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Water in my fuel, or could it be Decalin?

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Lately, I've been getting about a half-inch of clear liquid in the bottom of my fuel tester when I test the fuel before a flight. It only shows up on the first test of the day. I pour the stuff out, and immediately re-test, and the clear stuff does not show up again. It looks like water, but my plane is in a hangar, it doesn't get wet, the fuel cap is on and tight, and I can't imagine where water would be coming from. The hangar is in Santa Monica, so it doesn't get very cold, even at night. I do add the recommended amount of Decalin whenever I refuel the plane, so I'm now wondering (and hoping) that some Decalin settles to the bottom of the fuel tank, and that's what I'm seeing. Could that be it?
 
decalin is fuel soluble and it's hard to imagine that fuel tester is getting all the decalin in one test that you put in. isn't the treatment rate 1/2 oz/ 10 gal fuel?
 
Dunno how cold it is whee you are, and how full your tank is when you leave the hangar, but air contains water. Temperature makes it condensate. The less fuel, the more air/water. Fill the tank up before leaving should help a lot.
Beginning the day with a fuel tester however, is a wise thing.
 
water from condensation is a real possibility, especially when you see small quantities. The water separates and sinks to the bottom, so expect to only see it before first flight.

you can compare water amounts from different conditions. The fuller your tank is, the less water you should get from air based condensation. Same is true for atmo conditions conducive to condensation. drier air with lower temp drops at night should show less water. That would help to determine if it is coming from condensation.

The water could also be coming from your gas supplier. I also doubt that the declin is separating out.

Larry
 
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Santa Monica Airport is right next to the Pacific Ocean. You have on shore flow and high relative humidity, every single morning, as a result, hence foggy and low clouds on many mornings.

Your fuel tank cap is vented. There's your sign. Your fuel tank breathes air, every single day and night, with temperature changes. When it does, water condenses on cold bare surfaces. Don't provide any cold bare metal surfaces, always store with a completely full tank of fuel, clear up the filler neck. The only thing that should be in contact with metal surfaces inside your fuel tank is fuel, not air, during storage.

Always top your tank off with fuel before storing in the hangar, and you will minimize the problem. Premium MOGAS that's E-10 will absorb quite a bit of moisture and pass harmlessly through the engine, but you MUST fly 1x per week and burn 5 to 7 or 8 gallons every flight, to create fuel turn over so it doesn't become problematic using mixed e-10 with 100LL. Your Rotax is actually designed and engineered to run completely on Premium Mogas, not 100LL. Either that or you are buying "wet" gas from airports close to the ocean, all the time. You fly to CMR a lot, it's close to the ocean, could be wet gas there, too. Try doing a few full fillups at KSZP, inland @ $4.15 a gallon for 100LL currently, then top off with 4 or 5 gallons of premium Mogas back at the hangar at KSMA. Due to fuel prices, KSZP has a LOT of high fuel turnover. We've never once gotten a tank of wet fuel there in the past 13 + months. Gascolator always shows just faint blue tint of fuel, never even a drop of water.

Just a fact when you run fuel with ethanol. You need to have regular fuel turnover to prevent problems.
 
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Why guess? Get a straw, put your finger over the end, stick it to into the sampler, remove the finger, put it back, remove the clear fluid, empty into a test tube. Put a thermometer into the tube, heat it up, see if it boils at 212 F.
 
Why guess? Get a straw, put your finger over the end, stick it to into the sampler, remove the finger, put it back, remove the clear fluid, empty into a test tube. Put a thermometer into the tube, heat it up, see if it boils at 212 F.
When I got to ?Get a straw,? I thought you were going to tell me to taste the stuff.:eek: Glad you didn?t suggest that. Alas, what you did suggest would be very difficult for me, because I live in California from which straws have been banished.;)
 
When I got to “Get a straw,” I thought you were going to tell me to taste the stuff.:eek: Glad you didn’t suggest that. Alas, what you did suggest would be very difficult for me, because I live in California from which straws have been banished.;)

Get a free clean empty vial from one of your friends in the entertainment business. ;) maybe a syringe too.

Take a 50/50 sample from your fuel tester and put it in the vial, put the cap on and shake it all up, then let it settle out.

With syringe, pull sample off the bottom of vial, the heaviest fluid and put it on a paper towel, then smell the wet part of paper towel. What does it smell like? Water, 100LL or Decalin?
 
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Since our drain is at the gascolator and not directly at the bottom of the fuel tank I am wondering if we need to drain the complete volume of the fuel supply line each time in order to check for water. If so, then a full sample of a standard GasoholTester would probably not be sufficient. It takes more than one sample to drain the water.
Correct? Any opinion on that?
 
It seems like the water will migrate to the gascolator if you leave the fuel valve open. Not sure how long that takes though.
 
there is a paste you smear on the bottom of a dipstick and can then check the bottom of the tank for water. gas stations have been using it forever on the ends of their 10' sticks.
 
It seems like the water will migrate to the gascolator if you leave the fuel valve open. Not sure how long that takes though.

