Fuel Pressure Mystery

[Randy]

RV7A with I-O375 Aerosport with Silverhawk Injection.
About 150 hrs on the engine. Running 100LL all the time at this time.

I have posted about this before but thought I should do it again since the problem continues and I have learned more about it. I would really like to resolve this as it is not the kind of thing I want happening with passengers aboard, or even me...

Problem: I get fuel pressure warnings from the Dynon Skyview EMS. It seems to happen mostly at higher altitudes, like above 10,000? I have learned over time that if I do not respond by turning on the boost pump etc. the engine still keeps running fine and the fuel pressure fairly quickly returns to normal of 25 PSI. I have seen it drop to 13 PSI when it happens.

What have I done to fix it?

Replaced Kivilco fuel pressure transducer, purged fuel hose that feeds it.

Replaced all fuel system plumbing, eliminated many restrictive fittings and elevation changes in the system in an effort to reduce flow restrictions. Verified all flares are good with no cracks etc.

Installed shroud on mechanical pump with blast tube.

Added scat tube to route heat from heater down and away from the fuel pump to the cowl exit area.

Verified the fuel vents are clear, it is pretty difficult to blow through them (by mouth) but they are clear. I also blew them out with pressure hose. I even tried out the coil in the wing root version vent with 3/8? tubing but decided not to leave it in place.

Installed new fuel pump

After each attempted solution, it seemed to eliminate the problem only to have it return, sometimes many flying hours or later. It was approximately 50 hrs engine time since the last low fuel pressure event. This one happened high above the Sierra Nevada foothills at 12,000?. This time I did not respond other than immediately turning toward the nearest airport and watch the fuel pressure gauge etc. The engine kept running fine again and the fuel pressure recovered fairly quickly. I flew on about another hour to my destination in Redding CA with no issues. I did not turn on the boost pump.

Reviewing the Savvy Analysis engine data from the flight may have provided a clue and this is the reason for my posting this information. When I plot the fuel pressure and fuel flow on the same graph, the fuel flow falls off from around 11 GPH down to about 9 GPH, at which point the fuel pressure dropped from it?s normal 25 PSI down to about 15 PSI. At this point ?Betty? says ?Fuel Pressure? in my headset and gets my attention, all of it. I will try pasting an image here:


The image may not be clear enough to see what I am talking about so here is a link directly to my Savvy Analysis file if you prefer.

https://www.savvyanalysis.com/flight...f-4dffc2c38087

To see it clearly you need to choose Fuel Pressure on one side of the top chart and FF (Fuel Flow) on the the other. Then you need to select the area or period of time with the pressure drop and fuel flow spike in it, by clicking and sliding the curser across the area.

The fuel flow falls down for about 4 minutes, then the fuel pressure falls briefly and recovers, then the fuel flow gradually recovers back to near where it was before the event.

I have the red cube fuel flow meter.

Any ideas as to just what is going on here based on the chart? Perhaps this is classic symptom of vapor lock but it seems I have done just about everything I can to eliminate that. I suppose I could put more insulation on my red cube, but it has heat reflective tape on it and the muffler has a shield between it and the cube. Perhaps that is my next attempt at fixing this but I am hoping some of you folks might have ideas based on the information in the Savvy charts. Thanks for any ideas and help in advance.

[gasman]

Did you check and confirm every electrical connection?

[YellowJacket RV9]

Don't have a lot of ideas, but I do enjoy learning more and I tried looking at all your other engine data. One thing I did notice is that as your fuel flow was dropping, so was your airspeed (by about 15 knots), and you were also climbing, about 1500' total, during the event up until the fuel pressure drop. Also, during this time, the OAT dropped almost 10 degrees (still above freezing, however).

I guess what I am trying to say is that the reduced fuel flow seems to correspond with a climb and also a fairly rapid temperature change (10 degrees over 1000'). Maybe somebody smarter than me would know what that could suggest. I had maybe thought of ice somewhere in the fuel system, or vents, but at those temps I'm not sure it's possible.

Good luck, interested to hear what the smart people suggest...

Chris

[pujol]

I had the same thing from 8000
I tried everything and I ended up changing the mechanical pump 2 weeks ago
problem solved , the pressure is ok

[BillL]

Interesting, and surely annoying, situation, Randy. Just to state it out loud, and for review,


#1 pressure transducer replaced = pressure drop is real
#2 mechanical pump replaced = no mechanical pump problem.
#3 System "checked" = no bad joints ( not 100% sure about this)

Just for a thinking exercise, lets accept #1 and #2 above. Then think about what could cause perfectly good components to do this. What if - - a small gas bubble is collecting in the tubing somewhere else along the line?

To validate #3, you might run a pressure test with air. Disconnect at wing root, cap the fuel hose at the servo, purge the fuel and pressurize (closed system) with 30 psi air for 24 hrs. Take care when air purging as to prevent damage to the red cube.

It will definitely indicate any leak. Finding it is another matter. Snoop.

[Randy]

After studying the engine data more, I believe it must actually be a fuel vapor problem. Even though it is down stream of the mechanical pump, my red cube is not far from a muffler. The cube is between the mechanical pump and the servo. I have it covered with reflective aluminum material but will change that out and wrap it completely with fire sleeve material as most others have done. That could be it. I had discounted this idea since that part of the system is at 25 PSI, but it is time to try something I have not done. All other fuel lines firewall forward are fire-sleeved, the red cube is the only uninsulated item in the system

The data shows a corresponding rise in EGTs as the fuel flow was dropping, and then a sharp fall as the fuel pressure fell off. No doubt about it being real.

Thanks for the reminder of a pressure test of the fuel system. Even though I replaced all the fuel lines that does not mean there can't be a tiny leak somewhere. I have an idea for using nitrogen for low pressure test and then use snoop or equivalent.

[TS Flightlines]

Randy---does is happen only at higher altitudes?