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Fuel Smell Puzzle


207WF

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My 2007 CTSW has always smelled of fuel, but now its getting much worse. At the last annual the carbs were rebuilt, the wings were pulled for inspection, the sight tubes replaced and the rubber hose that runs along the windscreen from the tank to the firewall was replaced.

 

I have checked the fittings around the sight tubes and the rubber hose that goes to the firewall, stuffing long strips of rags down there, and after a several-hour flight they come out dry and don’t smell fuel laden. There is no sign of leaks at the firewall side of those hoses. I have checked the fuel filter behind the instrument panel – clean and dry. Roger Lee has tweaked the carb float levels and replaced the gaskets so the drip trays are now clean and dry. I still find a little fuel moisture around the air intake hose fitting at the back of one carb, but the air intake at the top of the cowl does not smell strongly of fuel. Today I confirmed that the fuel hose attachments to the carbs are tight.

 

I can smell fuel in the cabin and even the baggage bay when I unwrap the bird after sitting with the gas shut off and the canopy cover on. I can smell fuel underneath the cowl on preflight, but see no drips. The smell is strongest right as I start the engine when the vent window is open.

 

I am running out of ideas and inspiration. Anyone have any new ideas?

 

Thanks,

WF

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Maybe drain each tank and use a bore scope to inspect the insides? Wondering if there could be a pinhole leak in one of the fuel tanks that's allowing fuel to come thru the wing and into the cabin? One other thought is maybe the carb balance tube that runs thru the air cleaner. If this is filling with fuel it may bring this into the aircleaner? Don't know how this could fill with fuel but it is just a thought.

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A tiny fuel leak can vaporize before it shows up as a liquid. I've heard (never tried it) that you can find these sorts of leaks by taking a short length of hose, holding one end to the fittings and sniffing at the other end.

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By now, I have checked all the fuel line connections on the cabin side of the firewall and all of the hose clamps in the wing roots. All are tight and dry. There is no leaking around the nut that attaches the black fuel hose to the plate in the wing tank where the sight tubes attach. I am beginning to think that there must be an internal leak in the tank itself. The glider tape under the left wing root has developed a brown discoloration and feels soft, compared with the tape on the other side. Is is possible to diagnose this from the access panel under the wing and/or through the filler neck? Perhaps, the tank needs to be sealed or "sloshed" to fix this.

 

Does anyone have any experience with that?

 

WF

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Wayne...

That's a good way to describe the problem. I also had a little brown stain coming from under the tape.

Mine, Rog's, and a few others have done that. In my case, it was a little fuel leak in the root area (hose, NOT tank). Quick fix by Roger.

Tim

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Hi Wayne,

 

The leak is small enough not to see live fuel, but enough for your nose to pick up. Forget the tank slosh issue. If that was the case you would have stains on the underside of the wing in one spot and it would start to get soft over about a nickel sized area. Been there.

 

My $20 bet says it is still at the fuel bulk head plate at the site tube and it is still the fuel outlet fitting. You need to have the other Wayne pull that wing, but before he does have him give me a call. It's a fairly easy fix.

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  • 2 weeks later...

During the most recent attack on the fuel smell issue we found that one of the three washers on one of the fuel intake hoses into the carb was missing and it was spilling some fuel when under pressure into the drip tray. We replaced that. We took one of those tiny cameras for a super close-up view, and snaked it all around all the wing root attachments, including the nut between the black hose and the plate that goes into the tank, and checked the fuel hoses on the firewall and inside the cabin behind the instrument panel with it, too. They all looked clean and dry. There are no fuel stains under the wing. The fuel smell is still as strong as ever. I am completely at a loss on this. WF

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  • 2 weeks later...

I let the plane sit for 2 weeks at Christmas and the fuel smell in the cabin was diminished, but not gone, when I unwrapped the bird. On start up, and especially when closing the choke, the fuel smell was again quite strong. The next attack is going to be to replace the rubber tubing that runs from the back of the carbs to the air intake (these have been wet inside on past inspections) and to check the choke circuits in the carbs more carefully. Until we fix this my wife won't ride with me ! WF

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  • 1 month later...

Latest update about my ongoing fuel smell puzzle:

Last month we replaced the scat tubing that runs between the carburators and the air intake and replaced some parts of the choke circuit -- O rings and one needle valve. Today I taped over the holes where you access the sight tubes and spar bolts to see if that might make any difference. While the odor may be somewhat diminished, and less obviously associated with pulling the choke to off after start, the fuel smell was still strong. I can smell it under the cowling when I take the canopy cover off, and the whole inside of the bird including the tail cone smells of fuel. This is one pesky problem !

