Lower Fuel Pressure when climbing

[ducmot]

Generally, my 1966 M20C has normal fuel pressure on climb-out, cruise, etc. The airport where I fly out of is at 5,000 feet and I have a mountain range to the east that goes up to 10,000 within 15 miles. I have noticed when I am climbing out to higher altitudes to clear the top my fuel pressure will go down to the really small green portion around 2.5psi. It still works fine of course. I haven't tried turning on the boost pump.......especially since there is no noticeable signs of lack of fuel.

Does anybody else have this happen on theirs?

[SkyPilot]

I don't have any problem. I get up to 14,500' and have not noticed a low pressure. Maybe your steam gauge is FUBAR?

[Jerry 5TJ]

In my 1965 "C" I have noticed that when using a full-rich mixture during climb in warm temperature the fuel pressure drops to the lower end of the green above 5000 feet or so.  When I lean for cruise, the fuel pressure rises again.  

[eaglebkh]

ducmot,

You must have a C or a G.  The E's and F's should bottom out around 14psi.  Also, air in the gage line can cause screwy readings.  This happened in my E and the needle would oscilate between 15-40 psi rapidly at full throttle.

[rob]

Quote: ducmot

Generally, my 1966 M20C has normal fuel pressure on climb-out, cruise, etc. The airport where I fly out of is at 5,000 feet and I have a mountain range to the east that goes up to 10,000 within 15 miles. I have noticed when I am climbing out to higher altitudes to clear the top my fuel pressure will go down to the really small green portion around 2.5psi. It still works fine of course. I haven't tried turning on the boost pump.......especially since there is no noticeable signs of lack of fuel.

Does anybody else have this happen on theirs?

[Z W]

Happens on my M20C also.  It seems to be worse when in a nose-up climb attitude (80-90 MPH).  When I lower the nose for a cruise climb (100-110 MPH) the fuel pressure comes back up.  I've never had a problem, and it always stays in the green, but I've made a habit of leaving the boost pump on until I point the nose down and/or pull the prop back to 2500 RPM for cruise.

[ducmot]

Thanks. That is about what I am experiencing, although a little faster, but through 8-11,000 ft.

[Oscar Avalle]

Happens also to my M20C. But it varies from tank to tank...

[FLY4KIX]

I have an opposite problem. 65 m20c.  My fuel pressure suddenly started showing above redline.   I moved the plane from 1k to 6500 but I can't see why that would matter. Any  idea what the cause might be?    When the engine is off,  the electric pump will show pressure at about 5psi   which seems about right.

Thanks!

[jrwilson]

My 63 20C fuel pressure drops in climb too.  Depends on DA, but starts at about 3000' in summer and gradually goes down to bottom of green arc (used to got to zero sometimes).  I have spent about $8000 on this "problem."  Replaced engine driven pump, boost pump (2x), carburator, filters, fuel lines (all of them)...  Lots of research later, mechanic called Don Maxwell and Mooney.   Apparently this is a known thing, C's do this.  Not sure why, but it happens.  All the money I put in to it at least has stopped the pressure from dropping below green arc.  

If you check the vans RV (180 HP versions) and piper cherokee (180 HP) websites, it is also a common problem w/ them.  Something to do w/ low wings and lyc 0360s.  It is even in the cherokee frequently asked question list on their new buyer guide (fuel pressure drop in climb/  they reccomend climbing w/ boost pump).

So I wouldn't worry too much.  Keep and eye on it and if it bugs you, turn on boost pump during climb.  

[DaV8or]

I have the same issue with my F. Not that often, but sometimes at the top of a climb, the pressure will suddenly drop. Flipping the boost pump on for a few seconds always fixes it. Best theory is it's vapor lock. The thing is, the engine pump either works, or it doesn't. There really no intermittent middle ground.

