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How to Find Source of Fuel Smell?

Piper J3

Well Known Member
I just did the Fuel Tank Attach Modification Service Bulletin 13-12-19. I accomplished the modification by carefully removing the tank lid to gain better access to the lower front edges of the tank. All went well and tank was resealed with ProSeal. Tank was filled with 20 gallons 93E10 to calibrate the fuel gauge. The airplane sat for a full week with completely full fuel tank and no fuel odor was noticed when canopy was raised. I even left the fuel shut off valve in the open position to static test the fuel lines up to and including the gascolator. Very good.

Now I have flown the plane three times for roughly an hour each flight and have 5 gallons remaining in the tank. On the first flight I noticed fuel smell occasionally on climb out and occasionally in cruise flight with a fair amount of turbulence. Second flight was a little less fuel smell and the third flight was still less but smell was occasionally still there. I suspect the leak to be very small, and of course, difficult to find.

I see no evidence of a leak at the tank. It might be carb float bowl overflow but I don?t see evidence of residue in the drip pans. I have searched VAF and found a thread for RV-12 Fuel Tank Leak that?s not much help.

My question is? has anyone used a dye solution specifically made for detecting fuel leaks? I found a UV dye additive for auto fuel that fluoresces under black light. Anybody try anything like this?
 
Run a tank of 100LL .... it has blue dye in it ;)

A few gallons of avgas is ok for the Rotax
 
I'm not sure about the -12, but with the other RVs completely filling the tank can allow fuel to enter the vent tubes with their opening near the filler neck. Then when you fly the expanding air in the tank (not being used) pushes that fuel out of the fuselage vent outlet and you can often smell it in the cockpit.

Possibly the turbulence could allow fuel to splash down the vent in a similar manner.

When the tank is less full there is less likelihood of fuel getting into the vent.

Whether this applies for a single tank set up will depend if the air expands in the climb at a greater rate than fuel is being drawn from the tank.
 
I forgot to mention that tank is an early design with visual sight gauge on the side and vented fuel cap. I elected not to install the new vent line and instead stay with the vented fuel cap. The vent holes in the cap are fully open.

Also, as for carb bowl overflow possibility, during last condition inspection I weighed the floats and they are within spec. Both carbs were overhauled by Lockwood prior to my purchasing the plane.

Another data point - while doing the fuel tank modification the carbs sat for five months and most likely the fuel evaporated from the float bowls. I'm wondering if gaskets may have dried out.
 
Jim-

Five months is a long time for auto fuel to sit in a carb bowl compared to aviation fuel. You just may want to pop off the carb bowls and have a good look for gunk.

If you look carefully at the flight data recorder you may see a small indication of what is going on ... I would think if you are smelling fuel and it is coming from the float bowls, there should also be a small change in RPM, EGT, fuel pressure and fuel flow at the time of the "event" when you are smelling the fuel. If you can see the "event" reflected in the flight data, it would indicate you are not getting the fuel smell from your repaired tank ... its coming from under the cowl from a carburetor(s) overflowing.

As another data point, you may want to place 30% or more 100LL in the tank and make a test flight. You just may be seeing the results of winter blend auto fuel being used in the spring when the temperatures are warming up.

Keep us posted as to what your findings are.
 
The only time I get a whiff of fuel odor in my RV-12 cockpit is during climb-out with what was later to be determined were over-weight fuel bowl floats. You may or may not see any evidence of fuel residue in the carb overflow trays.
 
I checked my engine log book and carb floats were weighed 8 months ago as follows - 6.1g (R) 6.2g (L). So the floats are less than the 7.0 gram spec set by Rotax for SB-912-065UL. I guess I'll have to re-weigh the floats now that I'm having a problem.
 
In almost 600 hrs. of 12 time, the only time I have smelled fuel was when the floats were heavy.k
 
I checked my engine log book and carb floats were weighed 8 months ago as follows - 6.1g (R) 6.2g (L). So the floats are less than the 7.0 gram spec set by Rotax for SB-912-065UL. I guess I'll have to re-weigh the floats now that I'm having a problem.

