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02-18-2015, 06:28 AM
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Join Date: May 2005
Location: X35 - Ocala, FL
Posts: 3,679
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Yesterday we were all basking is the cold windy drizzle, but that gave way to sunny and cool today. 
__________________
Jesse Saint
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03-04-2015, 07:56 PM
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Join Date: Oct 2014
Location: Bellingham, WA
Posts: 21
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Pressure fluctuations
Just want to go on record that my RV-8 built in 2002 with an 400 hour since new IO-360 also sees these fuel pressure fluctuations. I always top my tanks when I fill up and it seems to be directly related to full fuel tanks in my case. Does not seem to be an issue after they burn down a bit. I hate it and wish it was not there but it is. Turning on fuel pump brings the pressure right back. Engine has never stumbled but I turn on the fuel pump as soon as the alarm goes off. Have seen it less than 7 pounds. Ed
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03-15-2015, 08:59 PM
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Join Date: Mar 2008
Location: Guelph Ontario
Posts: 267
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Fuel pressure drop Update!
Quote:
Originally Posted by rleffler
Ivan,
You've had the shroud on now for a month. How has it been performing since the change? The same or was there an improvement?
Hopefully, the RV-10, Dianne, and you are all basking in the Florida sun and not all the winter weather in Guelph.
bob
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I am sorry to have taken this long to get back here with my findings. The reason is two fold, first of all I don't tend to visit the VAF forums too often and the other is that I wanted to gather more data before I post my findings.
In a nutshell it has been an arduous journey to figure out what was wrong. To make a long story short I have come to the conclusion that nothing is really wrong with the system and that the problem is caused by heat.
The drop in fuel pressure occurs only in the climb and only after the boost pump has been turned off. The mechanical engine driven fuel pump is mounted on the back of the engine in a very hot environment where it is not only heated by heat transfer from a hot engine but also from hot air routed in the same area from the cabin heat valves when they are closed off to the cabin. In fact one of them is pointed right at the Mechanical fuel pump.

Fuel pump and cooling shroud.
A couple of changes I have done since this issue began, first of all I lowered the PSI setting at which the Master warning would come on from 20psi to 17psi. According to the Lycoming manual std. pressure for this pump is between 18-28psi. My VP-200 has the ability to turn on the Boost pump automatically and I lowered that setting from 18 to 15psi. Since this change the boost pump has not been turned on automatically which indicates that the fuel pressure has not dropped below 15psi.
Another change I made was to add a cooling shroud and blast tube to the mechanical pump. There is no doubt that this pump is running hot and heating up the fuel as it goes through it.
And lastly, I am now keeping the boost pump on much longer after take off than I did before. In the past I would turn it off as soon as I cleared the pattern but now at least through 3000'.
The changes in setup I made combined with the addition of this cooling shroud has not eliminated the problem all together but it is much reduced. This past weekend on an 88 degree F day here in central Florida I did have the Master warning come on a couple of times in a climb to 8500'. What is interesting is that these drops in fuel pressure were momentary in nature. A quick check I noticed that pressure at 16psi but it quickly built back up to normal pressure of 23-24psi and the Master warning went out. This was an exception because, I believe, the temperature was so high. On must departures now since these changes the fuel pressure stays put right where it should be.
During these events or at any other time in the past has the engine ever stumbled or starved for fuel. EGT's have never fluctuated which to me means that the mechanical fuel pump can deliver enough fuel at pressures as low as 15psi. or lower. I read on this forum someone stating that he has seen 10psi without any problem, some even lower than that.
I hope this helps and that it answers some of the questions.
Best regards,
Ivan Kristensen
__________________
Ivan Kristensen
Guelph, ON. Canada
RV-10 (C-GMDV) 1355hrs.
Dual GRT 8.4" HXr EFIS, VP-200, TT A/P, Avidyne IFD440, uAvionixX ADS-B out, Sky Radar WX and Traffic, Dual EI's (one Lightspeed & one SDS CPI)
RV-14A under construction.
Link to my build site:
https://ivankristensen.smugmug.com/B...ENTAL-aircraft
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03-16-2015, 08:21 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Random observations...
