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  #1  
Old 07-05-2022, 01:06 PM
MacCool's Avatar
MacCool MacCool is offline
 
Join Date: Aug 2020
Location: central Minnesota
Posts: 1,097
Default Fuel flow fluctuation when hot

edit: Sorry...the title is misleading. I meant to type that the gauge-displayed Fuel Pressure fluctuates when the engine is hot....

Background: RV-9A with IO-320 D1A. Fuel flow read from a Red Cube (new) driving an EMS part of an Advanced Flight 5400 EFIS. Red Cube mounted on firewall next to a Weldon 8163A boost pump. Engine driven pump is a Tempest, and was replaced about 70 hours ago.

Issue: When the engine is hot, I often get low fuel pressure when running on the engine-driven fuel pump while taxiing or on approach with manifold pressure at about 15 inches. No stuttering or anything from the engine, just low fuel pressure at the very bottom of the green arc according to the EFIS fuel pressure gauge. Doesn't do this during flight, nor when the engine is not heat-soaked. Works fine in cold weather and first flight of the day.

Trying to decide if this is a problem. The engine performs well otherwise. When I first picked up the plane, the engine-driven fuel pump went out...noted when the engine kept dying while taxiing. Replaced the original Lycoming engine-driven pump with a Tempest at that time. That was about 70 tach hours ago.

Any thoughts...?
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RV-9A, 2011, bought flying
IO-320D1A (factory new), C/S
IFR equipped
AFS 5400/3500, G5, IFD440 navigator,
bunch of other stuff

Last edited by MacCool : 07-05-2022 at 02:17 PM.
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  #2  
Old 07-05-2022, 01:35 PM
Majorpayne317641 Majorpayne317641 is offline
 
Join Date: Sep 2021
Location: Goldsboro, NC
Posts: 85
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Does the low fuel condition happen with the electric fuel pump on? Have you considered the proximity of the red cube to the exhaust? I would start with shielding the cube and fuel line in that area and see if anything changes. Running at idle lowers the fuel demand and it could be boiling the fuel in the line. Not sure what indications that would give you but it could be showing less liquid flow through the cube if vapors are also going through it.
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  #3  
Old 07-05-2022, 02:16 PM
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MacCool MacCool is offline
 
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Location: central Minnesota
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Quote:
Originally Posted by Majorpayne317641 View Post
Does the low fuel condition happen with the electric fuel pump on? Have you considered the proximity of the red cube to the exhaust? I would start with shielding the cube and fuel line in that area and see if anything changes. Running at idle lowers the fuel demand and it could be boiling the fuel in the line. Not sure what indications that would give you but it could be showing less liquid flow through the cube if vapors are also going through it.
Sorry, I titled this thread wrong and can't change it. I meant to say that fuel pressure fluctuates when hot.

You raise some important points though...the fuel pressure fluctuation does seem to be prominent only when the engine is hot and I suspect that that plays a role...might be that some insulation would help. I've noticed this on hot days (this is Minnesota, so we don't see much oppressive heat) and see it mainly on taxiing after a hot start, or on a real hot-day 5-mile approach when I pull back the manifold pressure to 15 inches to start slowing down the airplane. Although the EFIS fuel pressure gauge fluctuates and dips into the red, I've never seen the engine to stumble at all. Likewise taxiing...low pressure when hot and taxiing, but the engine never quits, or even acts like it wants to. This is unlike when the engine-driven fuel pump did die a couple of years ago...the engine wouldn't stay running while taxiing.

Pressure does come back into the green on hot-taxiing if I hit the boost pump. On take off, I generally flick off the boost pump at 1000 ft AGL and the fuel pressure remains normal.
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RV-9A, 2011, bought flying
IO-320D1A (factory new), C/S
IFR equipped
AFS 5400/3500, G5, IFD440 navigator,
bunch of other stuff
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  #4  
Old 07-05-2022, 02:35 PM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 6,657
Default Problem? Maybe . . .

Mac, Although it could be a marginal mechanical pump, IMO it is just heating of the fuel as you mention, some vaporization causing the lower pressure on the business side of the pump.

