. A report back at that point would be beneficial to many of us!

[Hank]

My fuel pressure gage is different. This is my C ar 9500 msl, I see thus a lot.

[47U]

4 hours ago, Hank said:

Does anyone know why the green stripe is different widths at different pressures?

Remembered reading this in the supplemental owner’s manual from 1965.

Bluehighway does have the wider band (like mine).  It’s just not as pronounced at Hank’s.

[kerry]

Mine does the same on climb out and cruise after turning off the electric pump.  Sometimes I turn my electric pump on for a few seconds when I see the mechanical pump pressure is going low which in turn seems to keep the mechanical pressure higher.

[mike20papa]

I had a similar problem with my A model.  It also seemed to get progressively worse and I finally admitted to myself not remembering the problem when the engine was low time.  I put a new mechanical pump on and now fuel pressure does not need to be supplemented with the aux. pump.  Don't ask me why.  In theory, I would have always told you that either a mechanical fuel pump worked, or it didn't,  not some diminished effectiveness.  Warning .. changing out the fuel pump on these engines without removing it is a very challenging task.

[moodychief]

On 6/11/2022 at 10:40 AM, ed said:

I have a 1963 M20C (a D that was converted to a C just after leaving factory).  180 HP, carb so the Low Pressure mechanical fuel pump (4 PSI).

Mechanical pump holds 4 PSI  in level flight.  In full power climbs FP drops to near zero on the gauge but engine runs OK.  If I turn on the electric boost pump on climb, then pressure is higher, but not quite the full 4 PSI.   On ground electric pump holds a full 4 PSI as does the mechanical pump.  Also seems like it uses more fuel thru fuel totalizer than actual.  I put on new mechanical pump and did not change anything. 

I am wondering if fuel could be going back thru the electric pump into the tank?  See picture. This is the fuel system on B, C , SN 1701 thru 2622

and D  SN 101-200.   Its an odd design, in that the boost pump path bypasses the gas-colator.  Also I don't see any check valve to prevent fuel from going back thru the electric pump.  So the elect pump must have some internal flow back restriction method.   Item 11 is a short fuel hose.  You can see that the T on the firewall connects the electric pump path and the mechanical pump path.   I think it has always been this way on the pressure but was curious what others see.   Only way that fuel could go back into tank would be if whatever device is in the electric pump to prevent flow back is leaking. 

Check the line from the gascolator to the mechanical fuel pump.  I had a similar situation and spent several months and $$ troubleshooting.  We ended up finding a crack on the top of the short section of fuel line that runs through the cabin (on left side of doghouse).  

[47U]

3 hours ago, moodychief said:

Check the line from the gascolator to the mechanical fuel pump.  I had a similar situation and spent several months and $$ troubleshooting.  We ended up finding a crack on the top of the short section of fuel line that runs through the cabin (on left side of doghouse).

This is the hard line moodychief references.  Mine was also cracked and broke when I removed it.  It did not leak and there was zero fuel smell in the cockpit.  I would have suggested checking the line to the OP, but my symptoms were more than low fuel pressure.  I had wild fluctuations in both pressure and fuel flow, yet the FP-5 total-used matched within a few tenths at the fuel island.

It broke at the elbow in the footwell that passes through the firewall.  I suspect that failure to use a backup wrench when tightening flared fitting on the hose to the mechanical fuel pump was a contributor, if not the cause.

[carusoam]

Strange FF readings are often caused by air bubbles entering a worn or broken part…

Air leaking in can occur before fuel can leak out… based on viscosity differences of each.

when fuel leaks out…. The light blue stains start to appear, turning darker over time…

PP thoughts only,

-a-