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Fuel pressure fluctuation at higher altitudes, M20J

jetdriven

By jetdriven
in Modern Mooney Discussion

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jetdriven Grand Master

jetdriven

Posted
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This has been going on for about a year now. Above 7-8K feet, the fuel pressure is lower and fluctuates.  Below that altitude, the fuel pressure is steady at 25 PSI.  As you climb higher, the fuel pressure steadily lowers to around 23, and above 7K or so,  it fluctuates between 20-23 PSI. Occasioanlly dips down to 18 PSI.  Running LOP the engine occasionally changes tone which sounds like it is leaning as air bubbles pass through the injection system.  We recently rebuilt the fuel selector and though we cured it, however, it is back now.

 

Turning on the boost pump the fuel pressure rises to 26 PSI and is steady.

 

Has anyone else seen this? Did you fix it?

N601RX Grand Master

N601RX

Posted
Posted

If the weep line to the mechanical pump was blocked could it keep the mechanical pump from regulating correctly? It really sounds like it is sucking air in though.

Piloto Grand Master

Piloto

Posted
Posted

Check the fuel caps O-rings and vent ports. In flight the fuel tanks are pressurerized by the air flowing into the vent ports. If the cap O-ring is leaking the internal tank pressure drops susbtantially due to the negative pressure on top of the wing. As the plane goes higher in altitude the problem gets worse. This is why jet planes have the fuel pumps inside the tank.

 

José 

N601RX Grand Master

N601RX

Posted
Posted

How about connecting a handheld vac pump to the line going to the mechanical pump and plugging the line that connects onto the bladder. Pull a vac and see if it holds or not.

  • Like 2
  • 2 weeks later...
jetdriven Grand Master

jetdriven

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Posted

Tom, brakes are fixed, and dual brakes as well now.

 

Jose, can you explain the function of the fuel pump vent line?

jetdriven Grand Master

jetdriven

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Video taken today. If the mixture is set to lean cruise, it varies a couple to 3 PSI. Enough to make the EGT vary a bit. Richening the mixture causes the fuel pressure to get dramatically lower. img.mov

Piloto Grand Master

Piloto

Posted
Posted

Tom, brakes are fixed, and dual brakes as well now.

 

Jose, can you explain the function of the fuel pump vent line?

 

The engine driven fuel pump has a vent line attached to the dry side of the diaphragm cavity. It serves two purposes. It allows the diaphragm to syphon the fuel and to keep the pump pumping in the event of a diaphragm ruputure. When the diaphragm ruptures fuel will come out of the vent instead of accumulating in the cavity and impairing pumping. If you see fuel coming out of the vent tube it is time to change the pump. Blockage of the vent tube will impair the pump from syphoning the fuel. There should be two thick lines going to the pump (fuel in & out)  and one thin line for venting.

 

José

  • 3 weeks later...
sailcail Apprentice

sailcail

Posted
Posted

We had a very similar problem on our J model after the installation of a reman engine.  We replaced the mechanical pump twice, rebuilt the fuel selector, rebuilt the gascolator, rebuilt the flow divider, replaced the boost pump, installed new fuel cap O-rings, replaced all of the flexible fuel lines, tank vents checked clear and had numerous pressure and vacuum checks of the fuel system by a number of shops.

 

In the end it turned out to be the fuel servo.  The shop that rebuilt the servo found a small piece of wire inside.  They think it might have come from a small wire wheel that may have been used to clean the parts. 

 

The fuel pressure fluctuations only occurred above 8K and the boost pump restored the pressure to normal.  In our case enriching the mixture did not affect the pressure.

kmyfm20s Veteran

kmyfm20s

Posted
Posted

We had a very similar problem on our J model after the installation of a reman engine.  We replaced the mechanical pump twice, rebuilt the fuel selector, rebuilt the gascolator, rebuilt the flow divider, replaced the boost pump, installed new fuel cap O-rings, replaced all of the flexible fuel lines, tank vents checked clear and had numerous pressure and vacuum checks of the fuel system by a number of shops.

 

In the end it turned out to be the fuel servo.  The shop that rebuilt the servo found a small piece of wire inside.  They think it might have come from a small wire wheel that may have been used to clean the parts. 

 

The fuel pressure fluctuations only occurred above 8K and the boost pump restored the pressure to normal.  In our case enriching the mixture did not affect the pressure.

Are you saying it was the servo for the fuel pressure gauge itself? I have had fluctuating fuel pressures on my 82j but have always had proper fuel flows on my gauge so I haven't worried about it. Absolutely no preformance issues what so ever. There is also another Mooney on my field that fluctuates fairly dramatically and he has thrown all kinds of money at fixing it. It would be great of this is the answer.

sailcail Apprentice

sailcail

Posted
Posted

The issue was the Bendix (Precision Airmotive) fuel servo which controls the fuel flow to the injectors.  The problem was intermittent and only occurred above 8k.  The pressure would drop/fluctuate for a short period of time.  This would change the fuel flow a few tenths of a g/hr, raise the EGT and few degrees and, if the engine was leaned, it would cause it to miss.  The line to the pressure transducer (gauge) connects directly to the fuel servo.  The change in pressure acted very similar to the one shown in Byron's video. Hope this helps.

 

Simon

  • 3 months later...
jetdriven Grand Master

jetdriven

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We changed the fuel pump and insulated the firewall-to-pump line with additional firesleeve.  Rock steady fuel pressure, and problem is fixed.

  • Like 1
  • 9 years later...
onautopilot Newbie

onautopilot

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Posted

I just had this issue on my '86 M20J and wanted to add my experience here for reference. In my case, fuel pressure was good at low altitude. Above about 9000 feet, I observed periodic drops in pressure that never left the green arc, but waivered around at lower pressures for a few seconds before returning to a normal relatively high reading within the arc. Using the boost pump always brought it back to full pressure at any altitude. I replaced the engine driven fuel pump and that fixed the issue.

  • Like 1
PT20J Grand Master

PT20J

Posted
Posted

Me too. I never noticed it with the factory gauges, but did when I installed the G3X with EIS. The engine and plumbing were unchanged, but the transducer is different. I added a snubber which improved it at lower altitudes, but at higher altitudes I can see it still wander. I think it might be heat causing vapor in the lines. The fuel pressure continually increases to an off scale reading after shut down with the mixture in ICO due to heat soaking. 

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