 

WF

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WF, some info you may use. My friend's CTLS had fuel stains in his LH carb drip tray. Inspection showed fuel stains were not on the carb proper but were seen on the firewall end of the rubber carb connector and on the rear clamp for this rubber connector. The carbs were removed and then rebuilt with the "kit" supplied for this. New diaphragm, new "O" rings, new float bowl gasket and new float needle. We took this plane to Sebring and after arriving, we inspected the engine after this long trip. We found fresh fuel in the LH carb drip tray. While at the Sebring show, I talked to the Rotax rep who was manning the tent. I told him about the fuel in the drip tray and the fact that the carbs had been serviced. Also, we seemed to see fuel at the rear (firewall) end of the rubber carb connector. He offered the opinion that the needle valve seat may have dirt or in some way may not be sealing completely. He said that the carbs actually run "downhill" going towards the firewall. If the needle valves do not shut off, this tech guy said he has seen where the overflow fuel from the float bowl will run out of the carb air inlet, towards the firewall, drip out of the rubber connector and into the fuel tray. He said that one can hold the carb upside down and then apply low air pressure to the carb fuel inlet (believe he said 15psi but you need to verify this). He indicated that one can listen for air leakage and there should not be any if the float needle is properly shutting off from just the weight of the float. He also said that if one has the "pump" checker with attached pressure gage used to check automotive emmission control systems, this would also work by observing if the air pressure was holding. He said the needle or needle valve seat is suspect if there is air leakage. FWIW.

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Can you not make up an attachment for the fuel cap to pressurise the system to check for leaks on the ground without the engine running ... we have just done a ct2k long wing in the UK that had fuel tank leaks that had to be pressurised to make sure they were ok it doesn't have to be much pressure to show up leaks in the system

 

Mike

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Mike, good idea. A word of caution from one who used to design and test automotive fuel tanks; be careful when adding pressure. Just a few psi and no more. Too much pressure on the full surface of the fuel tanks could crack or damage them. I suggest that the pump fixture with attached pressure gage that is used for auto emission testing is used to see if the pressure holds after pressurization.

 

Mike how did you fix the leaks you found?

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  • 1 month later...

We just might have resolved this at last! It seems that when the rubber fuel hoses that run from the wing root to the metal supply line running down the sides of the windscreen were replaced at the last annual, the hose clamps were not properly secured. Last weekend with Jeremy we pulled the wings and redid everything that connects to fuel in the wing roots, including new rubber fuel lines with good clamps. The fuel smell is gone! Yay! WF

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The fuel smell is back! :mad:

 

I am now sure it is Not coming from the wing root area: blowing air in there, the only smell is that of the air freshener I previosly sprayed in. It might be coming from the heater vents.

 

But, how could a raw fuel smell get into the heater vents? Very puzzling.

 

WF

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The fuel smell is back! :mad:

 

I am now sure it is Not coming from the wing root area: blowing air in there, the only smell is that of the air freshener I previosly sprayed in. It might be coming from the heater vents.

 

But, how could a raw fuel smell get into the heater vents? Very puzzling.

 

WF

 

Is there any fuel on or in the black air induction hoses from the carb to air filter box? If your needles are not holding and are leaking then raw fuel is ending up in these hoses where they attach to the carb. Then once they get good and saturated they drip down onto the drip trays and everywhere else possibly causing your fuel smell. This is even harder to see when and if you use 100LL due to it evaporating quickly.

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Here is the latest. After going over the carbs again (3 mechanics on 'em since September) we have new float needle valves and they hold air pressure on the bench like they are supposed to. This should address the just-mentioned fuel in the scat tubes problem - although I did have that issue before - and now the carb drip trays are dry and the scat tubes are new. Ruling out the wing roots as the source, I began to suspect the heater intake box. There are three fuel hoses on the firewall that could potentially drip into the top of the box and fumes could enter the cabin via the heater vent. Yesterday I checked each of those hoses at the firewall with a dry paper towel, which emerged dry and with no smell of fuel. I tried taping over the heater vents in the cabin and covering the canopy for a few days, and that seemed to make NO difference to the extent of the odor when I uncovered the bird. Ruling out the bulkhead plates on the fuel tanks to which the sight tubes attach and anything attached to them, which I think we have, is there some other way that fuel from the tank could communicate with the cabin or baggage bay? Along a spar, for example? I am still at a loss on this one! WF

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Hi Wayne,

 

How did you rule out the site tube area? The most common leak is the fuel outlet fitting. Some leak enough to see some are so slow you can't see or feel fuel, but the nose can pick up the odor. Have you ruled out all the fuel lines in the instrument panel?

 

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I ruled out the sight tubes or anything attached nearby after Jeremy pulled the wings, redid the rubber fuel hose, took off the bulkhead where the sight tubes are attached, re-resealed that bulkhead and reattached it with new lock nuts, cleaned and resealed all of the fittings that go through there and thoroughly inspected everything. With it all put back together I put some talcum powder down by the new black rubber fuel hose and let it sit a few days. Then, I blew air all around in there from a hair dryer and smelled what I got back out. Nothing but talcum powder.

 

I have been through all of the hose fittings that come into the cabin three times now, can't feel anything wet there, and my hand does not come out with a fuel smell on it. We also looked at them with a borescope. Couldn't see anything, either.

 

WF

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  • 1 month later...

WF or Jeremy, the 5 year hose replacement covers a wide area. Any idea what so ever where the leak may have been? This has been a long ongoing and perplexing problem for you. I really hope that you've soved your problem and can just start enjoying your CT.

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No idea whatsoever! I infer that there was some tiny leak, too small to feel moisture or see anything, yet enough to stink up the cabin. Last week was the second smell test and still no fuel smell! My wife said she will fly with me again. Palm Springs next weekend! WF

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