The International Cessna^® 170 Association

After several months of scratching our heads, I must ask for help.. (I must state no previous issue's to date with in almost a year of owning) In October on an 1 hour flight I just happen to notice the right tank fuel gauge was pegged fuel and the left was half empty. After landing at my destination, I immediately verified the fuel gauges were correct, and verified the fuel selector was draining fuel (from gascolator) off both the left, right, and both positions, and not in the off position. I figured the best coarse of action was to go ahead and eat lunch and let it cross feed back. After returning to lunch, the same situation was found. Right tank was full and left was about half??? After a call to my IA we decided to fly it back home and resolve the problem at home. Also during this time I had both caps off and after the phone call ended both tanks were equal, after a third check. The return flight home the fuel system acted normally and drained from both tanks as equally as I could tell. My IA and myself could not find any clogged vents on both caps or the main, and again checked fuel flow against the fuel selector valve, all positions were acted normal. Someone suggested replacing the fuel cap gaskets which I immediately did. Next flight with half tanks presented the same problem. Put the old gaskets back on, and the problem seemed to be fixed, for a couple of flights. My last flight I noticed the same issue, I took off with half tanks for a currency flight, and after 2nd trip around the pattern the right tank was full, and the left was at about a 1/4??? Myself and my IA are at a loss, we have verified all vents with compressed air. No apparent fuel leaks, fuel selector is working properly, rubber vent lines were replaced with-in the year. No apparent fuel spilling out the caps. Any Ideas??

Is the ball centered and A/H straight and level ?

Jason, you have described normal operations for most 170s. Seems like the only oddity here is it took you a year of ownership to see it. You've done due diligence looking to be sure the system has no restrictions and there is flow from both tanks.

Bottom line, it's normal.

Yes Ed, in level flight with the ball centered. Bruce it has certainly always drained a little unevenly. But never to this degree.

jmurtap wrote:Yes Ed, in level flight with the ball centered. Bruce it has certainly always drained a little unevenly. But never to this degree.

And you checked out the things you needed to insure no new issues other than your new experience. It's normal.

jmurtap wrote:Yes Ed, in level flight with the ball centered. Bruce it has certainly always drained a little unevenly. But never to this degree.

Usually running on both, my tanks hardly ever drain evenly. I sometimes run on just one tank to even things out....just have to remember to switch. Someone told me that it really gets your attention when the engine starts sputtering . I also remember you have the Javalin aux tank and I believe the plumbing for this goes to the right tank...just wonder if there is a little anomaly in that area...just a thought.

The aux tank does tie into the right side, it has a check valve and a tee. I have removed the tank and capped the line. Hmm.. Well I'll just keep flying her then, thanks guys!

I regularly see slight fuel level differences between tanks but I have never seen half-tanks fill one tank up full and drain the other to 1/4. That is not anything I would consider normal, but, as long as both tanks will drain completely in the end, it really does not matter.

It sounds to me like somehow the left tank is pressured up more than the right tank and is pushing gas from the left to the right. Do you have the center roof vent or the late style vent behind the left lift strut?

It has the center vent, both fuel caps are vented. Early last year we replaced the rubber hose's on the vent line. It is very unusual, with my previous experience with this AC. The right tank gets so full the gauge doesn't even bounce around. A strange sight.

OK, since you have confirmed the center vent is open to both tanks, they should be getting equal pressure while flying. Are your gas caps venting both ways or just vent into the tank to break the vacuum created by using fuel out of the tank? You need to check each one.

It now sounds like the left tank is not venting-out but the right tank is. The center vent will try to pressurize both tanks equally, but if the right tank is venting-out, gasoline will be pressured into the right tank from the left tank.

There could be another reason the pressure in the left tank stays higher than the right, especially considering you saw the equalized half-tanks turn into a full right tank in a couple of circuits of the pattern.

I think this is a differential pressure problem that appears to be intermittent since sometimes the tanks drain normally. I would suspect a bad cap gasket seal or bad vent seal of some sort on the right tank. Not necessarily the gasket itself leaking but it still could be. Has the neck of the right tank received any damage which would cause the gasket to not seal properly? It might not take much of a leak to make a big difference. Switch sides with the gas caps and see what happens.

Keep us informed.

My tanks draw unevenly too, and the blue stains on the wing suggest it's a cap venting problem. The vent seals don't seem nearly tight enough to prevent outflow. How tight should they be?

Jason, here is a fun experiment. Try flying the plane sitting in the opposite seat and see if the burn out of each tank reverses. Now that would be the true Stump the chump experiment!...lol

hilltop170 wrote:OK, since you have confirmed the center vent is open to both tanks, they should be getting equal pressure while flying. Are your gas caps venting both ways or just vent into the tank to break the vacuum created by using fuel out of the tank? You need to check each one.

It now sounds like the left tank is not venting-out but the right tank is. The center vent will try to pressurize both tanks equally, but if the right tank is venting-out, gasoline will be pressured into the right tank from the left tank.

There could be another reason the pressure in the left tank stays higher than the right, especially considering you saw the equalized half-tanks turn into a full right tank in a couple of circuits of the pattern.

I think this is a differential pressure problem that appears to be intermittent since sometimes the tanks drain normally. I would suspect a bad cap gasket seal or bad vent seal of some sort on the right tank. Not necessarily the gasket itself leaking but it still could be. Has the neck of the right tank received any damage which would cause the gasket to not seal properly? It might not take much of a leak to make a big difference. Switch sides with the gas caps and see what happens.

Keep us informed.

They are the vent in types, I have confirmed they do indeed vent in, I'm not sure how well they stop the outflow. The only time I see gas on top of the wing is when I spilled it. Both tank necks look great and don't leak. I will try your suggestions asap. Thank you for them!!

It could be possible that the vent hoses are getting old and when installed, the inside rubber was cut and now at times the flap restricts the air flow. It is tough to check the vent hoses both ways with pressure but possible to slightly pressurize the tank to check back flow.

John

John, they said they replaced the vent hoses in early 2014 so they should still be good unless during the install they left a piece of the hose loose, but still connected in one of the hoses.