The Cessna 120-140 Association Forum

Hi,
I am new to the group and new to 140 ownership...only a couple of weeks.
I completed an annual/prebuy before making the plunge.

First fillup I noticed that one tank held pressure the other did not. Called the IA and he said it can happen.
Second fillup same tank holding pressure. IA looked at it and said that this style (Cessna 150 style) of cap has a one-way valve and is working as it should the other much be broken.

Does this align with the collective knowledge of the group?

I would prefer to have both work the same, either hold pressure or not depending of the feedback I get.

Thanks in advance,

Rick

I believe you will need to post the serial number of your airplane, or at least state if your selector is a LEFT RIGHT or a LEFT RIGHT BOTH to be able to get the advice you seek.

There is a writeup on fuel systems here and fuel caps here in the Neal Wright Archives. Do not fly until you have sorted things out. Neither tank should be capable of holding pressure.

Thanks for the replies.

Serial # 14370
Both times I believe I had the fuel selector on off.

No, the reason I mentioned the fuel selector is that if it is a LEFT RIGHT BOTH system, then it has an interconnected vent, where as the LEFT RIGHT system does not. I believe the article John has pointed you to will show different fuel caps depending on which system you have.

Very interesting read John.

Seems I have half-vented caps with the silicone valve stuck on one side. I will switch the caps later on today to see if I can recreate the problem on the other side and then invest in a pair of Monarch caps or replace the failed half-vented Cessna cap.

Which fuel system do you have? It's been a while since I read up on the subject, but, IIRC, the Monarch caps are not without their own set of problems.

If one tank is pressurized and the other not then you can't have a vent line between the two tanks, and, as far as I know, you should have the plain caps with two vent holes.

Can you simply pull out the silicone bit?

I have attached what I believe is my fuel system, I will look more closely when I get back to the Hangar.
I have also attached pictures of my fuel cap that I found online.

I am not opposed to pulling out the valve but i don't yet feel like I understand what is happening.

I'm confused by your use of the word "pressurized" Do you mean positive pressure or negative? Positive would mean expansion due to heat, etc. Negative would mean you are trying to suck fuel out and no air getting in, which as John has said if thats the case stop flying for sure.

John, his serial number, is that an a/c with LEFT RIGHT BOTH? I do know that my A model requires the silicone one way vent as the vent tube on the cabin roof provides positive pressure to the tanks, so it would vent fuel out the caps if you took the silicone valve out.

First, if you have the vent interconnect, as shown in the diagram, and (assuming) it is not plugged, then it would be impossible for either tank to pressurize, positive or negative, without the other one doing the same thing.

I think we need to start back at the beginning. If you do have the interconnect, and both tanks were holding pressure, opening either one would release the pressure. This might lead you to believe the other one is not holding pressure. Is this the case? Are we dealing with positive pressure or a vacuum?

Remove the wing root fairing and look for the interconnect line. Do you have the "Both" setting on the valve? (If you do, "Off" will actually provide another interconnect between the tanks, forcing them to equalize.) If you do not, then you probably don't have the interconnect line, either, and both caps must be vented.

The 140a system should normally vent through the tube in the center of the cabin, so, even if both caps failed to vent there should be no pressure.

And, yes, a vacuum in the tanks will ruin your whole day.