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Fuel pressure oscillates harmonically

slaw76

I'm New Here
I know it was asked before but I figured I'd start a fresh thread with details pertaining to my situation.

Let's start with a brief video demonstrating the problem, I recommend watching to the end so you can get an idea of time for the whole sequence of down-up-down-up:
https://photos.app.goo.gl/SzGkqARbpCdVgjrw8

Context: just finished the built, new IO-540 engine at ~50hrs, on a hot day. This occurs only when the engine is hot and not in level flight. Never during the run-ups at start of day. Even when the gauge reports PSI is dropping, the engine seems ok; no signs of starvation that I can hear or feel. Boost pump stabilizes the pressure right away. Switching fuel tanks mid problem does not have a positive nor negative impact.

In a similar thread, builders suggested adding insulation - can anyone post a photo of their handy work insulating the relevant pieces and what materials were used? I gather the flow meter along with the short intake pipe sits right above the exhaust. On a slow descend the fumes are pushed up, heating up the floor, fuel lines, cube, etc. I want to start by wrapping the fuel lines/red box in insulation or lay insulation on the floor. If anyone can recommend ideal materials for the job go for it.

Safe returning home from Osh to everyone.
 

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Disconnected thoughts.

Is it possible there is a higher frequency dynamic and the electronics has an aliasing capture issue? The fuel flow does not seem to change. PostEdit - -A damping orifice should fix that re Brians comment below.

My brain can not connect with vaporizing fuel on the suction side going away and coming back. If it is hot, lower flow is going to make the fuel temp rise worse.

Do you have the electronic, time based data to plot?
 
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Sensor...

I was thinking it might be a sensor "problem" akin to the well known issue with MAP and the G3X @ 2400RPM.

You may want to put an Omega Engineering PS-8E on the fuel pressure line @/in front of the transducer.

B
 
The makers of the Red Cube (electronics international) recommend that the Cube should be placed after the mechanical fuel pump and before the carb or throttle body, not between the two pumps. (to stop the symptoms you describe) I’ve put mine as recommended before the throttle body, others have suspended in the fuel line between the throttle body and the spider, both positions are successful. In both those positions the fuel through the cube is pressure positive rather than fuel being sucked through the cube when it’s in the tunnel. When the boost pump is turned on you say the problems disappear.
 
Three recommendations:

First: check all the fuel line B nuts from the mechanical pump back to the tanks. Make sure all are tight. One time after removing and reinstalling my fuel filter I ran into an issue like that. I had fluctuating fuel pressure off the mechanical pump but with the boost pump on it went away. Turned out a B nut had just a very minimal looseness like 1/8 turn to tight. No fuel stain was seen but just enough that it sucked a molecule of air and cause cavitation., which effects the sensor.

Second, the fitting from the pump to the sensor, do you have a restrictor type fitting installed. A restrictor fitting does two things. Helps stabilize the pressure to the sensor and if your fuel line was to rupture it restricts the amount of fuel escaping.

Third and in regards to the red cube. The location in the tunnel per Van’s plan is incorrect. This has been an issue since the time the kit came out because the plans have it in the tunnel. Greg is spot on, It needs to be at least after the mechanical pump (this is also per the EI installation instructions which you should go over before reinstalling). The problem is the red cube needs to be on a pressure side not the suction side, which can produce cavitation. Which could be a cause. In the long game and much as it hurts to say I would relocate the cube just because it will be a problem (even if it’s not this time).

Hope this helps….
 
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What Rich said, plus make sure you have a continuous line with NO 90 degree fittings running from fuel tank to fuel selector. Get rid of all of the bends exiting your fuel filter to your electric pump. Move your fuel flow red cube to between the fuel servo and flow divider.
90 degree fittings on the suction side are not good! The output of your mechanical fuel pump to the sensor should have a restrictor built into it. If it doesn't, order one. They come packaged with all new tempest pumps.
 
Thanks for bringing this up. I'm not the only one with this crazy problem (see https://www.flysto.net/logs/9ti8v223?view=graph for a visualization - specifically Fuel Pressure and MAP graphs). Same here - hot days seem to be more prevalent than cold, and it'll sometimes just 'go away'. Thanks for the suggestions on the thread!

Notice how the variability in MAP tracks to changes in RPM?

The intake port opening/shutting sets up a pressure wave in the MAP pressure line which the transducer "sees" as changes in MAP.

I believe, although it hasn't been confirmed by Garmin to my knowledge, that the fuel pressure transducer suffers from a similar effect/problem.

Putting a snubber in front of the MAP transducer will resolve this issue. See G3X Touch installation guide for details on the snubber.
 
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