Fuel fumes in cockpit

[Chrj]

I recently purchased completed RV-8 (quick build kit), 170 hrs TTAF & E, NDH; inspection revealed no obvious problems. Flies like a dream - problem is I intermittently smell fuel fumes in cockpit - this does not occur every flight. This seemingly occurs at random, no particular flight attitude, power setting etc. Although, possibly coincidental, I seem to notice it most often when boost pump is on. There is no visible sign of a leak that I can find thus far, but obvious concern is if I can smell fuel, there are combustible vapors present. Anyone have any ideas about how to chase this down, or what the most likely source would be?

[N427EF]

Do you have a red cube fuel flow sensor in your cockpit?

[aviationone]

If you are not getting any visible staining from a fuel leak I would first look at the vent lines. If your plane was built per the plans they come in from the bottom of the plane near the gear and go into the gear towers where they loop around and go to the fuel tanks.
This would not explain why you smell it more with the pump on but it would be an easy place to start.
Good Luck.

[airtractor8]

Do you have the standard Vans fuel valve fitted? If so look around the shaft gland seal for a leak. You should see die from the avgas present if its leaking.

[vickruis]

I had the same symptoms, intermittend fumes, usually on downwind with the pump on. I discovered fuel stains under the pipe threads on the red cube, and made the assumption I forgot to put fuelube on the threads during the build. After I smeared the stuff on again, I proudly presented my efforts on the forum.
Walt Aronow replied me that I would be doing it again...... So I did last week. I used Titeseal this time, hoping it lasts a bit longer!

[Steve Barnes]

I have had this with both my RV4 and my RV6. As we fly along the fuel naturally sloshes in the tanks and creates a little positive pressure. If the fuel splatters up to the vent line in side the tank, the slight pressure in the tank pushes it down the vent line and splatters it on the belly. There is a slight low pressure in the cockpit and the fumes get sucked in through hole made for flap actuator and fill the cockpit with fumes.

I often get the smell during aerobatics, but not always. Some times it is random. This is more likely to happen on a hot day. I have about 2200 hours between my 4 and my 6 and this phenomenon has happened since the beginning with both planes.

Steve Barnes "The Builders Coach"

[Ironflight]

The -8 is well known for getting some fuel fumes sucked into the NACA scoop under the right wing and then into the cabin through the passenger vent. I find it happens with full tanks, particularly when the right tank is really full and I am descending in the pattern. If I burn ten minutes of fuel out of the right, I don't get the smell.

YMMV

[hydroguy2]

If all else fails...or you want to ensure you have no leaks or weeping fittings, here's what I ended up doing.

I also had "the smell". During phase 1, I found the vents were actually not tight and full tanks climbing to altitude gave me a strong smell. Fixed that, but still had smells while boost was on...sometimes, it was more noticeable when the heat was On. I searched and searched, but never could find telltale stains. I finally left my floor tunnel(RV-7) open and wrapped every possible fitting in white paper towel. Flew around for about 30 min, climbs, descents, boost pump, both tanks etc. got a whiff or two. Upon landing, I started pulling all the towels and inspecting each one. Almost the last towel I checked had a dampness to it, but hard to see any stain. BUT...TaDa! The bulkhead fitting had more of an ooze at the nut. A nut which is blue, it would have taken years to leave a big enough blue mark to see it. Problem solved.

I still get a whiff once in a while, but usually with full tanks and long climbs to alt.

[Chrj]

Thanks for all the great suggestions guys. I have yet to find any visible staining anywhere. I am going to have my A&P go through the fuel system and see what he can find. If that turns up negative, I will try the towels in the tunnel idea. I will keep you posted of any findings. I may be chasing my tail here, but I am paranoid enough, that I cannot let it go unchecked.

FYI - I also ran this by my father (retired. Prof. pilot) and this was his reply:

At this time, I think that Steve Barnes in probably correct. I have always thought that somehow the fuel vents were the problem. All fuel tanks are designed to have a positive pressure in them to ensure that fuel flow is not hindered by a vacuum. The fuel pluming is likely located in the lowest part of the tanks which is usually nearest the cabin. Considering the fact that most turns are made to the left, when the ball is not kept centered or is out a bit to the left, (slipping left turn) the slosh in the right fuel tank is such that the positive pressure would likely cause fuel to exit through the right fuel vent. (In a slipping turn to the left the fuel would "slosh to the inside of the tank towards the cabin which would cause more pressure in that area) As soon as the fuel exits the vent, it instantly vaporizes. This along with the fact that the airflow from the prop corkscrews around the plane would cause the airflow to literally "blow" the vapors under the plane where, considering the fact that there is a low pressure in all cabins, it is "sucked" into the cabin through any and all openings.

This would also answer the fact that the problem seems to be random. Regardless of the attitude when the ball is not centered the fuel in one tank is compacted in the area where the fuel "pick-up" is located.

Typically, fuel vents are located under the wings outboard of the fuel tanks. The fuel vents on the RV's are not only located on the inside of the fuel tanks, but are attached to the cabin between them. This arrangement just about ensures that that vapors from venting fuel vents will enter the cabin. I am willing to bet that if the fuel vents were under the wing outboard of the fuel tanks and out of the prop wash, that there wouldn't be a problem with fuel vapors in the cabin.

[flynwest]

My 8 does it also when the tanks are full. Burn off 10 min. of fuel and no more smell.