Lower Fuel Pressure when climbing

[carusoam]

Clarence,

I'm an emotional guy.  If I could block the fuel from coming through the side of the cabin, I would do that too.

- my 65C leaked vacuum when the MP line broken...

- it leaked rpm when the tach cable broke...

- it leaked a lot of fuel when the level sensor seal aged, soggy carpet under the kid's feet.

50 year old sensors are worth considering replacement as their time comes.

 

Dev, check the fuel level sensor seals during annual as well.  If you haven't done that before.

Best regards,

-a-

[Jim Peace]

My C does this as well. I even have a thread on this with a video. 

Dont buy a new pump. Probably will not fix a thing. You probably notice that your fuel pressure goes to normal once you set up for cruise.  After reading and thinking about this a lot I think it is just a mooney thing. 

[carusoam]

Unfortunately, I did not have FF or a JPI in my C.

From an engineering point of view... 0 psi and spikes in FF would have me working overtime to duplicate them to know what causes that to happen.

Are you able to detect the carb's second fuel nozzle opening and closing with the FP and FF?

the second nozzle is open at WOT and closes at slightly less than WOT. Wondering if that can be part of the observation. Typical instrumentation would detect the resistance to flow being removed as increased FF and possibly less FP.

as usual, I am only a PP.  not a mechanic.

Good luck,

Best regards,

-a-

[carusoam]

Jim,

I would settle for it being a Mooney thing if somebody could it explain it.  There are too many things that get ignored because they were considered normal. Like oil dripping off an O360.  Fuel smell in the cabin. I ignored them because old planes do that...

Might be a good idea to send a note to the guru a KGGG.  Since you are both seeing the same thing and it may be normal?

Best regards,

-a-

[DXB]

1 hour ago, Jim Peace said:

My C does this as well. I even have a thread on this with a video. 

Dont buy a new pump. Probably will not fix a thing. You probably notice that your fuel pressure goes to normal once you set up for cruise.  After reading and thinking about this a lot I think it is just a mooney thing. 

Jim- I just watched your video- it looks like your FP never gets close to the red line at 0.5 psi - it's basically the same drop I get normally every time to around 2psi, except without the oscillation.  What I'm talking about is an occasional event (maybe 1 in 10 flights) where it gets to the red line or even lower, leading me to soil my shorts and turn the boost pump back on.  Totally random event, has happened on either tank, different temps, different speeds, but always WOT and in climb.  Months can go by without it happening, leading me to stop worrying, and then it happens again.

[Andy95W]

I don't want to be like your MSC mechanic who is underwhelmed by your complaint.  I do want to point out that you are trying to measure with scientific precision a fuel system that basically throws fuel into the induction system and hopes for the best.

I'm assuming all other indications stayed the same, such as EGT, CHT, MP, RPM, and the engine didn't cough or hiccup.

Carburetor floats do bounce around which closes off the valve that allows the bowl to fill.  That could cause slight anomalies in your fuel flow as the bowl fills or stops, but the engine runs normally from the fuel left in the bowl.  

I wonder what would have happened if you had left the electric boost pump off, or if you hadn't noticed the drop at all.  Or how many times in the past has this indication dropped to 0 but was never noticed?

Good luck.  I'm really curious to hear the answer, if you find one.

[DXB]

42 minutes ago, N1395W said:

I don't want to be like your MSC mechanic who is underwhelmed by your complaint.  I do want to point out that you are trying to measure with scientific precision a fuel system that basically throws fuel into the induction system and hopes for the best.

I'm assuming all other indications stayed the same, such as EGT, CHT, MP, RPM, and the engine didn't cough or hiccup.

Carburetor floats do bounce around which closes off the valve that allows the bowl to fill.  That could cause slight anomalies in your fuel flow as the bowl fills or stops, but the engine runs normally from the fuel left in the bowl.  

