I took my 1967 172H to the local FBO, complaining that the left tank feeds about 3 times as fast as the right. When I refueled yesterday, the right tank took 4.5 gal, the left over 11 gal. The A&P man tells me that is not uncommon with the 172, and if one tank reaches about 1/4 full the other starts feeding until they equal. ??? I don't want any 'funny' things to happen aloft with fuel feed! Have only had the bird for 5 months. Any experience or thoughts on the subject would be appreciated.
The August issue of Light Plane Maintenance has a good article explaining the 172 fuel venting system and the problems/quirks associated with the design.
I dropped you a personal email with a scanned copy that should help to explain the situation.
I have a 172K and experience the same difficulties. I had the tanks and vent checked out by my maintenance shop and they could find nothing wrong.
What I do is manually switch to the right tank if I am going any distance. I may do that for 30 or 60 minutes depending on how far I am going. Then I go back to both tanks.
Check the position of the tank vent under the left sde wing. It must be in just the right spot behind the edge of the strut. If it is bumped as little as ,1 inch it can become a ram air pressure source or a low pressure venturi depending on airflow around the strut. This can have a significant effect on tank balance by slightly changing pressure in the tank.. Six years ago my AP tweaked mine slightly and improved tank balance a lot. Unfortunately I can't remember which side was using faster or which way he moved it, sorry.
I have a 1974 C172M, I have flown it around 200hrs. and noticed that the left tank runs down a little faster than the right but has not been a problem even when total fuel load is as low as 8gals. (The tanks were fairly even at that point), I have never run it lower than that and nor would I want to.
My 2001 172SP does the same thing. The reason is the under tank vent is on the left side only. Several A&P told me the same thing. I did not believe it so made a set up for the tank feed using fast food drink cups and straws. I used two cups feeding into a common sump with a crossfeed straw across top. One cup had a straw vent out the top and the other had a closed lid. Guess what , the one with the straw vent drained alot faster for the first 1/3 cup! There are vents in both fuel caps and you should verify these work but the ram air into your under wing vent is alot more air.
Cessna probably has had this fuel design in the Skyhawks forever. How come cessna has not fixed this in their newer models. I own a 2001 172s for almost two years and I never had flew it far enough to burn enough fuel at one stretch to empty out one tank until recently, just last week. As soon as I turn downwind from a trip back to my home airport the Low Fuel light came ON. I am guilty of not being prepared with the procedure of turning fuel-tank selection valve.
I juggled my memory for the procedure and I came up with the following steps, a) turn on the electric pump, b) turn the valve towards the right tank c) turn the electric pump off. Is this correct?
What will happen if I leave the valve on BOTH? Will the system start to draw fuel from the right tank when the left tank is empty? Or will the engine stall?
For years I have pulled my hair out concerning Cessna fuel tank venting and fuel feed, after a few long talks with Cessna Reps that is just the way it is. The biggest problem I hear is lateral imbalance, most of my customers just floy 30 minutes per selected tank (like a Piper).
KHAN: If the selector is on both the engine will run until both tanks are empty, providing the vents are working properly. This system is a gravity feed system and as long as the selector is on "BOTH" the fuel lines have a path to the engine, whether it be carbureted or fuel injected.
I have being flying a 1980 182 Q, and have experienced some trouble with the left tank. When the fuel selector is on both the right tank empty faster than the left about 2/3, and when I switch the selector to left the right tank keeps getting lower faster than the left, even though fuel flow is ok, the left tank doesn’t seem to be feeding at any time as it should. I have taken the airplane to the shop twice and they have checked everything they thought could be the problem (fuel selector, lines, vents, caps). One more thing to noticed is that when the tanks are full the right vent leaks some fuel but the left vent doesn’t leak any. Any help would be appreciated.
If I cruise on 'Both' tanks, my '76 182P (only one fuel vent - left wing) will feed down to less than 15 gallons (from 40 gallons) with almost NO decrease in the right tank. The pressure from the vent pushes the fuel across to the right wing - that one stays full!!
So, I use: Cruise checklist: Adjust Power, Close cowl flaps, Fuel Selector - Right Tank. And I leave it there until pre-landing - then selector to both per the checklist.
I get nearly even usage that way - until the left tank drops down to about 15 gallons, and then the right tank drains down.
Cessna vent system - love it or leave it?
Edited 1 time(s). Last edit at 06/17/2008 02:47PM by MiltonBill.