[DXB]

On 6/16/2012, 7:03:24, jrwilson said:

My 63 20C fuel pressure drops in climb too.  Depends on DA, but starts at about 3000' in summer and gradually goes down to bottom of green arc (used to got to zero sometimes).  I have spent about $8000 on this "problem."  Replaced engine driven pump, boost pump (2x), carburator, filters, fuel lines (all of them)...  Lots of research later, mechanic called Don Maxwell and Mooney.   Apparently this is a known thing, C's do this.  Not sure why, but it happens.  All the money I put in to it at least has stopped the pressure from dropping below green arc.  

 

If you check the vans RV (180 HP versions) and piper cherokee (180 HP) websites, it is also a common problem w/ them.  Something to do w/ low wings and lyc 0360s.  It is even in the cherokee frequently asked question list on their new buyer guide (fuel pressure drop in climb/  they reccomend climbing w/ boost pump).

 

 

So I wouldn't worry too much.  Keep and eye on it and if it bugs you, turn on boost pump during climb.  

 

I wondered if there is any new insight out there into this crazy problem?  I am dealing with it.  It happened twice when I first got the plane and then never happened again. Now it's happened two more times recently and scared the living $*&@ outta me- readings below 0.5psi in WOT climb that normalize with boost pump back on or dropping nose into cruise - no engine stumble, but I caught it immediately in all cases I think.   I'm going to have an MSC pursue it very aggressively at annual in two weeks because I can't tolerate seeing fuel pressure readings of 0.  Of course I fear that failure of the simple stuff (e.g. gascolator screen) will lead to expensive, time consuming stuff (e.g.replace engine driven pump and boost pump) with no benefit and the real risk of maintenance induced disasters.   I'd love to get some wisdom from a Don Maxwell-type person before going down what sounds like a frustrating road.  A detailed outline of my issue and the possibilities I've come up with using help from folks here:

Observation:  Fuel pressure when boost pump turned off during climb can drop to <0.5psi (below red line)

 

Details:

·         It happens only occasionally, not every time (4 times in about 50 flights)

·         Pressure returns to normal immediately by turning  boost pump back on

·         No drop when turning boost pump off in cruise flight

·         It has happened on my old FP gauge and the new JPI that replaced it, so it's probably not a gauge issue

·         The engine driven pump generates fuel pressure above the upper limit red line when idling

·         It has happened when running off either tank

·         I haven't noticed any fuel leaks

 

Possibilities?

·         Position of transducer, which sits well above the carb

·         Check valve in boost pump

·         Engine driven pump failing

·         Intermittent gascolator obstruction - debris against screen in gascolator?

·         Air leak in gascolator

·         Collapsing fuel line

·         Intermittent tank vent obstruction at climb attitude

·         "Carbed Lycomings in low-wing planes just do this"

 

 

[jrwilson]

I reported that I initially changed engine driven pump but problem persisted. Ended putting in a new new pump. That solved it once and for all. Maybe I got a bad pump initially or a bad install that let in air, not sure...

The other things I tried helped bring it up a bit but it looks like it can be solved.

Sent from my iPhone using Tapatalk

[DXB]

5 hours ago, jrwilson said:

 

I reported that I initially changed engine driven pump but problem persisted. Ended putting in a new new pump. That solved it once and for all. Maybe I got a bad pump initially or a bad install that let in air, not sure...

The other things I tried helped bring it up a bit but it looks like it can be solved.

Sent from my iPhone using Tapatalk

thanks- unless something else is found as a clear cause I'll probably just spring for a new pump and hope for the best

[KSMooniac]

My new pump is doing it as well...

Sent from my VS985 4G using Tapatalk

[jetdriven]

I changed the engine driven fuel pump for this as well. But I also did one more thing. Insulated the 124J silicon fuel hose with fire sleeve. Fixed it but maybe this is a  vapor lock issue not a fuel pump one. Pumps push much better than they pull, and these planes must lift the fuel 2 feet and the hoses passes right behind the muffler...hot air.... Heat soak...pump suction...altitude lowering ambient air pressure...

[KSMooniac]

Did you insulate the hose feeding the engine pump, or the one exiting? I'm not sure there is enough room around the input hose due to the cowl flap mechanism...at least on my installation.