I wonder what would have happened if you had left the electric boost pump off, or if you hadn't noticed the drop at all.  Or how many times in the past has this indication dropped to 0 but was never noticed?

Good luck.  I'm really curious to hear the answer, if you find one.

Thanks for input.   I'm trying to walk a balanced line between not pushing enough to investigate and acting like a total spaz with the mechanic, who certainly does have more perspective than me on this issue.  The imprecision of the carb system is clearly a cofounder here- but I'd think that a FP near 0 just before the carb at some point would make the bowl run dry.  EGT, CHT, MP, RPM do all stay the same, and no stumble in the engine.   But thanks to the JPI alarm , I react immediately by turning the boost pump back on, so the carb bowl may not have had time to deplete and make the engine stumble- too much of a wimp to wait it out.   I've verified that it happens only occasionally by pulling the data from all the flights logged on the JPI, and it hasn't happened unnoticed at least since I've had JPI in place the last couple of months.  

[carusoam]

DP

[carusoam]

 

Anyone know what the valve in the Marvel carb looks like or how it operates? (Proportional based on float position?)

does it stick open when it gets dirty or aged?

where are the FF and FP gauges in relation to that valve?

This may be interesting....

M20C uses MA-4-5 Marvel-Schebler carb according to the '76 M20C POH.

http://www.insightavionics.com/pdf%20files/MA-4%20Carb%20Manual.pdf

They have a trouble shooting guide that may shed some light on what could happen.

check when the carb was OH'd last.  Look to see if the float has been updated to a plastic one.

the usual, I am a PP, not a mechanic.

Best regards,

-a-

[mike_elliott]

17 hours ago, M20Doc said:

"I would prefer an electronic sensor in place of the mechanical one.  Fuel and oil brought into the cabin when modern planes use electrons"

Fuel and oil in the panel is an emotional issue only.  All of the fuel in a Mooney is right behind your side upholstery panels, there is no fuselage skin between your bum and the inner fuel tank rib, and the fuel supply lines from the tanks to the selectors/strainer is under a thin aluminum floor board.

Clarence

I have often thought a simple fire wall at the wing root to possibly mitigate fuel draining into the cockpit in the event of an off airport landing and resultant compromising of the tank integrity may save a life or two.

 

[tony]

I read the POH and it doesn't say but I was always taught to leave the boost pump on until you level off at cruise.  It will increase the pressure up to the engine insuring there is no vapor lock in the fuel system.  Remember our tanks are below the engine which is why, I think, we cant use mogas in a Mooney.  They got a vapor lock at some conditions when trying to add an M20 to the STC. 

 

[DXB]

4 hours ago, tony said:

I read the POH and it doesn't say but I was always taught to leave the boost pump on until you level off at cruise.  It will increase the pressure up to the engine insuring there is no vapor lock in the fuel system.  Remember our tanks are below the engine which is why, I think, we cant use mogas in a Mooney.  They got a vapor lock at some conditions when trying to add an M20 to the STC. 

 

Tony- your point is interesting and generally makes sense to me.  But the Dukes boost pump supposedly isn't the most robust thing, unlike the engine driven pump, and isn't rated for continuous duty.  I was taught to turn it off at 1000ft agl, I think to help preserve it.   This also makes sense to me because the fuel pressure behavior in my system makes me wonder if I'll really need the boost pump in a pinch one day.  Also using it continuously for long climbs might mask emerging fuel system issues until they become severe enough to be a problem in cruise.  Usually I get assurance everything is fine when I turn it off in climb- pressure drops but stays well above the 0.5psi alarm lower limit- I imagine value was set based on the pressure at the transducer that's required to ensure max fuel flow into the carb bowl is available with a wide margin.    Then about 1 in 10 flights it scares me by going to less than 0.5, even though the engine doesn't stumble.  Fuel is one of those things I'd like to have very wide margins around- both how much I carry and how much I can get to the engine.  Maybe I should have got a C172 instead  where gravity works for you...just kidding