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  #11  
Old 01-30-2017, 07:59 AM
airguy's Avatar
airguy airguy is offline
 
Join Date: Oct 2006
Location: Garden City, Tx
Posts: 5,147
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Here's another datapoint on the FT60 location, I'm chasing a problem now on my 9A and I am pretty sure I've got it figured out but haven't had a chance to confirm the final fix in flight, gimme a few days.

I mounted my FT60 on the fuel line just forward of the pump shroud, immediately aft of the firewall, just below the heated air inlet to the cabin. All was well until this winter when I started flying with full heat on pretty regularly. Coincidentally with that I started seeing fuel flow variations on my EFIS, sometimes swinging by a 2-3 gph over a few minutes, that had me strongly suspecting either a fuel leak somewhere or a problem with the fuel injection servo. Datalogging and graphing the egine CHT/EGT against the fuel flow quickly indicated that I was definitely NOT burning that extra fuel - and I could find no evidence of a leak. I watched the fuel computer closely for total gallons burned versus total gallons pumped back into the tanks, and realized the fuel computer was short of fuel everytime. The extra flow is not real. I finally connected the dots and figured out that hot air was blowing directly on the FT60 where I have it mounted, and I am about 98% certain that I was heating that device sufficiently to make vapor bubbles in it, accounting for the additional flow.

I have modified the heat-directing aluminum vents just above the FT60 where it penetrates the firewall, and I anticipate that will solve my problem.

Bottomline - the FT60 is subject to vapor bubbles just like the mechanical pump - don't put it someplace where it will get too hot. Mounting it in the upper deck area above the cylinders should be just fine, that will be the coolest spot FWF in flight.
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N16GN flying 700 hrs and counting; IO360, SDS, WWRV200, Dynon HDX, 430W
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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  #12  
Old 01-30-2017, 08:19 PM
lr172 lr172 is online now
 
Join Date: Oct 2013
Location: Schaumburg, IL
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two points to consider in your diagnosis. The mechanical pumps are variable in flow and pressure and after a certain flow rate they are inversely proportional. However, we are using under half the max flow rate, so we don't see much pressure drop and our flow rates. However, a worn pump will potentially show these traits, as the max flow rate is reduced.

Second, the sensor is dynamic and operates on a similar principal to your ASI / pitot static system. If the static pressure vent is partially blocked or very slow to bleed air, your fuel pressure reading will decrease during rapid reductions in air pressure (i.e. climb). Once level and stable pressure, the bleed port will allow the pressure on the back side of diaphragm to eventually equalize and you'll see the measured pressure come back up. This was the problem Dynon had with the initial Kavlico's (they issued an SB). The seal was too tight and they wouldn't equalize or were slow to do so. Most of these parts are made for applications that don't see rapid pressure change.

Larry
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  #13  
Old 01-31-2017, 05:24 AM
mahlon_r mahlon_r is offline
 
Join Date: Jan 2005
Posts: 1,025
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I have heard of this issue on all RV's and I don't think it is a vapor or fuel pump issue.. I think it is inherent to the design of the wing and tank venting system, as it happens a lot to all versions of the plane. Don't know this as fact but if you search the archives I am sure you will see it come up several times. I supplied many engine driven pumps under warranty to many of my customers just to prove that the pumps weren't an issue. Old pump same results as the replacement every time.... Redoing fwf fuel cooling and heat protection didn't have an effect either. Happens on the low pressure engine systems as well.

Good Luck,
Mahlon
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  #14  
Old 01-31-2017, 07:22 AM
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bret bret is offline
 
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Location: Gardnerville Nv.
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Diesel tech by day here, a long shot but you may want to check this off of a possible cause, I have seen in the past, a fuel pressure problem caused by air being sucked into the fuel supply line while under a high volume demand, ( full fuel ) and then the problem goes away at idle ect, the problems I have seen is where a fuel line connection would pull air in and not leak fuel out while static. very hard to find. I would check all fuel supply lines carefully for this.
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  #15  
Old 01-31-2017, 09:26 AM
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MikeyDale MikeyDale is offline
 
