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Discussion Starter · #1 ·
I can't seem to get this straight. Upon removal of my 1500 gas cap, there is a pronounced "hissss". Some say it's pressure releasing and some have told me it's "air going in" due to a partial vacuum (like opening a new can of coffee).
Anyone familiar with this? Is it supposed to be doing this? If not, what's the cure (other than a new $65 gas cap)? Thank you!
 

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It aint rocket science
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The fuel tank is designed to hold a pressure and so is the cap. Many mistake the hissing sound as air rushing into the tank from a vacuum being formed within.

Next time you fill up unscrew the cap and while holding downward pressure slowly release your down force, the hiss will occur as pressurized fuel vapors are released.

If there was a vacuum in the tank when cap is unscrewed you would physically have to pull the cap up to break the seal.:)
 

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Discussion Starter · #3 ·
The fuel tank is designed to hold a pressure and so is the cap. Many mistake the hissing sound as air rushing into the tank from a vacuum being formed within.

Next time you fill up unscrew the cap and while holding downward pressure slowly release your down force, the hiss will occur as pressurized fuel vapors are released.

If there was a vacuum in the tank when cap is unscrewed you would physically have to pull the cap up to break the seal.:)
Thank you, that makes sense... So if there were some sort of gas cap problem at all, it would be if no hissing were heard. Yes? I'm trying to figure out a possible fuel flow problem, and will now look towards the fuel filter. Thanks, again
 

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It must let air in as well otherwise you would have a vacuum and very little fuel flow or a crushed tank.
 

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It's normal.
 

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It aint rocket science
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So if there were some sort of gas cap problem at all, it would be if no hissing were heard. Yes? I'm trying to figure out a possible fuel flow problem, and will now look towards the fuel filter. Thanks, again
Air must be allowed to enter the tank via the cap when for instance fuel level drops, but should not normally vent fumes to atmosphere. EPA gets mad when that happens.

What is the problem you are experiencing?
 

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Yeah, the horrors of having a directly vented gas tank, why all the spotted owls, California Condors and those little smelt would immediately roll over and die. So without a direct vent, there will always be a difference in atmospheric pressure in the tank vs. air outside.
 

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Discussion Starter · #8 ·
...What is the problem you are experiencing?
Recently, for the 1st time ever, while on "cruise" in the 55+ speed range, the engine asks like it's running out of gas. Pull in the clutch (kill the cruise) and let the engine idle a few seconds (while still rolling along) and the engine resumes running normally. It's as if the gas had a chance to "refill" whatever was running dry. Then after a few minutes (+ or -), it does it again. Running slower (requiring less gas flow), everything seems OK.

I was trying to start with basics asking about the gas cap, but will now move on the changing the fuel filter. As far as I know, it's the original with 48k miles.

Thank you to everyone for your help. If anyone else has an additional idea, let me know... It has to be something easy (like the fuel filter).
 

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X-Roughneck
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That sounds exactly like a filter plugging up.
Aslo make sure your choke is off.That can effect the way criuse control adjusts.

Stick a straw in your favorite drink,put your thumb iver the end of it,and try to suck the drink out of the other end.



BUT,when the engine is shut-off,and the drawing of fuels stops,and the heat builds up in the tank,and THEN you take the gas cap off,You might hear Pressure.Sometimes,on a cooler day,you might NOT hear any pressure.Even the amount of vaccum is controlled.A certian amout of vacuum is necessary to maintain a more steady pressure flow through the pump.That would become a varible if relying on Pressure,altough SOME cars DO rely on pressure vs vacuum.
 

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When you get the old fuel filter off take a breath and blow a little air through it the same direction as fuel flow. If you meet resistance with your breath through it, it is plugged. You will have your new filter alongside for comparison.

Next and most likely item after that is the auto fuel valve.
 

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Hey,

Also check the vacuum line that is attached to your petcock. Check it all the way down to the engine – from one end to the other. Any cracks or nicks can cause a vacuum leak and cause the petcock to work improperly… resulting in restricted gas flow.

Tim.
 

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Before you change the fuel filter try riding with the gas cap loose.
 

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I don't often find myself on the other end of the stick with Paul, but if you don't know when regular maintenance items were done change it now noting date and mileage.
 

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The purpose of the suggestion was to try to isolate the problem. It's more common for fuel tank vents to fail than fuel filters to plug up. I try to eliminate the easy to check possibilities before digging in deeper. No harm in changing the filter of course but I've seen some high mileage 'Wings still using their original filters with no problem. My '93 was still running well with it's original filter at 170,000 or so miles on the odometer.
 

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Discussion Starter · #15 ·
Mystery Solved

Big THANK YOU's go out to all who made suggestions. Removed "old" fuel filter and tried to blow through it while it was still "wet". I don't understand how ANY gas got through this thing - it was soooo pluged. Unbelievable! I obtained one of those "humungo" NAPA type filters recommended here and elsewhere, installed it, then went for a 90 mile "ride around the block" (including Interstate Hwys on "cruise control"). No signs of "running out of gas".

Everything is now A-OK and back to normal. Thanks, again for all the suggestions. :claps:
 
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