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Discussion Starter #1
Last fueling air wooshed from the tank upon removing the cap. A check later and no pressure release. Which is the correct condition? I'm guessing no pressure. What could cause pressure in the tank? Thanks.
 

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1966-2013
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13,741 Posts
No whoosh is what you're after.

There's several threads on that, and it's relative to the vent and valve up in the front part of your trunk behind the flap where that plastic box is.

Hang on, I'll see if I can dig one up quick-like.
 

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1966-2013
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Ahh, here we go.

Double check the routing of your hoses if there's any doubt, and if correct then items #2 and #3 are likely where your issue lies.

#2 should allow air to pass from the outside world into the system, while #3 is a light spring loaded check valve that prevents reversion of fuel in the event of a tipover. The arrow embossed on the side of it should be pointing in the direction of the hose that heads up into the engine compartment, not the plastic tank in the trunk.

Blowing through it should result in it only passing air in the direction of the arrow and not the other direction. (if it won't pass air at all or seems sticky, clean it with carb cleaner)

The way the two valves operate, the fuel tank should always remain at atmospheric pressure with no pressurization or vacuum buildup.

 

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the oil vapor seperator can plug up and give you this. pull it off and clean it with carb cleaner or even gasoline. number 14 on above diagram.
 

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But Mad North-Northwest
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It's normal for there to be *some* whoosh, especially if the tank is near empty. The check valve requires a bit of pressure to open (not much...less than 1 PSI) but since there can be a lot of volume in the tank when it's empty this can seem like a lot of air.

It's never a bad idea to clean the check valves and check flow through the vent system valves and hoses, though.

If in doubt, you can pump air into the open end of the check valve #2 and make sure it escapes through the check valve #3. If the tank pressurizes significantly when you do this then you've got flow problems. I think there's a procedure and pressure specs in the service manual.
 

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I think I do have a problem-

It is quite a lot of pressure that builds up- It affects fuel flow(I believe) when I have less than half a tank. I also live at elevation and routinely drive it between 5000 and 8500 feet. I believe heat also exacerbates the problem. So- there is procedure outlned in the service manual? I will check it out when I get back to my car in a week. Thanks!
 

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But Mad North-Northwest
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If it's affecting fuel flow you may have an issue, yes.

The procedure is on 4-15 of the manual. Looks like the opening spec for valve 3 is 0.36 PSI, which means that's the most pressure the tank should see.
 

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1966-2013
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It affects fuel flow(I believe) when I have less than half a tank
I'd think that would be due more to a faulty connector hose between the in tank pump and the sending unit. Or at least that's about the only thing that directly affects fuel flow at specifically 1/2 a tank or less.


To confirm if there's pressure or vacuum problem in the tank, it doesn't take much more than driving around for a while with the filler cap off.

If it works ok, then chances are fair you've got one of those problems and can chase from there. If it acts the same way, then it's something else.
 

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Whoosh

I've got a really big whoosh when I undo the fuel tank cap. I'm guessing one of the check valves on the vapor thingo are blocked.
Does anyone either know the part number or how to clean these guys.
Thanks
 

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1966-2013
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13,741 Posts
The metal one-way valve can be pulled and soaked/flushed with carb cleaner until it functions.
IIRC, the plastic one can be prised apart and cleaned out that way, or some gentle flushing with kerosene or gasoline might do the trick. (based on its construction, I dunno if it would take something as caustic as carb cleaner, but certainly it should stand up to fuel)

Make sure they both are installed in correct order and orientation, otherwise it'll all be for naught.
 

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1966-2013
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The metal one has an arrow embossed/stamped onto it.
That points the way along the hose that goes to the engine.

The plastic one goes on the gas tank side of the metal one.

Net result is overpressure passes the plastic and goes to the engine via the metal while vacuum pulls through the plastic back to the tank.
 

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But Mad North-Northwest
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Note that if it's a '91 you do not have a metal check valve, you have two plastic ones.

The valve that is not open to atmosphere is designed to hold a little positive pressure in the tank. So it's normal to sometimes get some air out of the tank when you open it, especially if fuel is low (a little pressure over a big volume can produce a reasonable amount of flow).
 
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