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Every now and then my fuel psi will fall for no reason on my IO 360 engine mounted pump. A few seconds with the boost pump always corrects it. It has never happened during climb. Only when Ive been cruising a while usually LOP and more often than not, shortly after refueling. The last time it happened was several months ago and I had been cruising straight and level for about an hour and made a 360 deg turn to get a second look at something. A year ago, I moved my Red Cube from the firewall between the mechanical pump and the servo to the engine mount in a more level position and replaced a 90 degree elbow entering the Red Cube and insulated it with fiberfax and aluminum tape. It helped some but it has happened a couple of times since. No leaks in the fuel lines and the air vents are clear. You can do a search on my situation and get more on the story. I do not have a shroud on my fuel pump. The fuel could be heating up in my tunnel but it has never happened during descending when the flow is very low. Its a puzzler for sure but I have gathered from reading these forums and talking to RV pilots, I'm not the first one with this situation.
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  #16  
Old 02-03-2017, 08:54 AM
Kent Ashton Kent Ashton is offline
 
Join Date: May 2010
Location: Concord, NC
Posts: 116
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Some of the fuel pressure problems here may be due to ground loops--sender and gauge grounded at different points, small current induced in the loop that interferes with the tiny sender values. I pulled my hair out over erratic fuel pressure indications for a long time. They were sometimes consistent but weird; sometimes just weird. Switching the grounds to the same point solved it.
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  #17  
Old 02-03-2017, 08:05 PM
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Gash Gash is offline
 
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Location: Goodyear, Arizona
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MikeyDale:

Same exact thing is happening with my RV-8. In a strange way, it's comforting to know that I'm not the only one. I actually started wondering if my engine driven fuel pump was going bad. But a quick search of VAF posts on the subject helped me to realize that I'm just one of many.

Anyway, in my case (and perhaps yours), it seems unlikely that fuel temperature is the cause. I cruise LOP, usually between 7,500 and 9,500, fuel flow is about 7.5 gph, and CHTs are all around 340 F. TAS is 170 kts at these settings, so there's plenty of cooling air flow in an engine compartment that's relatively cool already.

The only thing I can think of is the reduced mass air flow due to higher pressure altitude, but then I wouldn't think that cruising at 9,500 ft would cause sufficient mass air flow reduction to decrease the cooling effect and allow the fuel to heat up.

This is a head scratcher indeed!

Quote:
Originally Posted by MikeyDale View Post
Every now and then my fuel psi will fall for no reason on my IO 360 engine mounted pump. A few seconds with the boost pump always corrects it. It has never happened during climb. Only when Ive been cruising a while usually LOP and more often than not, shortly after refueling. The last time it happened was several months ago and I had been cruising straight and level for about an hour and made a 360 deg turn to get a second look at something. A year ago, I moved my Red Cube from the firewall between the mechanical pump and the servo to the engine mount in a more level position and replaced a 90 degree elbow entering the Red Cube and insulated it with fiberfax and aluminum tape. It helped some but it has happened a couple of times since. No leaks in the fuel lines and the air vents are clear. You can do a search on my situation and get more on the story. I do not have a shroud on my fuel pump. The fuel could be heating up in my tunnel but it has never happened during descending when the flow is very low. Its a puzzler for sure but I have gathered from reading these forums and talking to RV pilots, I'm not the first one with this situation.
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  #18  
Old 02-24-2017, 02:25 PM
Jimzim Jimzim is offline
 
Join Date: Mar 2009
Location: Arvada, CO
Posts: 118
Default Same issue...

I have been seeing a loss of fuel pressure since the first flight. A couple of things to note, it happens on on initial start at low rpm, or during an extended climb. I have an RV 7 with a stock o-360. I have replaced the mechanical pump, insulated all fuel lines fwf, added a cooling shroud and blast tube, all to no effect. I can resolve the issue instantly by eith lowering the nose, or turning on the boost pump. It doesn't seem to matter which tank is in use or how much fuel is in the tank. I'm just not sure where to go from here! By the way, my fuel system is per Van's plans, and I had the same issue before adding a red cube. I'm sure the pressure drop is real, as I have experienced momentary rpm drops when pressure falls below 2 psi on the EFIS. Frustrating! Any additional thoughts appreciated!
Thanks, Jim
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