Jim,
Simultaneous failures (tank and floats) are unlikely. The last thing you worked on prior to the fuel smell was the tank. I would close that issue before galloping off in other directions. Ben Barron had a tank leak that was hard to find and he did find it with dye.
Check out this this thread for the dye he used: http://www.vansairforce.com/community/showthread.php?t=142601&highlight=dye&page=3
 
After rebuilding my tank and applying SBs, I had one slowly weeping rivet down in a corner. Fortunately I was able to get to it and seal it up. However, it was a very slow seep -- maybe a drop every few minutes. You could smell it in the cockpit after closing the canopy for a while. I think that would be true of any tank leak, except maybe the collar where the filler neck attaches to the tank. The return line pumps fuel into that area, so I suppose it's possible it could be the source -- although you'd almost certainly smell it while fueling. If you fill yours halfway up the filler neck like I do, it would be obvious after sitting for a while.

So if you're smelling it only during flight, especially on climbout, I'd be more suspicious of a firewall forward source. The carb floats do sound like a likely source. I would maybe try some slow flight at high angles of attack at idle and part throttle to see if it happens with certain engine speeds, certain loads, or just certain attitudes. That might help nail it down. If the float bowls are overflowing during flight I wouldn't be surprised to not see anything in the drip trays -- there's probably a good bit of turbulent air in that area to blow stuff around.
 
If my floats have become "heavy" and need replacement, has Rotax come up with their new float design yet? I can't believe how expensive these little buggers are. If current PN is 861-184 then it looks like $93.50 will buy a single pair of floats. Of course there's two carbs so twice that amount to make things right.

I'm going to buy AC Delco 10-5045 Multi-Purpose Fluorescent Leak Detection Dye and a 100 LED UV Blacklight Scorpion Flashlight to survey the tank for leaks. This will be the starting point for troubleshooting the fuel smell.

Thanks guys...
 
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Also make sure that the fuel vent mod line is still attached behind the baggage bulkhead. I'm not a fan of that design.
 
I have also been trouble shooting a gasoline smell in the cockpit of my RV-12. After failing to find any leaks, I finally looked behind the bulkhead behind the tank and found the problem. There is a "T" in the overflow tube where the 1/8 " clear tube is connected to kill any tendency to siphon. This 1/8" tube had pulled out of the "T". Furthermore, this part of the "T" was angled downward, so that when I filled the tank to overflow, fuel was dumped into the fuselage via the open part of the "T". Also, it looks like I'll have to fabricate a new 3/8 aluminum overflow tube because the old one is too short and terminates inside the belly grommet. This could also possibly dump some fuel inside the fuselage. This tube is suppose to extend through the grommet to the outside by 1/2 " with a 45 deg bevel facing forward. I may can just do a small amount of bending and get it to protrude through the belly. It's awfully hard to access that area though
 
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Fuel Valve Assembly Leakage

We had a similar problem on my -12 where a raw fuel odor seemed to occur on initial climb out and dissipated soon thereafter. Like others, we originally suspected that filling the tank into the neck may have been the cause, but avoiding that condition changed nothing. The odor was soon present throughout much of our subsequent flights and we grounded the plane until we could find the source.

When I finally looked at the bottom of the fuselage (duh...), I found fuel stain streaks that began aft of the firewall, suggesting the problem was not coming from the engine. So we openned fuel valve floor panel and discovered no standing fuel but residue was clearly present. Although the fuel valve line connectors seemed sufficiently tight, we disconnected them and added fuel-appropriate thread sealer to the lines and reassembled.

That solved the problem for us.
 
We had a similar problem on my -12 where a raw fuel odor seemed to occur on initial climb out and dissipated soon thereafter. Like others, we originally suspected that filling the tank into the neck may have been the cause, but avoiding that condition changed nothing. The odor was soon present throughout much of our subsequent flights and we grounded the plane until we could find the source.

When I finally looked at the bottom of the fuselage (duh...), I found fuel stain streaks that began aft of the firewall, suggesting the problem was not coming from the engine. So we openned fuel valve floor panel and discovered no standing fuel but residue was clearly present. Although the fuel valve line connectors seemed sufficiently tight, we disconnected them and added fuel-appropriate thread sealer to the lines and reassembled.

That solved the problem for us.
Good job!!
 