That's exactly my point. Since there's very little or no gravity between the fuel pick up and the gascolator I doubt that water finds its way there on its own.
Just asking.
 
Since the OP stated that this has only been occurring lately perhaps as previously suggested, he may be getting bad fuel.

I've had aircraft based at Carlsbad and Gillespie for years and never once had an issue. My RV-12 is based in the humid and sometimes cold South Carolina coast and never had an issue.

I also top-off the tank after every flight.

Just a side note, depending on your fuel tank mods, you may have a vented or non-vented fuel cap.
 
That's exactly my point. Since there's very little or no gravity between the fuel pick up and the gascolator I doubt that water finds its way there on its own.
Just asking.

From the POH:
? Fuel Sample ? CHECK for water or sediment
contamination with fuel pump ON.

Since the OP stated that this has only been occurring lately perhaps as previously suggested, he may be getting bad fuel.

I've had aircraft based at Carlsbad and Gillespie for years and never once had an issue. My RV-12 is based in the humid and sometimes cold South Carolina coast and never had an issue.

I also top-off the tank after every flight.

Just a side note, depending on your fuel tank mods, you may have a vented or non-vented fuel cap.

If you have the non-vented fuel cap and the new vent and fill the tank completely, a temp increase in the hangar will cause some fuel to vent to the floor.
 
From the POH:
? Fuel Sample ? CHECK for water or sediment
contamination with fuel pump ON

The reason for operating the fuel pump is to move fuel/contaminate (water) along the horizontal fuel tubing from tank to Gascolator. The flow of liquid occurs because of the return flow from the upper fuel distribution rail back to the fuel tank. Don?t have to run very long ? perhaps 30 seconds will get a current cohesive sample in the Gascolator bowl. Best to shut off pump before sampling to prevent fuel spray.
 
there is a paste you smear on the bottom of a dipstick and can then check the bottom of the tank for water. gas stations have been using it forever on the ends of their 10' sticks.

What are you using for a dip stick? Rigid sticks don?t get past the bend in the fuel tank neck, and flexible ?sticks? (like plastic tubing) don?t seem to reach the bottom of the tank.
 
What are you using for a dip stick? Rigid sticks don?t get past the bend in the fuel tank neck, and flexible ?sticks? (like plastic tubing) don?t seem to reach the bottom of the tank.

Actually, it doesn't matter what type of dipstick or material in this instance. The bottom of the fuel tank is slanted like a swimming pool with "deep" end in the front so every drop of fuel exits the fuel line fitting. The filler neck is located at the back of the tank which is the "shallow" end. With no measurable fuel at the "shallow" end the tank may still have 1-2 gallons of fuel at the "deep" end. So, looking for water presence at the bottom of the tank below the filler neck is no applicable...
 
Best to shut off pump before sampling to prevent fuel spray.

Leaving the pump on produces the best result. If it is off, the same vacuum that makes it difficult to remove the bowl during maintenance will allow very little flow.
The pressure is low....... it will not spray.
 
A rigid stick does work. I have two made out of Sitka Spruce. Measures from 2 gallons up to 19.8 and above to full filler neck
 
A rigid stick does work. I have two made out of Sitka Spruce. Measures from 2 gallons up to 19.8 and above to full filler neck

Sitka Spruce is flexible enough to get past the bend in the filler neck, but stiff enough to reach the bottom of the tank? I?d love for you to describe how you made yours.
 
Sitka Spruce is flexible enough to get past the bend in the filler neck, but stiff enough to reach the bottom of the tank? I?d love for you to describe how you made yours.

Easy to do. Use table saw to make 1/8" x 5/16" stick about 48" long. Cut end of stick at 45 deg angle to help get past the aluminum fuel return line that is turned vertical inside the base of the filler neck. Takes a little fiddling each time to get stick to enter the fuel tank. Stick will bend nicely to pass through the bend in the filler neck.

Next completely drain the tank and add fuel in two gallon increments. After each pour you insert the stick. Withdraw the stick and use a lead pencil to make a horizontal line for every two gallons. You'll end up with (you guessed it) ten lines. Graphite mark from the lead pencil will not wash off with gasoline.
 
I use a piece of the nylon brake line to sound my tank. It?s easy to push down the filler neck and gives a clear sounding. I calibrated it when I filled the tank the first time.
 
Although it appears that this issue is being addressed through other testing, I want be sure no misunderstanding occurs with the use of Decalin.

Decalin has a high affinity for dispersing, meaning that if you put any amount in 100LL fuel, it will completely disperse within the fuel over a short period of time with no mixing required.

It will not come out of solution in fuel.

Even though your cap is on tight, there are two o-rings, so if there is water intrusion, it could be the oft ignored center stem o-ring is allowing water to come in. A little spray lubricant on the stem, and they cycle it a couple times, can help that. Or just replace both, especially if it's been a year.
 
Although it appears that this issue is being addressed through other testing, I want be sure no misunderstanding occurs with the use of Decalin.

Decalin has a high affinity for dispersing, meaning that if you put any amount in 100LL fuel, it will completely disperse within the fuel over a short period of time with no mixing required.

