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Gas cap
#1148952 Mon Feb 08 2016 02:48 AM
Joined: Jan 2010
Posts: 4,610
D
DADS50 Offline OP
Shop Shark
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?

Re: Gas cap
DADS50 #1148972 Mon Feb 08 2016 04:40 AM
Joined: Jan 2013
Posts: 1,426
J
Shop Shark
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.

Re: Gas cap
DADS50 #1149022 Mon Feb 08 2016 12:19 PM
Joined: Jan 2010
Posts: 4,610
D
DADS50 Offline OP
Shop Shark
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.





Re: Gas cap
DADS50 #1149050 Mon Feb 08 2016 04:07 PM
Joined: Dec 2013
Posts: 889
Shop Shark
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.


Andy

His: 1947 Chevrolet 3104 [flic.kr]
Hers: 2008 American Saddlebred [flic.kr]

"I proudly Stand for the Flag and Kneel for the Cross" Unknown
Re: Gas cap
DADS50 #1149051 Mon Feb 08 2016 04:13 PM
Joined: Jan 2013
Posts: 1,426
J
Shop Shark
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.

Re: Gas cap
DADS50 #1149147 Tue Feb 09 2016 12:33 AM
Joined: Mar 2010
Posts: 9,449
5
Master Gabster
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

Last edited by 52Carl; Wed Feb 10 2016 03:28 AM.
Re: Gas cap
DADS50 #1149199 Tue Feb 09 2016 03:26 AM
Joined: Jan 2010
Posts: 4,610
D
DADS50 Offline OP
Shop Shark
Dont think I want it to vent out.
Already had one incident where it self siphoned at work. Little dribble out the filler neck.



Re: Gas cap
DADS50 #1149284 Tue Feb 09 2016 05:09 PM
Joined: Jan 2013
Posts: 1,426
J
Shop Shark
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.

Re: Gas cap
DADS50 #1149286 Tue Feb 09 2016 05:20 PM
Joined: May 2005
Posts: 9,146
G
.
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.


1951 GMC 250 in the Project Journals [stovebolt.com]
1948 Chevrolet 6400 [stovebolt.com] - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
1952 Chevrolet 3800 pickup [stovebolt.com]
---All pictures [picasaweb.google.com]---
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Re: Gas cap
DADS50 #1149311 Tue Feb 09 2016 09:14 PM
Joined: Jan 2010
Posts: 4,610
D
DADS50 Offline OP
Shop Shark
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

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