Question... Does anyone know the weight of a set of new floats fresh out of the box?

Jim - not sure about of the box, but I checked the weight of both the floats in each carburetor on my engine prior to installing it on the RV-12 firewall. The floats measured in at 3 grams each.

Since you are in the carbs .... make sure the float arms are parallel with the body of the carb when the carb is held upside-down. It is possible for otherwise good floats to allow excess fuel into the carbs if the float arms are also not adjusted correctly. Visually, they should be parallel ... but there is an $85 gauge that is supposed to be used to make the adjustment perfect.
 
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I had the same smell on climbout only. Removed the tunnel cover and found real small leak on blue fitting used to connect fuel return line. I had been using some avgas, so trail to fitting was obvious. A & P added microthin washer to fitting and all good. My guess is that during climbout, the gas moved down the tunnel to where the fuel shut-off is and a small opening exists where the handle comes thru that also allowed the vapor to exit. Never smelled gas during long flites, or taxi or landing, only on climbouts. This happened during the time period when we were weighing floats, waiting for new ones, so i thought the smell was float related also.
 
Update... I bought AC Delco 10-5045 Multi-Purpose Fluorescent Leak Detection Dye and 100 LED UV Black Light Scorpion Flashlight to help locate source of fuel smell. Yesterday I mixed 1 oz. of the dye with 12 gallons of fuel in the tank and flew for half hour. Today I will open all access panels and search for source of leak with black light. The dye fluoresces a brilliant orange color so it should be easy to spot a leak.

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Good luck. I just did the 5 year hose replacement, and I am chasing a coolant weep that equates to 1/2" of overflow bottle level in 12 hours of operation. I put The dye in my coolant Friday and will check with a UV light Monday after 5 hours of flying this weekend.
 
Found the leak with black light and fluorescent dye... works a charm. Two flush rivets at internal strengthening plate on front bottom right side have very slight (miniscule) leak. Real problem is single rivet on top cover plate in rear right corner has enough seepage to wet your finger when you wipe over it. This rivet is leaking when the plane is in a climb and fuel sloshes to the rear.

I also removed cowling to check carb overflow. They are dry as a bone...

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Since they are small have you considered goofing over them with Pro Seal?
^^^ This. You have absolutely nothing to lose by trying it.

I had a weeping rivet in the lower inboard forward corner after I had rebuilt my tank with all the SBs applied, painted and re-installed it. I cleaned the area and mixed up a 10cc syringe of sealant, then used that to fill in the rivet and put a little around the edge. I put the syringe in the deep freeze for later use -- I assume it's no longer usable but I haven't needed it again. It's been completely leak-free for about year and a half now.
 
Well(d)

I just asked a pro to weld me a tank out of 1,5mm aluminum.
Will let you know the results once I have it back.
New tank, sloshed with tankslosh, fully covered with RTV where needed. Left it loose for some eight months or so with auto gas (10% ethanol over here): 8 months no smell, just laughing at your posts. Then suddenly a leak. I turned over the tank on the other side: small leak as well. Probably eaten by ethanol? Who knows. Anyway: I'm fed up with it. The design is OK, the new tank is about that size.
 
I visited a fellow builder today and looked over his work in progress. He has a factory built tank. It certainly has a lot less external goop than mine, but then again: mine hasn't leaked in 5 years!
 
Nice detective work Jim! Only un-fun part now is fixing it.

Tony - I'm going to remove tank for 100% inspection and repair.

Quick side note... I'm now flying the D-180 Dual Display I bought from you back in November. Flew right seat several times and lovin' it. My 12 has two EFIS, 496, and mini iPad running ForeFlight. Four color displays - feels like I'm flying a biz jet.

Jim
 
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Tony - I'm going to remove tank for 100% inspection and repair.

Quick side note... I'm now flying the D-180 Dual Display I bought from you back in November. Flew right seat several times and lovin' it. My 12 has two EFIS, 496, and mini iPad running ForeFlight. Four color displays - feels like I'm flying a biz jet.

Jim

Glad to hear life is good in Ohio! I've had my tank out and apart a couple times for the SB work as well.

Not a big deal. Yeah, I liked the dual display setup too. Will probably put in a second SV one of these days.
 
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