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Mechanical fuel pump cooling ?

L

Larry DeCamp

Well Known Member
Above 90F ambient, the fuel flow drops and CHT rises. Boost pump returns operation normal. Only one 90degree fitting at selector valve and problem exists on both tanks. So, I assume I have a mechanical pump cooling opportunity. Any suggestion how to proceed are welcome.
 
I'd recommend going farther to define the problem, maybe there's a simpler solution. I looked at cooling shrouds but they got very mixed reviews and passed on that as a fix.

Here's what I found from my test flight after taping thermocouples to various points along the fuel system:

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MacCool said:
I'd recommend going farther to define the problem, maybe there's a simpler solution. I looked at cooling shrouds but they got very mixed reviews and passed on that as a fix.

Here's what I found from my test flight after taping thermocouples to various points along the fuel system:

View attachment 123973
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What's your location of Gascolator & Red Cube ?
 
supik said:
What's your location of Gascolator & Red Cube ?
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Both are on the firewall, as is the boost pump. Lines are un-insulated and have some 90° bends. Not sure why the firewall-forward builder chose that method or location.

1784814790527.jpeg 1784814841491.jpeg 1784815118822.jpeg
 
You don’t mention carb vs fi. You also don’t mention what happens to fuel pressure. Heat related issues in the system tend to drop pressure but not flow. I would say your issue goes beyond just getting the mech pump too hot. Lots of folks see pressure fluctuations when tempsget high but very few see flow drop. You clearly have an issue, but doubt it as simple as the mechanical pump getting hot. Also very different issues between carb and fi when discussing over heated fuel.
 
Larry's observations are apt... I should add that my engine is fuel injected. When under-cowl temp is high, I get variations in fuel pressure and a long taxi in those conditions will get me some engine variability which smooths out just fine when I advance the throttle for takeoff. CHTs are always under 400. I also get hard restarts when it's that hot...a problem I solved by active power venting of the engine compartment when the airplane is on the ramp for short turn-around flights (as in re-fueling or Young Eagles flights). That seems to cool things down enough to address my main problem with fuel boiling...difficult hot restarts. Even on a really hot day, such venting after a fuel stop even for just the time it takes to refuel allows for much less frustrating restarts. It's a simple device...about $20 worth of parts from Amazon.


1784821016247.jpeg 1784821086330.jpeg
 
MacCool said:
Larry's observations are apt... I should add that my engine is fuel injected. When under-cowl temp is high, I get variations in fuel pressure and a long taxi in those conditions will get me some engine variability which smooths out just fine when I advance the throttle for takeoff. CHTs are always under 400. I also get hard restarts when it's that hot...a problem I solved by active power venting of the engine compartment when the airplane is on the ramp for short turn-around flights (as in re-fueling or Young Eagles flights). That seems to cool things down enough to address my main problem with fuel boiling...difficult hot restarts. Even on a really hot day, such venting after a fuel stop even for just the time it takes to refuel allows for much less frustrating restarts. It's a simple device...about $20 worth of parts from Amazon.


View attachment 124004 View attachment 124005
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How to measure if mechanical fuel suction is inadequate to do the job ? I am not excited about flying until the problem occurs even though boost pump pushes the problem away and engine runs normal.
I could circumvent the tank vents to eliminate that question, but that would only say the boost sucks harder than the engine pump. The vent lines in tanks do have more resistance to blowing air through them than 1/4” tubing does. Is this normal ? I did not build the wings.
 
FYI, boost is in the fuse floor rear of Firewall. FWF lines all teflon with firesleeve. Mechanical pump input is oringed fitting with FP hose going to FP sensor on firewall.Red cube is vertical between servo and flow slitter.
 
I'm no airplane engine guy, I've just had heat related fuel issues that I've had to deal with. Take my observations with a grain of salt and let's hope that Larry weighs back in.

I will observe that, based on experience, if you have a fuel injected engine that is showing pressure variations while flying, the engine is acting up, and that's all corrected by turning on the boost pump, you might want to be prepared to conclude that your fuel pump is dying. I had that happen too, on my ferry flight home from buying the airplane several years ago. It was completely corrected by replacing the fuel pump. I've also had a period of wide variations in fuel flow, without any discernible engine performance issues, and that ultimately turned out to be a dead red cube.
 
gasman said:
May have been a PA-28 owner and just copied what he was familiar with.
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Dunno. It was farmed out to Don Swords at Don's Dream Machines when it was in Atlanta. A different Don owns the shop now, and it's in Alabama somewhere.
 
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