yeah, but Greg---your system isnt exactly stock, but we know it works.
Tom
Tom
yeah, but Greg---your system isnt exactly stock, but we know it works.
Tom
I posted this at the end of another thread today but decided after further research it would be more appropriate in this 11-page classic
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RV-10 in early phase 1 with IO-540 with AFP mechanical injection (no return lines) , Andair valve, EFII boost pump and red cube in the tunnel, 40 micron JEG's billet filters under the seat pans between each tank and the selector valve which were just inspected and cleaned after the first flight two weeks ago. Since the first flight on 100LL, running 93 octane no-ethanol mogas. No shroud/blast tubes on pump. One (right) heat muff and stainelss valve, left side muffler has a 60% radiant heat shield from Vetterman but no hoses or firewall flapper valve for heat. Y-connector in tunnel for front and rear heat from the ingle muff. Tunnel does not feel hot-- I did a fibrefrax and SS foil sandwich on the firewall.
All flights have been nominal until today, 3 hours into phase one, when in level cruise 80% power 16.5 gph indicated about 15 minutes into the lazy circles at 4000' I got the "Check fuel pressure!" command and saw 14 psi and falling. Boost pump brought it back to 25 psi immediately. This behavior repeated itself several times on three flights today and has seemed to increase in severity. It will drop very slowly to 5 or 6 psi. One time it caught me off guard and threw all 6 cylinders into the EGT red zone on the Advanced monitor and the engine lost power. (I hope it went quickly through the "red box" detonation zone because I was cruising at 80% power for break-in when it happened.)
I've had enough flight time today to narrow a few things down about this. It only happens on the left tank. The boost pump typically builds the pressure back quickly but the pressure has dropped to critically low values (although much more slowly) on the left tank even with the boost pump running. Switching to the right tank relieves the low pressure immediately and lastingly (or else I wouldn't be writing this until after SAR plucked me out of the Virginia mountains). Selecting Right tank fixes the issue every time, with or without the boost pump running.
The behavior certainly makes me think of a partly obstructed vent line on the left tank pulling vacuum on it gradually, as I can repeat the cycle over and over again. After running on the right tank awhile I can switch back to the left with no boost pump and it will take a minute or so before the pressure drop repeats itself. Back on the ground, I've pulled wing root panels and swiveled the vent lines around to where I can blow into them - and with the fill caps loose I feel like it takes the same effort to blow air through either side. As others have noted, there is considerable resistance to blowing through that length of 1/4" tubing!
Maybe it's a filter that needs cleaning and it takes a while for the engine driven pump to cavitate after switching over to the side with a partial filter blockage.
I plan to chase the vent lines with weed eater monofilament and be sure there's no plugging material. I'll do fuel flow tests again on both tanks tomorrow (initial values were 40 gph both sides before first engine runs). I might even fly with a temporary perforated tape covering over the filler neck on the left side to assure that tank is adequately vented.
May end up with my transducer getting relocated to the FWF area...
I did verify today that fuel flow with the boost off is a nominal (as-yet uncalibrated) 16.1 gph in full rich cruise and 16.3 gph (once it stabilizes) with the boost pump on.
Seems like I'm in good company, here. Lots of work ahead: testing, measurement, shrouds, new vent locations, maybe even abandoning mogas
I appreciate DanH's opinion that leaving the boost pump on continuously is merely emulating the norm in the automotive world, but it still makes me uneasy being dependent on that thing. And I've seen one pressure dip today that happened on the left tank while the boost was already on. That part concerns me greatly.
Just curious, why not use the boost pump on takeoff? I also have mine on for landing - one less thing to worry about close to the ground.... I take off without boost pump on, for at least the last 300 hours. Pressure stays at 25-27 thru all phases, either tank. Climbs to 12.5 no problem. (Hi as I go) I use boost pump for cold start only. 100LL AvGas only.
they are finer than spec for any injection system at 40 microns but don't have a very large filter surface. They are rated for plenty of flow, but that's at pressure, not on the suction side... At the time I was looking for ways to go finer, trying to block this crud from reaching the injectors. It seems to have all been flushed out now.
Updates: flew to town and grabbed some 100LL - 2/3 ratio in each tank with balance 93 mogas. Left tank will now keep pressure with boost pump on, and loses prime on the engine pump much more slowly than before. Right side still runs fine, as before.
Definite improvement related to vapor pressure, I assume...
Steve and Tom are making hoses for me to plumb the left side with a flex line from the tank straight to a Holley paper element 10 micron filter under the seat and another flex line through the tunnel side to the Andair valve, eliminating two fittings, and with them the possibility of cracked flares.
In the meantime, I'll be flying with avgas or a high avgas blend - which I certainly hope is only a temporary necessity.
After Phase 1 flight testing in my RV-10 I ran one tank of ethanol free mogas with the other tank 100 LL exclusively. I never experienced a difference in fuel pressure when I swapped tanks during flight. I did use a fuel pump cooling shroud and insulated the bottom of the tunnel to reduce tunnel heat. I am not a fan of paper media fuel filters at all in airplanes.
Updates: flew to town and grabbed some 100LL - 2/3 ratio in each tank with balance 93 mogas. Left tank will now keep pressure with boost pump on, and loses prime on the engine pump much more slowly than before. Right side still runs fine, as before.
Definite improvement related to vapor pressure, I assume...
Have you noticed anything with regards to the rate of pressure loss versus fuel consumption? You'll always see increasing vacuum as measured at the pump suction with increasing flow. If you've got a moderate obstruction on one side, you should clearly see a difference in the vacuum between the two sides, but also should find that the severity your problem varies with fuel flow rate. Clearly not a solution to the problem, but measuring that vacuum and how it responds to different conditions should confirm whether this is a restriction issue or not. If on the other hand the vacuum is lower (closer to ambient pressure) on the problematic side, you have a leak and you're drawing air.
When it comes to measuring vacuum, you also may want to consider using a differential gauge/sensor (other side open to atmosphere) rather than a single port. With single port you'll need to consider your current density altitude when comparing values.
Best luck getting to the bottom of this.
One thing to consider, is that fuel tank fill height will change that amount of suction required to pull fuel. The fuel pump draws a good bit less suction when pulling fuel from a full tank than a near empty tank. Something to consider when reviewing symptoms. I would expect a small air leak to show more significant symptoms on an empty tank compared to a full one.
Larry
I also like to use these Del Fitting Seals when I final assemble aluminum fuel and brake lines. They are a bit pricey but help make a leak free sealing joint.
https://www.aircraftspruce.com/catalog/hapages/del37fittings.php