Jump to content

Fuel pressure fluctuation at higher altitudes, M20J


Recommended Posts

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?

Link to post
Share on other sites

*Members that donate $10 or more do not see advertisements*

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 

Link to post
Share on other sites
  • 2 weeks later...

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

Link to post
Share on other sites

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

Link to post
Share on other sites
  • 3 weeks later...

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.

Link to post
Share on other sites

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.

Link to post
Share on other sites

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

Link to post
Share on other sites
  • 3 months later...

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.