Pump body temperature is a secondary interest at best. The real interest is the inlet-outlet rise in fuel temperature, or better, the tank-to-pump-outlet rise.
The pump body is not the only place fuel can pick up heat. In the case of the RV-10, we hear reports about high tunnel temperatures, and a lot of the fuel system is in there.
Everybody's pump body temperature is basically the same, i.e. equal to engine block temperature. If a particular pump install actually does run hotter, it's because there is an additional heat source. It makes a lot more sense to remove the additional heat source than to try cooling the pump.
A blast tube is operating at about the same pressure delta as a heater box, so running them in opposition isn't very effective.
Fuel flow and fuel temperature rise would have an inverse relationship. The pressure problem appears in climb, high power, i.e. at high fuel flow. High flow equals less fuel dwell time in any particular hot component, meaning less temperature rise than at low fuel flow. Heat may be a factor, but it is probably not the only factor.
We all agree that vapor bubbles cause pumping problems. It would be worthwhile to consider all means of vapor bubble formation, in particular mechanisms that work in concert with heat, for example, an elbow fitting with a sharp internal corner. Hoses are available with curved tube end fittings for a good reason.
BTW, I vaguely recall reading about a study done at Piper, in which a complete fuel system was constructed with clear tube for visual observation, and that everyone was amazed at all the locations in which cavitation was evident at high fuel flow. Anybody else recall that?
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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03-16-2015, 09:53 PM
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Join Date: Aug 2005
Posts: 97
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Quote:
Originally Posted by DanH
Random observations...
Pump body temperature is a secondary interest at best. The real interest is the inlet-outlet rise in fuel temperature, or better, the tank-to-pump-outlet rise.
The pump body is not the only place fuel can pick up heat. In the case of the RV-10, we hear reports about high tunnel temperatures, and a lot of the fuel system is in there.
We all agree that vapor bubbles cause pumping problems. It would be worthwhile to consider all means of vapor bubble formation, in particular mechanisms that work in concert with heat, for example, an elbow fitting with a sharp internal corner. Hoses are available with curved tube end fittings for a good reason.
?
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It seems the major variables between planes are in no particular order:
Type of fuel valve and height above the floor, type of boost pump, filter and filter location, type of fuel lines in the tunnel, and what, if any efforts have been made to keep tunnel temps down.
Fuel vapor occurring in the fuel selector would give the mechanical pump fits. While fuel should siphon right through the selector regardless of height, I don't know that it does.
In hot places some folks remove hose from heater valves and divert to somewhere near cowl exit.
__________________
Kelly McMullen
A&P/IA
EAA Tech Counselor
KCHD
RV-10 40866
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03-16-2015, 10:37 PM
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Join Date: Jan 2005
Location: Clarion, Pennsylvania
Posts: 550
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In my own evaluation, there are several good reasons to leave the right heater valve off and go with a single valve. Heating fuel, at the pump and in the tunnel, is one. With just the left valve installed and plumbed to the rear heat outlet, the airplane is shirtsleeve warm at altitude when surface temperatures are down to 30 F or so. Below that you might need a light jacket, no big deal in my mind. But, I sleep in the snow for the fun of it, YMMV.
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-Andy Turner
RV-10 N784JC
Last edited by aturner : 03-18-2015 at 05:57 PM.
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03-17-2015, 06:37 AM
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Join Date: Nov 2014
Location: Fayetteville nc
Posts: 23
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Boost Pump Suction Filter
Might already have been said (as I read these posts briefly) but open up and check your filter ahead of the electric boost pump. Particularly on new tanks and installations, some debris could be collected in this filter which would cause the symptoms you describe. I would recommend opening this filter anyway soon after the first few flights / engine runs. I was talking to a factory representative yesterday about this very scenario and the symptoms match having restriction in the boost pump suction filter.
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03-18-2015, 03:27 PM
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Join Date: Mar 2008
Location: Guelph Ontario
Posts: 267
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Quote:
Originally Posted by Kellym
In hot places some folks remove hose from heater valves and divert to somewhere near cowl exit.
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This is one thing I have been giving serious though to. I rarely use the front heater anyway as it is just too hot. Even at -15C the rear heater seem to be enough to keep the cabin comfortable.
