Fuel Distribution

[n2582d]

Jason, A couple of years back Miles figured that the fuel head pressure at the fuel valve should be around 1.4 psi. The fuel tank is less than 6" deep so with one tank full and the other empty the difference in head pressure between tanks is less than .156 psi or next to nothing. If the fuel cross vent line between the tanks is open I don't understand how there could be a differential in pressure between tanks, even if one cap was not vented. But, as it is always the left tank that seems to be lower than the right, my suggestion would be to swap fuel caps and see if the problem follows the cap.

[blueldr]

That guy Gary is a swift one! Why didn't I think of that?

[edbooth]

That guy Gary is a swift one! Why didn't I think of that?

Our brains are getting really old BL and the usefull information is being diluted by all the useless information floating around in there.

[blueldr]

Getting old is only the half of it. I was talking to a guy the other day that is a retired Air Force pilot and is now flying for an airline. He told me that he has NEVER flown an airplane with a propeller on it.

[c170b53]

When I read the problem, like Bruce, I would say the fuel may not drain at an equal rate from each tank in cruise. But like Richard suggests, I don't believe I've experienced one tank remaining at full and the other at 1/2. I did have one gauge with a fuel-logged float which read low but that was consistent fault.
To recap if I understand things (big if) on the ground the fuel gauge indication was confirmed by removing the caps and looking inside the tanks. Was a dip rod used to gauge ( ) whether the gauges individually are reading correctly ?
The checking of the flow: how was that done? I'll guess maybe each source was checked to see the time required to fill a 5 gallon gas can.
There's no check valves in the system so you would think that the fuel would eventually balance its self out if the wings where level and also imbalance itself if parked on uneven ground with the fuel selector in both.

[n2582d]

That guy Gary is a swift one! Why didn't I think of that?

Blind-Squirrel-Finds-a-Nut-small.jpg (73.77 KiB) Viewed 23435 times

[n2582d]

I've replaced both of my fuel caps with with p/n C156003-0101 caps per SEB 92-27 .

If I'm not mistaken the old style vented caps allowed air to flow in and out. The umbrella valve on the bottom of the new style cap acts as a check valve, allowing air in but not out. (Edit: According to the '77-'86 172 Service Manual this “filler cap incorporates a vent and safety valve that provides both vacuum and positive pressure relief." This doesn't make sense because in a note on the same page it says "Check valve shall open at or before 4.0 inches of water vacuum pressure, and be able to withstand .5 PSI positive pressure without leakage." At over .5 PSI positive it should vent??) The umbrella valve is not available from Cessna as a separate part. Doing some sleuthing online I think I've found where they may be available though. I think one p/n for the valve may be B100145. That number lead me to miniValve's website. I'm guessing the umbrella valve might be their UM 145.003 SD. (EDIT: According to Service Letter SE 76-08 the umbrella p/n for flush caps is 4708.) So Jason, if you find that the tank imbalance follows the cap when they are swapped between the left and right tanks and you have this style cap it might be worth trying to replace these umbrella valves. For what it's worth here's Cessna's instructions for cleaning the cap:

Click to Enlarge

The other thing I was thinking -- grasping at straws perhaps -- is that it might be worth trying to clear fuel out of the overhead vent line in flight by doing a slight side slip with the tank low wing raised. The overhead vent line is, or should be, retained by two clamps which should also keep them nearly flat. (These NAS 437 clamps are not shown in the IPC.) At the most I would think you could have 2-3" difference in height if that line was installed incorrectly -- hardly enough to account for the difference you are seeing between your tanks.

Finally, I know you said your gooseneck vent was clear but to keep it that way I really liked George's idea:

Also, (some of you guys are gonna love this one...) I have an "undocumented" modification to my fuel system. I discovered one day (while fooling around with an empty Alcor TCP can) that the screen/strainer at the bottom of the pickup tube in that can is EXACTLY the correct size for the gooseneck fuel vent in a 170. The screen is "hat" shaped...just like a top-hat.... so it's body fits right down into the tube with a nice friction fit, ...but the "brim" prevents it from going on down into the tube. It's made of fine-mesh stainless steel screen. No bugs larger than a molecule of air will get down there. It's been at work on my airplane now for about 4 years.

[jmurtap]

Well Guys,
Since the conversation always comes back to the fuel caps, I went ahead and ordered two new vented caps. I really couldn't find anything wrong with my old ones. I'm cautiously optimistic at this point I installed the caps and flew her a half hour today, and the fuel system was acting normally again!! I've got to say the new ones are tight!!! It took some (a lot) effort to get them closed against the stops, I guess maybe mine were a little worn out.. lol

Thanks for all your input, I sincerely appreciate it. Hopefully I can get some good hours on her soon and know for sure, but things are looking good so far!!

[170C]

Hope that cures your problem. I don't know if there is a way to adjust the tension on those caps or not. Maybe other members will chime in. One of my caps is sometimes so tight I have to use a wrench to open it. A PVC "T" with slots cut in it makes opening or closing easy. Another forum member posted this solution several years ago.

[daedaluscan]

I have made a little wrench out of a length of 1" X 1/4" flat stock to allow my wife to get the fuel caps on and off. Lives with the fuel stick in the back of the seat pocket.

[blueldr]

I landed for fuel at Fallon, NV. some years back and a delightfuly happy lady of about sixty five years came out to fuel my airplane. She had a fuel cap wrench which was a short length of 3/4" PVC plastic pipe with a slot cut in one dide to to accomodate the ears on the fuel caps. A very slick tool, indeed.

P.S. Fallon had aeronautical charts at a very attractive price too.

[bagarre]

I have made a little wrench out of a length of 1" X 1/4" flat stock to allow my wife to get the fuel caps on and off. Lives with the fuel stick in the back of the seat pocket.

You get your wife to fuel the plane???
I am truly impressed. I'm lucky if Daniela fuels her car