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  #21  
Old 09-06-2011, 03:09 PM
L'Avion's Avatar
L'Avion L'Avion is offline
 
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Location: Memphis
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Default fuel odor in cockpit using mogas (no ethanol)

Using mogas may have been coincidental with a fuel odor in my cockpit. Odor seemed to be related to boost pump on. Checked several times inflight boost pump on/off - odor was with boost pump on. On the ground later, no fuel odor; turned boost pump on -> smelled fuel; cowling off, boost pump on; found minute leak at fitting where fuel line comes through firewall and connects into gascolator. I loosened fitting and re-tightened, still very small leak. I then completely removed fitting, slid the ferule (sp) back, and looked for a crack in the fuel line flare - no crack. I re-inserted the fitting and tightened it again; boost pump on - no leaks; pulled and pushed on gascolator to simulate vibration/movement with boost pump on (several times) - no leak.

I've flown the bird three times since the above, no fuel odor, no leaks. Needless to say, I checked all fuel fittings. All fuel fittings had been checked for tightness at condition inspection in May --- one never knows....

I report this here to remind others to not overlook what one's airplane may be trying to tell you.
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  #22  
Old 09-06-2011, 03:41 PM
Tom Martin Tom Martin is offline
 
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Check the stem of your fuel valve with a kleenex or talc powder as someone else has suggested.
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  #23  
Old 09-06-2011, 03:57 PM
prkaye prkaye is offline
 
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thanks guys, all good suggestions. I will definately inspect the entire system including all lines, fittings, fuelp pump, fuel selector etc using talc and running the fuel pump as suggested.

I will not use any goup.

But nobody has yet offered any theories as to why I experience the smell mainly at low power settings - after startup and taxi, and again when i throttle back for descent / approach. Any ideas on what kind of leak would account for this seemingly backward behaviour (i.e. stronger smell when the system is drawing less fuel)??
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  #24  
Old 09-06-2011, 03:59 PM
Kyle Boatright Kyle Boatright is offline
 
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Location: Atlanta, GA
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Quote:
Originally Posted by David-aviator View Post

Note. The vent system is open to atmospheric pressure, there is no such thing as a "high vapor pressure" causing a leak, that is total nonsense.
David,

Your other advice was good, but you're mistaken here. Fuel with a higher vapor pressure (typically autogas) will evaporate faster than lower vapor pressure (avgas) blends. That means more fuel vapor will be exhausted out of the fuel vent over time. That will create a stronger fuel smell.

Is it enough to cause the issue of concern in this thread? I have no idea. But it is a possible cause.
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  #25  
Old 09-06-2011, 04:14 PM
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Vlad Vlad is offline
 
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Quote:
Originally Posted by prkaye View Post



But nobody has yet offered any theories as to why I experience the smell mainly at low power settings - after startup and taxi, and again when i throttle back for descent / approach. Any ideas on what kind of leak would account for this seemingly backward behaviour (i.e. stronger smell when the system is drawing less fuel)??

Could it be main gear legs fuselage penetrating points? There is a gap there I remember a builder reported strong drafts coming from those wells. I prosealed mine. Could it be a fuel tank drain o-ring not properly set and sipping or vent discharging excess fuel, then airflow from lower power settings picks up those fumes and draws them via wells to the cockpit.

You asked for theories we have them plenty
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  #26  
Old 09-06-2011, 05:48 PM
prkaye prkaye is offline
 
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Quote:
then airflow from lower power settings picks up those fumes and draws them via wells to the cockpit
Interesting - so if the leak came from outside the cockpit, then you're suggesting that maybe only at relatively low speeds is the fuel able to travel to a place where it can get in... i.e. at higher power speeds maybe it just gets blown back out of the way. I'll plug up those gear-leg holes and my (rather large) flap pushrod holes, for good measure.

Quote:
You asked for theories we have them plenty
And every theory gives me something else to check. The curious aspect of the smell only at low power settings does seem to me like it should be an important clue. This fact seems incompatible with a typical fuel leak inside the cabin. So any other theories that would be compatible with this clue (smell only at low power settings) will be much appreciated!
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Last edited by prkaye : 09-06-2011 at 05:56 PM.
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  #27  
Old 09-06-2011, 05:55 PM
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Jamie Jamie is offline
 
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Phil:

Are you sure it's low power settings that'st the trigger? The situations you enumerated seem to closely align with times that the boost pump would be running or would have just run.

Also, the times that you listed are when the airplane is slow. Being slower = less air circulating through the cockpit. It could be leaking vapor continuously and you're simply not smelling it when moving fast.

Pull your wing root fairings as well and check your tanks. It doesn't take a lot of fuel leaking in the wing root to get vapors to waft into the cockpit. I know -- been there, done that.
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  #28  
Old 09-06-2011, 05:59 PM
prkaye prkaye is offline
 
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Quote:
Are you sure it's low power settings that'st the trigger? The situations you enumerated seem to closely align with times that the boost pump would be running or would have just run
Doesn't seem to be correlated to boost pump. I throttled back without putting the boost pump on and got the smell. Could be related to speed, as you suggested.

Will definately remove the wing root fairings and inspect those covers.
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  #29  
Old 09-06-2011, 08:36 PM
David-aviator David-aviator is offline
 
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Location: Chesterfield, Missouri
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Quote:
Originally Posted by Kyle Boatright View Post
David,

Your other advice was good, but you're mistaken here. Fuel with a higher vapor pressure (typically autogas) will evaporate faster than lower vapor pressure (avgas) blends. That means more fuel vapor will be exhausted out of the fuel vent over time. That will create a stronger fuel smell.

Is it enough to cause the issue of concern in this thread? I have no idea. But it is a possible cause.
Yea, I know mogas evaporates quicker than 100LL. I was using the stuff a few years back and checked its VP before every flight. It was usually good for about 15,000' vrs over 20,000' for avgas, depending on temperature.

But mogas or any other fuel vapor does not build up pressure in the vent system. I don't see how it could unless the vent exit was blocked.

The way the Vans vent system is designed, a slight pressure build may occur due to ram effect. I get more than a little ram pressure build up because a couple forward facing vent line inlets. That should force any liquid in the lines back into the tank. I think Phil has the same set up. Its a good idea because that little head of pressure gets fuel moving toward the engine better than no ram pressure. But I don't think it will cause a fuel smell in the cabin, at least I've never had it with my beast over a 7+ year relationship.

The fuel vapor smell at lower power could mean vapors are being drive by prop air flow and when there is less of it due to the prop coasting, vapor could be entering the cabin rather than blowing by. The question remains, where are the vapors coming from?
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  #30  
Old 09-06-2011, 08:56 PM
Captain Avgas Captain Avgas is offline
 
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Quote:
Originally Posted by prkaye View Post
Today I was at a small airport and at the gas pumps they had both 100LL and V-Power (91 Octane auto gas by shell with no ethanol). I decided to try the V-Power, and i filled my right tank.
91 Octane seems to be a very low octane mogas to use in a Lycoming.
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