It will not come out of solution in fuel.

Even though your cap is on tight, there are two o-rings, so if there is water intrusion, it could be the oft ignored center stem o-ring is allowing water to come in. A little spray lubricant on the stem, and they cycle it a couple times, can help that. Or just replace both, especially if it's been a year.

Thanks for this. I wondered whether my o-rings were letting moisture in, because they were more than a year old. As a result, I replaced both o-rings a couple of days ago, but haven't checked for water in the fuel since.
 
Thanks for this. I wondered whether my o-rings were letting moisture in, because they were more than a year old. As a result, I replaced both o-rings a couple of days ago, but haven't checked for water in the fuel since.

Good deal.

You might consider using a little DC-4 (Dow Corning #4) silicon grease on the large o-ring and a shot of a medium weight spray lube on the center stem once in a while (I do it monthly) to help the o-rings stay clean, lubricated and last longer.
 
Good deal.

You might consider using a little DC-4 (Dow Corning #4) silicon grease on the large o-ring and a shot of a medium weight spray lube on the center stem once in a while (I do it monthly) to help the o-rings stay clean, lubricated and last longer.

I checked the fuel this morning for the first time since changing the o-rings. There was no water in the fuel! Now I'm going to get some DC-4 and spray lube too. Thanks!
 
I checked the fuel this morning for the first time since changing the o-rings. There was no water in the fuel! Now I'm going to get some DC-4 and spray lube too. Thanks!

Try 1/4 tank of fuel in storage, and let it sit through some foggy mornings for a week or two, and then let us know the results, if your sample is still water free. Then you'll know for sure if it was the seals or not.
 
Phase Separation

One possibility I did not see mentioned could be phase separation if the fuel is Ethanol Mogas. I am not an expert by any means but I have recently read a couple of articles about the different types of fuel we use, and old E10 can absorb quite a lot of water, and then when it reaches a saturation point it will separate, just as the OP is describing.
 
One possibility I did not see mentioned could be phase separation if the fuel is Ethanol Mogas. I am not an expert by any means but I have recently read a couple of articles about the different types of fuel we use, and old E10 can absorb quite a lot of water, and then when it reaches a saturation point it will separate, just as the OP is describing.

No, I don't use Mogas. I use 100LL, and add Decalin.

I thought I'd solved the problem by changing the o-rings on the fuel cap. This morning, before adding fuel, I checked my fuel, and again had no water in it. But I then added fuel from the self-service pump, and when I checked after doing that, I had the same 1/2 inch of water that caused me to start this thread in the first place. A follow-up test had no water in it. There wasn't anyone around to ask whether others were finding water in the self-service pump fuel, but that's the latest suspect!

Here's another piece of the puzzle: I had the fuel pump on when I tested first thing this morning. But I didn't have the fuel pump on when I tested after adding fuel. I know the POH says to test with the fuel pump on, but what effect would that have on water in the fuel?
 
No, I don't use Mogas. I use 100LL, and add Decalin.

I thought I'd solved the problem by changing the o-rings on the fuel cap. This morning, before adding fuel, I checked my fuel, and again had no water in it. But I then added fuel from the self-service pump, and when I checked after doing that, I had the same 1/2 inch of water that caused me to start this thread in the first place. A follow-up test had no water in it. There wasn't anyone around to ask whether others were finding water in the self-service pump fuel, but that's the latest suspect!

Here's another piece of the puzzle: I had the fuel pump on when I tested first thing this morning. But I didn't have the fuel pump on when I tested after adding fuel. I know the POH says to test with the fuel pump on, but what effect would that have on water in the fuel?

As I suggested earlier, go fuel up at Santa Paula airport, or somewhere 15 to 20 miles inland of the Pacific Ocean. See if the fuel is dry from other airports. Or go fill up with Premium car pump gas, from Costco or such.

N836BL is stationed at KWHP, 15 miles from you, and running fuel from KSZP or the Costco in Pacoima, CA, near KWHP, we have never seen water from fuel sampled out of the gascolater since 2016. Costco in our area is Conoco/Phillips/ 76 brand fuel out of the Carson, CA, refinery I believe. They have extremely high turnover of their fuel, due to very competitive pricing, their fuel is always fresh. You just might want to give it a try. Top off with it from portable 5 or 6 gallon tanks you fill up before you go flying, and check for water in the fuel. $3.50 / gal currently is a lot better that $5.50 for 100LL at KWHP
 
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I know the POH says to test with the fuel pump on, but what effect would that have on water in the fuel?

The electric fuel pump will move fuel from the lowest part of the fuel tank (sump fitting) to the Gascolator and then to the fuel rail on top of the engine. From there, fuel returns to the tank at about one gallon/minute with only electric pump running. This is enough flow to move water and sediment to the Gascolator where either component will settle out. Water and sediment are both heavier than gasoline.

Water may or may not find its way to the Gascolator without a positive flow of fuel. Sediment will definitely not move along the horizontal fuel run without liquid flowing to move it along.

It 's possible the FBO is selling you water...
 
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