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Lo pressure runup and climbout

dick seiders

Well Known Member
It’s lback afte r two yrs. se repla ced sensor again but it’s still there.
What am I overlooking or is it just that rv12. Driver just need to learn to ignore the worry of it?
Dick Seiders
 
I presume you are talking about the fuel press. I have flown three years with a fluctuating pressure. I bought a new sensor and had the same problem untill I found the Dynon technical service bulletin. I removed the gasket from the connector and have a good reading ever since.
 
Thanks for the. Feedback but what Dynon tech bulletin to are you referring to?
Also unclear on removal of gasket Can you elaborate please?
Dick seiders
 
That is the one I was referring to. With the gasket installed my fuel press was just in the green during start up but as soon as I started the take-off roll my pressure was decreasing and indicating zero just after take-off. During the descent the pressure was rising again.
After removing the purple gasket the fuel pressure is around 5psi and is stable during the entire flight. I left the gasket off.
My fuel pressure sensor is in the serial number range mentioned in the bulletin.
 
I do not have a Kavlico sensor . Equipment. In 2009 was VDO. I never received instruction to change that I. An recall. So what are the other old timers
out there doing?
Dick Seiders
 
I took off today in PHX at 0645. It was 89F on the ground with a 2 thousand foot thick layer at 94F to climb through before the lapse rate kicked in. I have a switched fuel pump that I usually turn off after leaving the pattern. Today every time I switched it off I got a low fuel pressure and high fuel flow alarm indicating vapor lock. It was an hour cold soaking at 68F at 9500 FT before I could turn off the electric pump and maintain normal fuel pressure.
 
I took off today in PHX at 0645. It was 89F on the ground with a 2 thousand foot thick layer at 94F to climb through before the lapse rate kicked in. I have a switched fuel pump that I usually turn off after leaving the pattern. Today every time I switched it off I got a low fuel pressure and high fuel flow alarm indicating vapor lock. It was an hour cold soaking at 68F at 9500 FT before I could turn off the electric pump and maintain normal fuel pressure.

And this is the reason Vans designed the electric fuel pump to run continuously and not be switched on/off indiscriminately by the pilot.

Vapor lock is a show stopper - literally.
 
True, but it is only a problem at high fuel temperature. 8 months of the year in PHX switching the pump off makes no significant change in fuel pressure. I like the option to switch it off so that my run up demonstrates the satisfactory operation of the mechanical pump. If the mechanical fails with a constant running electric pump you won?t know until the electric fails. It is a difference in operating philosophy. I can see both sides of the argument, and my choice is for the switch. I?m sure that others choose the Van?s approach, and that?s their business.
 
Pretty sure I can use the Kavlico. Would appreciate any help in sending useful info to make the conversion. Thanks
Dick seiders
 
Turn on electric pump (master switch) and note fuel pressure.
Start engine and look for increase in fuel pressure.
Now you know that both pumps are working.
 
Pretty sure I can use the Kavlico. Would appreciate any help in sending useful info to make the conversion. Thanks
Dick seiders

There were a number of posts in past on how to make the conversion. I know you've been searching, but here is what I remember.

The Kavlico will mount on the same screw as the VDO but you need a different bracket:
i-jqs685X.png


This plan sheet shows the installation, you will need to run 5 volts from somewhere as the Kavlico has a third wire. Joe Gores might be your best bet for how he did it:

i-HSjPPZD-L.png


You will need to set the Sensor Type on the D180 for the Kavlico. I don't remember what the type for it is, maybe "2", check with Dynon if you can't find that by searching posts here.

i-TrCzp73-L.jpg
 
Hi Dick,
I still have the original fuel pressure sender.
Exactly what information are you looking for? If you are looking for a 5 volt source, it is available at the manifold pressure sensor, WHT/RED wire.
Connect 5 volts to Kavlico Red wire.
Disconnect the two brown wires from the VDO sender. Turn on the D-180 and determine which of the two brown wires has voltage on it. Connect that wire to the Kavlico green wire.
Connect the other brown wire to the Kavlico black wire.
Change the D-180 setup.
Read THIS Dynon document.
 
Thank you Joe. I?ll make the conversion. to Kavlico if further testing doesn?t assure the VDO. I can?t believe your original sensor is
still working.
Dick Seiders
 
Joe Gores, I have noted your fuel pump pressure test. Question: have you checked for decreasing psi on run up and climb out? If so what are the readings?
Thanks
Dick Seiders
 
I actually have two checks for fuel pump in my preflight checklist. I guess you could add a 3rd check for fuel pressure right after the ignition check while engine is still at high speed.

I have dual-screen D-180 EFIS and both screens are configured for 100% EMS at power-up. Makes engine start and initial monitoring a charm...
-
2ceot3o.png
 
The fuel pressure senders are notoriously inaccurate. The actual numerical value of the pressure is meaningless and will vary both in my plane and in other planes. Pressure drop is directly proportional to flow rate. At zero flow rate, there is zero pressure drop. As the flow rate increases, so does the pressure drop. So it is normal that pressure will drop at high RPM. Aren't there gravity fuel systems with about 3 feet of head pressure? That is about 1 psi. So fuel pressure of 2 psi or more is good enough. If the fuel flow rate passes the test described in Van's manuals, there is nothing to worry about. Just compare the pressure with electric pump only to the pressure with both pumps running. If the pressure goes up, then both pumps are working. Like I said, the actual values are meaningless.
An RV-12 owner installed a mechanical fuel pressure gauge below his instrument panel. It required a firewall penetration. And the gauge was plumbed with rubber hose. Even though he knows exactly what the fuel pressure is, I consider my RV-12 safer because there is less of a fire hazard.
 
