Gas Tank Vacuum

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BkrDude001

BkrDude001

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Went to go riding around Saturday. Started my '73 Duster 340, it ran about a minute and run out of gas. I know there was about a quarter tank, but the gas gauge doesn't work right so I went and got some gas. When I took the gas cap off, there was a vacuum. Once the gas got to the carburetor it started right up. I drove it around for a while before stopping to fill up. There was no vacuum this time. Saturday was the first time I noticed this. Is there supposed to be a vacuum or is there a vent I need to find?
 
BkrDude001 said:
Went to go riding around Saturday. Started my '73 Duster 340, it ran about a minute and run out of gas. I know there was about a quarter tank, but the gas gauge doesn't work right so I went and got some gas. When I took the gas cap off, there was a vacuum. Once the gas got to the carburetor it started right up. I drove it around for a while before stopping to fill up. There was no vacuum this time. Saturday was the first time I noticed this. Is there supposed to be a vacuum or is there a vent I need to find?
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Wander yourself over to MyMopar.com and download the 73 manuals, free. There are guys right here who GOT some of those manuals posted there. Read the section on fuel, tanks, and emissions. I think we are talking about chapter 9? and 20 something, way in the back. There's a 1/4 tube that comes from the tank up to the engine bay that ties in with the carbon can evap system, and whether that system is intact or not that tube MUST be high up and open so to vent the tank. If it has somehow been eliminated, you could also modify the filler neck to imitate the 69/ earlier vent, which is simple.

The evap controlled tanks, beginning with some 70 models in CA, has had SEVERAL different filler caps but NONE OF THEM are truely vented. They have (or had) what was called "pressure vacuum" caps, meaning, that they had relief valves both for pressure and vacuum to vent extremes, but they were NOT meant as "vented" for normal driving.
 
73 has a vent in the tank above the sending unit. There is a line that goes to the front of the car to the charcoal canister under the hood. Also this car should have a vented cap. That is the first thing I would check.
 
My Duster doesn't have any of that emissions crap on it. I'll look at MyMopar.com and see what I can find.
 
I think what 67Dart273 was getting at was you have possibly the wrong cap for your car. If fuel goes out air has to replace it or you get vac and eventually it runs out of fuel or the tank collapses- something has to give.
 
BkrDude001 said:
My Duster doesn't have any of that emissions crap on it. I'll look at MyMopar.com and see what I can find.
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If they removed the canister did the plug the line coming from the tank. 71's have a nipple on the breather cap you could use for that line.
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BkrDude001 said:
My Duster doesn't have any of that emissions crap on it. I'll look at MyMopar.com and see what I can find.
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It had it originally. There were no 73's made that did not have evap, that I ever heard of. I don't know about, say, Canada or somewhere
 
BkrDude001 said:
Went to go riding around Saturday. Started my '73 Duster 340, it ran about a minute and run out of gas. I know there was about a quarter tank, but the gas gauge doesn't work right so I went and got some gas. When I took the gas cap off, there was a vacuum. Once the gas got to the carburetor it started right up. I drove it around for a while before stopping to fill up. There was no vacuum this time. Saturday was the first time I noticed this. Is there supposed to be a vacuum or is there a vent I need to find?
Click to expand...

For a stock, unmodified system, the 1973 A-bodies employed both a fuel tank vent line and a pressure/vacuum fuel filler cap.

A single vent tube at the upper center front of the tank connects via a rubber hose to a metal tube that runs to the front right corner of the engine compartment. An overfill limiting check valve is formed within the end of the tube there. This end of the tube connects to the charcoal canister as part of the evaporative emission capture system.

The vent tube is the primary vent for the system. However, the fuel cap contains a pressure/vacuum vent in the event of significant over pressure or vacuum conditions.

If the vent tube is blocked and the fuel cap vent valve fails or incorrect cap is used, high pressure or vacuum can be held in the tank. If the vent fails, but the cap is good, mild pressure or vacuum may be present.

For failure to vent in a stock 1973 A-body system, check the following:
  • Sticking or blocked overfill limit check valve
  • Canister hose to metal tube for blockage or collapse
  • Blocked canister
  • Vent tube connector hose at the tank for blockage or collapse
  • Metal vent line itself blockage due to clogging or damage
  • Internal tank vent tube blockage
  • Fuel cap pressure/vacuum valve. This should still relief vent if the canister/tube/valve is blocked and pressure or vacuum gets high enough unless it is also bad.
The check valve end on the metal tube at the charcoal canister can be replaced. Original part is # 3642263. See the factory service manuals for replacement procedure.

Original 1973 pressure/vacuum cap on my Duster:
PXL_20250305_163058304.jpg


Overfill limit check valve end of vent tube:
PXL_20250305_163509054.jpg


Charcoal canister vent tube connection:
PXL_20250305_163527130.jpg
 
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Thanks everybody that responded. It's raining today. But I'll get out tomorrow and see what I can find. At least I now have someplaces to look.
 
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