I noticed some actual FF and pressure wandering at cruise and installed 5 thermocouples from the fuel entry into the cabin to pump inlet. It increased much more than I had expected. Sorry don't remember the numbers, but your exposed pump FWF will certainly add to the temperature rise.

Is it a "problem" ?? . . . . if the standard boost pump procedures don't completely address it, yes. Otherwise just follow the procedure. Insulation of the pump head, cube, and tubing will certainly help lower the threshold for the pressure drop occurrence. You can get sheets of insulation at a Speed shop and make covers with RTV seams pretty easily. If you wanted to experiment, or more margin.

A source for firesleeve. Lower prices, several steps in the order process and and slower to fulfill but good prices and products
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Last edited by BillL : 07-05-2022 at 02:57 PM.
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  #5  
Old 07-05-2022, 02:48 PM
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MacCool MacCool is offline
 
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Location: central Minnesota
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Thanks guys. I am inclined to suspect that fuel is boiling, but I've yet to see any actual functional problem with fuel flow or pressure (other than hard starts when hot). I'm just a little gun-shy since I did have the engine-driven fuel pump go TU on my first flight while ferrying it here from St. Louis. I am going to figure out an insulation scheme and give that a try.
__________________
RV-9A, 2011, bought flying
IO-320D1A (factory new), C/S
IFR equipped
AFS 5400/3500, G5, IFD440 navigator,
bunch of other stuff

Last edited by MacCool : 07-05-2022 at 02:51 PM.
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  #6  
Old 07-05-2022, 03:25 PM
Majorpayne317641 Majorpayne317641 is offline
 
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Location: Goldsboro, NC
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Two ways to do it, stand off heat shield on the exhaust and insulation wrap on the pump and lines. I do both whenever I have exhaust near fuel lines. You can get the stand off shields from ACS and one brand is anti splat. Theirs works well.
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  #7  
Old 07-05-2022, 03:54 PM
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Carl Froehlich Carl Froehlich is offline
 
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Location: Dogwood Airpark (VA42)
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Mac,

I suggest you have some basic issues that need to be fixed before you go chasing possible fuel line heat causing your pressure drop.

Looking at the photo, I note that you have the fuel line going into the (new) red cube, then into your electric boost pump, then I assume to your mechanical fuel pump. I also note a bunch of 90 degree fittings.

I refer you to the EMS install instructions on placement of the red cube (as in between the engine driven fuel pump and the engine and not mounted on the engine) and fuel line considerations on the input and output of the cube.

Carl
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  #8  
Old 07-05-2022, 05:15 PM
Freemasm Freemasm is offline
 
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Location: Orlando
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Gonna add a little to the comments. As mentioned, add radian shields where appropriate. Closer to the source would probably be better here; accessibility, no hinderance to local airflow, etc. Insulating the lines is fine. Insulating the pump is not. Even with fuel flow, pressure relief recirc, good mechanical efficiency, etc. it generates heat as a byproduct. Insulating it will affect its life.

Gonna open myself up here but I suspect the upper line is too short. I've never found a spec but it would be very reasonable to assume minimum length should be no shorter than 1/2 max unsupported length. Short and straight is the worse combo. I'd personally ditch the 90 and steam loop it or get a flex line from TSflightlines.

Keep us informed.
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  #9  
Old 07-05-2022, 07:22 PM
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Walt Walt is offline
 
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I run basically the same Weldon pump but mounted in the cabin due to heat concerns, even with a purge valve I didn't feel comfortable with the pump FWF.
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  #10  
Old 07-05-2022, 08:03 PM
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MacCool MacCool is offline
 
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Location: central Minnesota
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Quote:
Originally Posted by Walt View Post
I run basically the same Weldon pump but mounted in the cabin due to heat concerns, even with a purge valve I didn't feel comfortable with the pump FWF.
I replaced that one just last year. The first one started leaking a little after 10 years. It does get hot, so I limit the amount of time I run it...pretty much only in the pattern.

I did just order a heat shield for the exhaust...need to get some hose clamps. Anybody know the diameter of a Vetterman exhaust used on an IO-320? Save me a trip to the hangar...
__________________
RV-9A, 2011, bought flying
IO-320D1A (factory new), C/S
IFR equipped
AFS 5400/3500, G5, IFD440 navigator,
bunch of other stuff
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