The Stovebolt Site Forums The Shop Area Engine Shop Gas cap

Last year my new spectra fuel tank developed a leak at the solder joint at the filler neck.

Took the tank to a shop and had it repaired

Since the repair
Ive noticed a problem with what I think is the tank not venting while driving
Ive been using this brand of gas cap ...pic...

I remove the cap after a drive and can hear the air rushing in



Ive tried 3 gas caps (same brand) with same results


Is there a better cap out there?

I have noticed that all the "vented" gas caps I've seen lately are actually 1 way vented to let nothing out and a strangely strong spring on a one way valve for air to come in. Environmental protection.

I dis-assemble them and take the spring out.It is then vented both ways with good control against splash out.

I think my issue is that air is not being let in properly.

After about 20 minutes of driving I can hear the tank thump. I release the cap When I get to my destination.

I had the same experience last summer. The truck would actually start running very rough and wouldn't idle. When I removed the gas cap, I could hear that air rush in. I've also taken the same approach as Jim. Without the spring on the 'vented' gas cap the tank is always vented to the atmosphere and that solved the problem.

exactly what that overly strong spring will cause.
Since they are making them so nothing can escape, they made a monster.
If the spring was very weak (1/10 psi check valve)a full tank in a cold condition would suck in air as the gas contracted. Then in the full sun the gas would expand and create a lot of pressure (probably has a warning sticker that says be careful of pressure when opening)so the strong spring doesn't let the air in until a substantial vacuum is created. The whole design is dumb.
Original vented caps had a tiny vent hole so the tank could breathe without any effort.

Mine sucks air when I take the cap off. (classicparts.com). No issues with the ancient glass top fuel filter supplying fuel, but I know what to do should I ever have a fuel delivery problem. I am hesitant to do anything yet because I don't want to risk gas gushing out with a full tank on a hot day because a jimmied up the cap vent.
Carl

Dont think I want it to vent out.
Already had one incident where it self siphoned at work. Little dribble out the filler neck.

These trucks, and millions of others, have been running with vented gas caps (little hole in them) for 60 years. The idea of a valve and spring cap is new (no gas fumes in the air) idea.
Gas doesn't "self siphone", the tank was over filled.
Gasoline in underground tanks is 40ish degrees. As soon as it gets pumped into your gas tank it starts to warm up to the air temp,70,90,...
Liquid expands as it is heated. That's why you don't fill the gas tank all the way up into the filler neck. It needs room to expand.
With an unvented tank it can build a lot of hydraulic pressure in the fuel system.

I agree with Jim, the tank has to be vented. The original method was through the cap.
If the caps today are not freely venting they need to be modified. Or find a 60 year old NOS cap if you don't like the idea of modifying one.

The tank was not overfilled
Parked level



It was probably just over half full, and noticed the leak at the end of work.
I found the rubber seal cracked back when this happened a few years ago.


I'll look for nos

What is the small tube that comes off the filler neck?

Are these filler parts used with your fuel tank?

...yes...

nice photo

Do both metal tubes which are attached to the top of the tank end at the top of the tank? Or, does one or the other metal tubes extend into the tank?

They both end at the tank

It's the fill vent pipe...when filling up, as gas goes in the big pipe air comes out the small which helps eliminate gas burpping back.

Mike B

MIke,

How would gas siphon out of that tank?

In my mind that can't happen (expansion overflow, yes)...someone a lot smarter than me would need to explain that one.

Mike B

It can't siphon out of the filler neck, the filler neck is the highest point.
Gas sure can expand and then overflow through the vented cap. I've had it happen on several occasions with a full tank or mostly full but parked sideways on a hill. I learned not to park filler neck downhill or fill the tank very fill on a hot day unless I'd be driving right away to reduce the fuel level enough to not worry about it getting warm and burping.

Gas tanks always have an air space for expansion, or they are not supposed to be filled plumb full, leaving some % for expansion.

I do not fill my fuel tank (especially in the morning), unless I am going for at least a 15 minute drive.

The temperature rise of the gasoline (in the ground-tank vs in the truck tank) might result in a 1-2% expansion of the gasoline, depending on the outside temperature. That might result in a full-tank expansion (15-18 gallons) of 0.15 to maybe 0.5 gallons (depending on temperature rise). The overflow would be a mess, but an original-design old-truck tank would not have the gas siphon out.

Great conversation. I am not sure then why have a spring on the gas cap vent. Seems a few small holes would keep things from getting out of hand?

The spring is more than likely there to protect against roll over leaks and liability issues for the manufacture.

Mike B

I think that it is the fumes which "siphon" out of the tank when the tank interior becomes hotter than the outside air. After the fumes make it outside to where the air is cooler than what the interior of the tank is, the fumes then roll down the filler neck and onto cab.
Carl