Sent from my VS985 4G using Tapatalk

[jetdriven]

The hose from the firewall to the pump inlet. It's just fire sleeve, it's no bigger in OD than the original hose I replaced, that was fire sleeved. In fact, that's what I got it from. 

[nels]

I don't own a C model but have a few ideas, only ideas, nothing concrete.

I do think it is a fuel delivery problem to the pump or at the pump not from the pump to the engine. At 10k ft the outside pressure is about 10 psi not 15 psi as on the ground. The air pressure outside must push the fuel to the pump before the pump can pump it to the carb. If the vent in the tank is obstructed the pressure in the tank could be reduced even further than 10 psi, maybe a lot further than 10 psi. If you have a leak in the filler cap "o" ring or you have a leak at the top of the wing skin into the tank the low pressure on top of the wing will also be seen inside the tank. This would also be seen during cruise but while in climb you could see an awful lot more fuel being drained from the tank that need to be replaced with air at the same rate. The fuel does need to go up hill to get to the fuel pump ( I think ) which means there must be more pressure on the tank side or in the tank than the intake side to the fuel pump. 

Also, and as I said I'm not a mechanic and not familiar with C models, but typically there needs to be a vent hole in the fuel pump housing above the diaphram which allows the pump to deliver fuel at the same rate no matter what the outside pressure might be.  If the hole is plugged or inadequate, maybe the pump just starts delivering less fuel which would be lower pressure.

Just some ideas not suggesting it's the cure.

[DXB]

And so I did two steep climbs today to reproduce, one up to 8500 with boost pump off and couldnt reproduce the issue.  I feel an expensive wild goose chase coming, but no safe alternative I suppose.

[DXB]

On 11/27/2015 at 0:25 AM, jetdriven said:

I changed the engine driven fuel pump for this as well. But I also did one more thing. Insulated the 124J silicon fuel hose with fire sleeve. Fixed it but maybe this is a  vapor lock issue not a fuel pump one. Pumps push much better than they pull, and these planes must lift the fuel 2 feet and the hoses passes right behind the muffler...hot air.... Heat soak...pump suction...altitude lowering ambient air pressure...

Plane is in annual now, and MSC mechanic is underwhelmed by my concern - has the perspective of "eh...carbed Mooneys just do this."   I don't feel quite so chill about it yet.  Byron's point is an interesting one - an air/vapor lock issue in the fuel hose up to the transducer might explain why a drop  to <=0.5 psi only happens to me once in a while when the stars align, not every flight. Also seems like a strange design to have the pressure transducer mounted much higher than the fuel connection to the carb.

 Anyway, per my reading, the engine driven pumps on the 0-360s are actually fairly robust and rarely fail.  Maybe I should just replace my old Dukes boost pump with a Weldon, since I may not have the guts to turn off the boost pump any more until I level off. 

[carusoam]

DP

[carusoam]

The C has a noticeable or significant fuel pressure drop during the climb.

Byron may be right. It could be a bubble or something else.

Consider while it is in annual to get it's tube cleaned or replaced preemptively.

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

Check the age of the hose at least.

You need to be comfortable that it is working properly.  Outside of the green arc is not a good sign.

Best regards,

-a-

[Guest]

"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

[DXB]

7 minutes ago, carusoam said:

The C has a noticeable or significant fuel pressure drop during the climb.

Byron may be right. It could be a bubble or something else.

Consider while it is in annual to get it's tube cleaned or replaced preemptively.

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

Check the age of the hose at least.

You need to be comfortable that it is working properly.  Outside of the green arc is not a good sign.

Best regards,

-a-

Actually the fuel line to the transducer is only 1 year old.  I had the same issue intermittently on the old gauge and the new JPI, whose transducer is mounted away from the firewall, but still much higher than the carb.  Here's the only trace that I've captured from the JPI that illustrates the problem - here FP drops to 0 (!) before I turn on the boost pump.  There was a slight drop in fuel flow associated with this but no change in engine peformance.  The bubble/vapor lock would explain the intermittent nature of the problem but maybe not the slight drop in fuel flow.