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rv-10 fuel pressure drop solved ( for me)

Lan Vinh Do

Well Known Member
No fuel pressure drop for me anymore.

I had fuel pressure drop after climb as many other rv when i was turning off my aux fuel pump. As many other i had to keep my aux pump 1 or 2 min after level flight to prevent too much pressure drop. It was going down from 26 psi to 12. I thought before that it was the sensor that was doing this until i had a very perceptible loss of power for a fraction of a second when i was turning my aux fuel pump off.

So i can't pinpoint exactly what was doing this as i change many thing during my annual inspection. What i try before the inspection was the cooling shroud on the mechanical fuel pump and it wasn't changing much things.

At the annual, i did new fuel line from fuel selector to firewall. I change my electrical fuel pump to the new afp fuel pump. I Screw the filter directly in the fuel pump assembly ( with the modified fitting from Don ) and i remove the fuel flow sensor from the tunnel and put it after the throttle body. With this i pass from 6 fuel line in the tunnel to only 2 ( probably much less heat with less time in the tunnel and a lot less fitting )

I also did a air pressure test of my system and found a really really tiny leak from my gascolator .

So i did a long climb today to 10 000ft, turn off the fuel pump immediatly after levelling and no drop at all. It stay at 25-26 psi all the time.

So was it the fuel flow sensor that is now on the pressure side ( could induce vapour lock on the succion side) was it air suck by the gascolator, was it a defect in my old aux fuel pump, was it too much heat pick up by long loop of fuel line in the tunnel? I will never know but it was one of these.

It was a long annual inspection put i was glad of the result.
Also as many already reported, the fuel flow sensor is now rock stable and no variation at all with the fuel pump on or off
 
Good Hunting

Great job of sticking to it!
I had a similar issue with my low pressure system while climbing and keep looking and replacing, and found a bad transducer. Very satisfying to find the issue.
Thanks for sharing your work and results.
Congrats.
 
Do not always assume a bad sensor

The main goal of my writing was that i was assuming a bad sensor in the past and now, the fuel pressure doesn't drop at all even after a rapid climb to 10 000ft. So I had a real low fuel pressure problem.

So never take for granted that it's just a bad sensor or a bad sensor reading .
 
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You will likely see this again. in warm to hot weather and with warm fuel, climbing through 6-7000' you hit a zone where ambient pressure combined with the up hill suck for fuel, lower fuel flows in the cruise (assume LOP) all conspire to heat the fuel up and create vapour locking.

Just to make it worse any leaks or other things like sharp corners etc do not help.

Just be aware that some really hot days you may need your pump on for 10-30 minutes in the cruise, especially when the OAT is crazy warm.
 
I used to see that pressure drop on a climb to higher altitudes when I had a FloScan sensor in the tunnel.
I replaced the FloScan with a redcube and mounted it between the fuel servo and the spider. Fuel pressure during climb is rock solid now at 26 psi.
 
I flew with Raymo this past weekend (RV7A, IO360/180) and we didnt notice any significant pressure drops either during climbout, or during tank changes. His cube is mounted at the spider as we have been doing for clients for quite a while now.
Don Rivera at AirFlow has all the flow data on this stuff!
Tom
 
Guess I'm in the club

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. After first flight on 100LL, running 93 octane no-ethanol mogas. No shroud/blast tubes on pump. Tunnel does not feel hot-- have 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. Once 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 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 - as long as the system has had time to "rest" by feeding from the right tank I get good pressure from the left tank for a minute or so, over and over again. 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.

So 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.

I'll take any ideas on this. Can't have a fuel system that is electrically dependent on a single backup fuel pump to access half the fuel payload. That's just not acceptable.
 
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air leak

On my case, i think the main thing was air leak ( Air aspirated in the system on the succion side of the mechanical fuel pump. )

watch this video. All of it but the air leak is explain around 11 min.

https://tempestplus.com/product/fuel-pumps/

It may not be your problem but it was mine. Pressure drop after boost pump off.

I am not an expert. It took me couple of years to find the exact problem. Mecanic always said to me that everything was fine. I only found the leak when i empty my tank and pressurized the fuel system with air (not the tank but first fitting after) Found tiny air bubble at one fitting. No problem after. And no , there was no sign of fuel leak. My mecanic always said to me that there would be blue stain but no. It was just enough to aspirate air when on the succion side of the mechanical fuel pump when the boost pump was off. same thing. Pressure was dropping bellow 14 and returning to normal with boost pump

Now, steady 25 to 27 psi

Are you sure you can run a 540 on 93 octane?. I was always thinking that the detonation margin on the 540 was too narrow.
But i don't know anything on that.
 
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Appreciate your thoughts, Lan. This is going to be a fun exercise, I can tell.

I'm going to copy and paste my post above to another thread started 7 years ago by Bob Leffler as it seems more appropriate there...
 
I used to have fuel pressure drops on climb out and sometimes right after climb in cruise. My fuel system was stock with the exception of the FF transducer between servo and spider. Also I had added a 90 degree fitting where the fuel line leaves side of fuse. I decided it had to be turbulence and air vapor in the 90 degree fittings. I fabricated two new aluminum lines without any 90 degree fittings. One runs from the tank outlet to under the seat, and the other from under the seat to the Andair fuel selector. They are connected with a straight coupler under the seat.
This totally eliminated my problem. It is my belief that anything you insert, including 90 degree fittings, on the suction side of the fuel system is asking for trouble, the FF cube included.


It does sound like you may have blockage in your air vent
 
Bill I had the same thing occur last year. Lan Vinh Do has that link for the Tempest fuel pump video (thanks Lan) Watch it. Check all B nuts and connections on the suction side of the fuel pump back. With me it was the fitting between the filter and the pump had maybe a quarter of a turn on the nut. Just a tiny micro amount of air sucked into the line causes the symptom you have. Didn’t even have visible fuel stain, but enough for air to get sucked in.
 
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