Why might my fuel be BRIGHT YELLOW?

Is there any logical reason for it not to be blue?
 
Because something recently changed? (Is this a test?)

New fuel line/fuel pump?
Leaking mechanical pump introducing a bit of oil into fuel?
Fuel additive?
Knowing the airframe might help.
 
My planes been sitting for a little while so I'm doing an incredibly thorough preflight. There was a mud dauber nest inside this sump tube. I cleaned out the mud dauber nest and the first sample is what you saw in those photos that day glow yellow color20260920_085754.jpg
 
Previous POA thoughts on green gas.

JScarry

Thread 'Why is my fuel green?'

My Cherokee sat since March while I tried finding an A&P to do the anuual. I finished the annual at the beginning of August and flew three times in August. Before I flew on the 31st, I filled the plane and it has been sitting on the ramp since then. Fuel color was normal.

Yesterday the fuel in the wing tanks was normal but the fuel in the gascolator was green. I did change the gasket in the gascolator during the annual but otherwise did not mess with the fuel system.

The smoke has been doing weird things to the light, but it doesn’t affect the color from the wings, just the gascolator...

I'm guessing it was just from sitting.
 
We've been looking for "green" aviation fuel for years. And you've developed it! Congrats, you'll be a um...

Well, congrats!
 
I have yellow stains on my hands. I'm wondering if the fuel that was sitting in that little sump receptacle there had an issue in the dye broke down or something

20260920_090145.jpg
 
Did you pee in it? It’s also not St Patrick’s day yet…
 
Ever run auto gas? It’s yellowish, would make a green color… maybe heavier and settled towards bottom?

Just checked the gas tank in my 70s bobcat restoration project. Prolly hasn’t had fuel in it since the late 80s maybe 90s?

Clean as a whistle, but stinky and amber… got a gallon, I think it’ll burn in my lawnmower… prolly predates ethanol, six bucks is six bucks!

But av gas pooling usually results in thick gooish dark blue, never seen it turn yellow or green. Had to leech some yellow out of something if no autogas.

Routinely parked the fuel truck at Pioneer field (EAA museum) from Oct to April, stuff never changed color a tiny bit.
 
I’ve found gasoline to be a TERRIBLE solvent for mud dauber stuff…

Has only ever resulted in sediment for me, never a real change in the gas itself.
 
Tools said:
I’ve found gasoline to be a TERRIBLE solvent for mud dauber stuff…

Has only ever resulted in sediment for me, never a real change in the gas itself.
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Sure - the bug itself. But what about all of its retching and puking during its death throes? :p
 
Response from Claude, so take with a grain of salt:
Bright green isn't a normal 100LL color, but it's a fairly common thing to see in stale, stagnant fuel. My best guess is dye drift.
100LL gets its blue from a blue dye, often with a little yellow dye, so the shade can shift toward green when the balance changes. In a tank that's sat for weeks, the fuel at the sump point can end up with a different dye balance than the rest. The blue dye can fade or drop out, leaving yellow dominant, and the two together read green. That fits what your friend saw: a jar or so of green from each side, then normal blue once fresh fuel from higher in the tank reached the drain.
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Jim K said:
Response from Claude, so take with a grain of salt:
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This is what I am hoping.
I sumped until it was normal looking and went and flew.

But then I lost my alternator which I think is unrelated to the gas but I somehow managed to get it replaced same day and flew some more.
Alternator was the normal color.
 
Years ago. The little yellow plunger broke off on dads plane and the piece came out during pre flight test. Over time it had broken down (heat or fuel?)and gave similar yellow on hands.
 
SixPapaCharlie said:
This is what I am hoping.
I sumped until it was normal looking and went and flew.

But then I lost my alternator which I think is unrelated to the gas but I somehow managed to get it replaced same day and flew some more.
Alternator was the normal color.
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Alternator belts tend to change color when they slip and heat up due to friction when newly installed. Did you check the tension on the ground both before and after you flew? -Skip
 
Figured it out.

I have aux tanks that have not been filled in probably 2 years (I know. Its time to replace them anyway).
The mud dauber meant no fuel was coming out when I sumped the mains. So I went through all 4 tanks, then inspected the tube, found the bug, removed it.
There must have been some very old fuel in the aux tank that made it into the sump.

When I get back from my current trip, I am going to drain the aux tanks via the sump and I expect it to all be yellow.
 
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