One thing is for sure, the temperature of the air coming out of the heater vents is VERY hot. Eliminating one of these heat sources from the space behind the engine would/should be a good thing.
__________________
Ivan Kristensen
Guelph, ON. Canada
RV-10 (C-GMDV) 1355hrs.
Dual GRT 8.4" HXr EFIS, VP-200, TT A/P, Avidyne IFD440, uAvionixX ADS-B out, Sky Radar WX and Traffic, Dual EI's (one Lightspeed & one SDS CPI)
RV-14A under construction.
Link to my build site:
https://ivankristensen.smugmug.com/B...ENTAL-aircraft
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04-05-2015, 12:26 PM
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Join Date: Feb 2012
Location: Prosser,WA
Posts: 272
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Replaced mechanical fuel pump under warranty and so far (10 hrs) no fluctuation. As well is in cruise it will maintain 23 psi where is old one was 21 to 22 psi. I dissected the old pump before sending back but no obvious reason for the drop in pressure. Our issue only appeared in cruise but I also leave electric on till level off.
__________________
Jack Maljaars
Rv10
N819DJ flying. 350hrs +
Dues 2015 pd
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04-05-2015, 07:25 PM
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Join Date: Jan 2005
Location: Sedona Arizona
Posts: 349
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This has been a great thread for me to read.
I have a little over 40 hrs on my new engine install. IO-375 on RV7A.
After reading this thread I realize that in both instances that I have experienced loss of fuel pressure it was with full tanks and during a climb to relatively high altitudes.
The worst one happened at 16,000', I was still climbing to get over some weather. Fuel pressure alarm came on, (Dynon Skyview, Betty says "Fuel Pressure" in head set, love it). The pressure dropped to 12 PSI and I felt the engine losing power, boost pump cured it. I descended and turned the boost pump off to see the pressure drop again slowly. One more time on the boost pump then off again and the pressure stayed good for the rest of the descent. I was heading for nearest airport. The OAT was well below 0F up at 16,000' so I was thinking ice...
Spent the night in Bend OR. No water found from sumping tanks again.
I topped off the tanks before leaving Bend the next morning. At 10,000' the pressure dropped again. Boost pump again and descended to lower altitude and flew on to Central CA. Changed out pre filter as precaution, old one was not clogged at all.
I have tested the plane up to 17,500' and it ran fine, but the tanks were not full during the test. This has only happened two times, both times with full fuel and climbing at altitude.
Now I believe the problem my be what others have discovered. Full tanks in a climb puts fuel in the vent lines, that plus the low ambient pressure up high makes me think vapor lock as probable cause. A little extra restriction in the system plus fuel in the vent lines may be enough to cause the problem.
FYI, I went through my fuel system looking for leaks. All flares are fine with no cracks. All fittings were snugged up, nothing found loose.
I used a pre existing hole in my firewall for my fuel line bulkhead fitting that was pretty close to my cabin heater valve.
Changes made to try to help situation:
I moved my firewall penetration for fuel line further away from the cabin heat valve. In the process I eliminated a 90 degree fitting in the system, managing to bend some alum tubing for the connection from boost pump to firewall.
I added another 2" flange to my cabin heater valve and routed it scat down to the cowl exit area. The heat was coming out pretty close to the mechanical pump.
The pump has always had a shroud and blast tube.
When testing the fuel vents, I was surprised at how hard it was to blow through them with the fuel caps off. It was difficult to move air through the tubing but I could get air to the tanks. (Try it and I bet you will be surprised also!) With thousands of RVs flying with stock fuel vents and no problems, I think I should leave them alone, but, it is tempting to do something with the system to get better venting. Any ideas?
Now I know to test the system with a sustained climb and full tanks to see if I can replicate the problem. Maybe accepting the idea of needing to run boost in those conditions is the answer? I would prefer to have a system that is less prone to vapor lock as I would like to run auto fuel but this experience has me staying with 100LL for now.
I can't help but consider putting pumps between the wings and fuse to push rather than pull the fuel...
I may find a way to eliminate a few more fittings with less restrictive high dollar free flowing types.
Randall in Sedona
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