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  #1  
Old 10-06-2017, 08:23 AM
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JackinMichigan JackinMichigan is offline
 
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Default Fuel line leaks - how to find them.

We recently had to replace a number of fuel lines in some very hard to reach areas (in the forward part of the tunnel). After completing this arduous task we leak-checked the system by capping off the wing tubes and pressurizing the system with air thru the firewall to 50psi.

With an artist brush and a bottle of children's soap bubble compound I painted all the fittings and could not see any foaming anywhere, but the system pressure dropped about 3psi over 30 minutes. We decided to leave it overnight, and this morning it was down to 20psi (approx. 14 hours later). The total volume involved is equivalent to probably 12 feet of 3/8" aluminum tube.

Does anyone have any tips on how to locate such a slow leak? Soapy water, a flashlight and an inspection mirror don't seem to be cutting it.
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Old 10-06-2017, 09:24 AM
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Default Talcum powder

Put on a light dusting of talcum powder. It will turn bright blue in the presence of av gas. You can vacuum up the residue when finished.
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  #3  
Old 10-06-2017, 10:01 AM
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You can "diaper" each fitting with paper towels and pressurize the system with AvGas. The diaper that turns blue is your culprit.

As for dealing with a bad flare, you might try a Del Fitting Seal rather than re-making a piece of tubing.
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Old 10-06-2017, 10:06 AM
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Your post sounds like you believe you have a leak even after you leak tested with soapy water. Why do you think you have a leak? If you are basing that decision on the psi change note that the leak detection is NOT watching to see if the pressure in the lines decrease or not. The leak detection process is the soapy water and looking for bubbles.

The pressure within your lines can move up or down solely based upon the temperature and baro pressure in the environment from one day to the next. So, it is erroneous to think of the psi change as indication there is a leak. If you have concerns about leaks, test with the soapy water again. In fact, do it multiple more times until you are satisfied. Just don't automatically assume you have a leak because the pressure dropped.
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Old 10-06-2017, 10:29 AM
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Quote:
Originally Posted by RVbySDI View Post
Your post sounds like you believe you have a leak even after you leak tested with soapy water. Why do you think you have a leak? If you are basing that decision on the psi change note that the leak detection is NOT watching to see if the pressure in the lines decrease or not. The leak detection process is the soapy water and looking for bubbles.

The pressure within your lines can move up or down solely based upon the temperature and baro pressure in the environment from one day to the next. So, it is erroneous to think of the psi change as indication there is a leak. If you have concerns about leaks, test with the soapy water again. In fact, do it multiple more times until you are satisfied. Just don't automatically assume you have a leak because the pressure dropped.
I can see barometric pressure or temperature changes accounting for maybe a 10% change, but a 60% change tells me air is escaping, just not fast enough to be readily detectable with the soapy water test (these fittings are in a difficult to reach area hard against the floor, so it may be easy to miss some foaming). It was my hope to eliminate all leaks, no matter how slow, before reintroducing AvGas. I just need to pinpoint them.
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Old 10-06-2017, 10:32 AM
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I understand your reluctance to introduce fuel into a leaking system but it does sound like the previous two suggestions could quickly find your leak source.
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Old 10-06-2017, 10:47 AM
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Hi Jack,

If you haven't already, check the seals on the fuel valve. They aren't always designed to hold more than a few psi of pressure.

David
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Old 10-06-2017, 11:28 AM
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Pressurize with compressed air and use a mechanic's stethoscope to look for the leaks. Works well.
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Old 10-06-2017, 11:28 AM
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Is the gauge perhaps leaking? Soapy water is about as sensitive as it gets. The large viscosity difference between air and avgas might prevent a leak of air into fuel or fuel into air, due to the surface tension.
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Old 10-06-2017, 12:13 PM
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Quote:
Originally Posted by AlexPeterson View Post
Is the gauge perhaps leaking? Soapy water is about as sensitive as it gets. The large viscosity difference between air and avgas might prevent a leak of air into fuel or fuel into air, due to the surface tension.
That was the first thing we checked. Several times.

It's quite possible I'm taking this further than it needs to go, but if there's improving to be done I want to do it.
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