Hey Jimmie, nice to hear from you. Good preflight list only things I don’t have are one notchflaps as Inever use them for takeoff except on short field.
Thanks for your interest
Dick Seiders
 
Last edited:
Joe, I appreciate your input. I suspect my problem is knowing what is happening to fuel psi on takeoff unnerves me a bit. I will try a bit more
Of the trials.
Thanks
Dick Seiders
 
Bled fuel line to sensor and went flying trying trying
To ignore alerts. Pressure drops to 1 or so on climb out, and comes up to 4 when I drop nose. Flew that way for weeks.
Ok it’s back. Pressure went to zero on climb and climbed to1.5 on level so bright it down and rebled “.Preemsure holding at 4.3 on wot run up. Now hav zero flow rate. What is going on ?
Dick Seiders
 
Low fuel pressure today too

I have had issues in the past. I?ve replaced the mechanical pump and sensor. All seemed to be working great for the last 50 hours until today when she screamed in my ear ?low fuel pressure? during climb out. Got down to 2 psi for 17 seconds, below 3 for 58 seconds (data log and Savvy are great).
Landed, dropped off my wife ( who handled it well but was not interested in flying any more today) and did several ground runs to try and recreate the issue..nada..took off for a close to home test flight and all is well...very disconcerting but the ROTAX has never skipped a beat..still looking for that solid solution too.

Has anyone out there changed to the bigger fuel pump? What pressures are you seeing?
 
Thanks B2.
I am thinking of both fuel pump and floss an. Mostly floscan as that is a new issue. In addition if flows an had debris that could slow its fuel flow rate that could explain low fuel pressure in that a blockage even partial coild impede flow rate to engine pump and reduce pressure to sensor. Fuel flow rate of zero is either indicating some blockage or defective reporting of fuel pressure. I am thinking of looking at floscan fist and 3 year old engine pump is
next.
Any thoughts are appreciated
Dick Seiders
 
Not sure a bad Floscan would create a low fuel pressure symptom. I had a low fuel pressure problem some time ago and, in my case, it was a bad fuel pressure sensor.

You might check the fuel hose between the spider on top of the engine to the fuel pressure sensor. I seem to remember reading that line can mess up fuel pressure readings if it's not purged regularly (should be done as part of the Condition Inspection).

None of that explains the zero fuel flow though. As far as I know, assuming the engine is still running, that has to be the Floscan.
 
before you do anymore swapping out you really need to put a mechanical pressure gauge on there to validate what you are seeing from the digital.

On the Rv12 a friend asked me to look at they had swapped several components like you, I took the AN adapter off the mechanical pump and that was blocked with plastic residue on the outlet side, have no clue where that came from.
 
Tom, thanks for the thoughts. I did bleed the sensor line twice on Sat. It
Was following that the flow rate dropped to zero.
Azijulian I will try a mechanical gauge to confirm sensor readings before taking out the floscan. Thanks
Dickseiders
 
Bleeding the fuel pressure line does no good whatsoever unless the line is plugged and debris needs to be flushed out. It doesn't matter if the line is full of air. The air will be at the same pressure as the fuel and exert the same pressure against the fuel sender diaphragm. The air will eventually be absorbed into the fuel. I have never purged my fuel pressure line. I have been lucky in that my sender has not failed . . . yet.
 
Joe, you are fortunate that you have experienced no problems with the fuel pressure issue. I have replaced the VDO twice. On the bleed issue my fp went from 1.5 to 4.3 # after bleeding line
to sensor.
Still uncertain about fp and now have transducer problem as well.
Dick Seiders
 
Bleeding the fuel pressure line does no good whatsoever unless the line is plugged and debris needs to be flushed out. It doesn't matter if the line is full of air. The air will be at the same pressure as the fuel and exert the same pressure against the fuel sender diaphragm. The air will eventually be absorbed into the fuel. I have never purged my fuel pressure line. I have been lucky in that my sender has not failed . . . yet.

The MM indicates flushing the fuel pressure hose at Annual or 100 hrs.

I read about the rationale some time ago but can't to find it now. I think it had something to do with the fuel pressure sensor being below the spider but don't recall the symptoms it caused.
 
You are right Tom about the maintenance manual specifying flushing the fuel pressure hose at annual or at 100 hours. It would be interesting to know the reason for doing that. My philosophy is, if it is not broken, do not fix it.
 
Just spit-balling here... maybe because that hose is dead-headed and residual fuel doesn't get used and goes stale so-to-speak?
 
The scary thing in this case is that the debris wasn’t in the hose it was in the coupling from the hose fitting to the pump, so flushing the hose didn’t clear it
 
Interesting feedback,thanks. My plan is to pick up a mechanical fp gauge tomorrow and pull the floscan following psi test to determine what its problem might be.
Dick Seiders
 
Test using 0 to 15 fuel psi gauge clearly showed good fuel psi. Facet alone was 3.2 lbs. Rpm of 2000 was 5.88, 3000 4.8, 4000 4.0 4800 3.3 on static ground runs.
VDO was cleaned with brake cleanerand on subsequent tests ran just a little less than the 0to 15# instrument
at each stage.
Conclusion is that Joe Gores was right in his statement about fuel psi. But I may switch to a Kavlico for the greater reliability


Floscan problem turned out to be a loose ground wire.

Dick Seiders
 
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