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Discussion Starter · #1 ·
A while ago I posted about my engine idling rough and having throttle hesitation, but when at speed all is fine. Well, I poked around, didn't find anything obvious, and after a while the problem went away. Since then the proablem has reoccurred a couple times then gone away. Yesterday right after I filled her up the problem came back. This morning the tank is down about 3/4 and all is well. I now realize that each time this happens, it's right after I fill the tank, and by about 3/4 tank the problem goes away.

Ideas??????

By the way, it's a 302 with Holly1650, Tri-Y's, C4. Thanks
 

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There may be dirt or water in the tank. When you fill it up it stirs up everthing in the tank. By the time you get to 3/4 things have settled back down. Just a thought.

Scott
 

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Throwing darts here, but when's the last time you changed the fuel filter? Maybe you're stirring stuff up in the tank when you fill up, and the filter is pretty close to clogged anyway.
 

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There may be dirt or water in the tank. When you fill it up it stirs up everthing in the tank. By the time you get to 3/4 things have settled back down. Just a thought.
D'oh! Beat me to it.
 

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Did you give your stang mexicain food before it got that gas? a couple of rolaids and beanos might do the trick!
 

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Have you checked your vent in the cap?
 

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I like the other two posts, fuel contamination would be my first suspicion as well. Water in the tank can get stirred up during a fill up and pass right through the filter. At idle it's affecting the engine, but at speed, it burns right through the system (evaporates).

That eliminated, then this might be a possiblity if I correctly understand the older fuel systems (probably where jims6t6 was going with his post):

I am assuming the old systems aren't supposed to create a vacuum like the newer fuel systems. The older fuel systems were allowed to "vent".

What might be happening is that your full tank doesn't have much "head room" (no air in the tank) that can expand as fuel is sucked out, and you cap may not be letting air into the tank to replace the removed fuel, causing a vaccuum and your fuel pump has to work overtime until there's some air in the tank that can allow for a "buffer" until your cap vents enough air into the tank to replace the removed fuel.

Next time it acts up, try popping your cap to see if the problem goes away.
 

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I was going to suggest checking the vent also. Junk in your tank can also clog things up a bit, as pointed out.
 

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as modifiedmustang stated

That eliminated, then this might be a possiblity if I correctly understand the older fuel systems (probably where jims6t6 was going with his post):

I am assuming the old systems aren't supposed to create a vacuum like the newer fuel systems. The older fuel systems were allowed to "vent".

What might be happening is that your full tank doesn't have much "head room" (no air in the tank) that can expand as fuel is sucked out, and you cap may not be letting air into the tank to replace the removed fuel, causing a vaccuum and your fuel pump has to work overtime until there's some air in the tank that can allow for a "buffer" until your cap vents enough air into the tank to replace the removed fuel.
Exactly right. The older systems would actually stop if the cap had a good seal and the vent hole was plugged up.
 

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Discussion Starter · #10 ·
Two trains of thought going here: crud/water in the fuel and cap venting:

CRUD/WATER: I replaced the fuel filter when this first occurred, so I know thats OK. Does filling the tank really stir this up more than hard driving? The only way I can see for water to be the issue is if the gas I'm buying has water, but it's the same gas I've always bought (usually Arco premium) and this problem is new.

VENTING: I didn't know the cap was supposed to vent. However, everytime I remove the cap there's no hissing (like in a newer car) so I assume it's venting OK. Also, if this were a problem I would suspect all would be fine right after a fill-up when the cap was just opened and the displaced air volume is less, then worse as the tank empties. Am I off on this?

Thanks again for the suggestions. Keep 'em coming!
 

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Do you always get gas from the same place? Maybe it is related to the gas you're buying that has high water content as others have mentioned. Water is heavier than gas and will sink to the bottom.

Maybe try buying gas from another place and see if the symptoms change?
 

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I worked at a gas station many many years ago. At that time, there was a certain amount of water that was allowed to accumulate in the holding tanks. As 65Topless pointed out, water will settle on the bottom. Water becomes a problem when it is stirred up by filling the tank or when the tank is close to running dry.

Scott
 

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The water in the gas theory doesn't sound plausible. Either it's the cap not venting, or it's coincidence that you notice it happening when the tank is full. Still easy to check. Just pop the cap the next time it starts and see if it doesn't help. Thie isn't just all theory. I've seen this air-lock condition several times. It's a good "joke" to play on someone you're not too fond of.

You had asked about why it would have this effect when full, rather than when empty. When the tank is full of fluid, there is no air inside to buffer the vacuum being pulled. Take a straw, place it into a glass of water half the length of the straw, then put your finger over it. With the straw only half full, if you shake the straw up and down, you can make the fluid level inside it move. This is due to the air in the top half. The more you fill the fluid toward the top of the straw, the less the fluid will move when you shake it. When it's full, nothing can give since no air is present. This is what happens inside the fuel tank when a vented system can no longer vent. (Just call me Mr. Wizard, wasn't that fun?)

The reason you don't hear a big "whoosh" when you remove the cap is due to very low vacuum pressure being required to fluid lock the tank. Your tank is on a slight vacuum with the vent plugged on the cap. The sound you hear from your fuel injected car is a positive pressure escaping, so don't expect them to sound alike.

I'm just disappointed that nobody has blamed the solenoid or coil. How can there be a problem exist that isn't somehow connected to one of these two components? :)
 

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Not sure if this is any help, but how old isyour fuel tank?

I had a '70 chebby pickup with a similar problem. It took me a while to figure out that the gas tank had set for a while about half full of fuel. Condensation formed in the top half of the tank, causing rust. It didn't cause any problems when i had less than a half tank of gas, but when I filled it, the gas loosened up the rust, pumping it into the fuel lines. If your filter is clogged, this might be the source.

Just a thought.
 

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Hello,
To me it seems more likely that vacuum in the gas tank will sooner cause a problem while driveing than idling shouldn't it??
This is a long shot and should not really make a difference when using a fuel pump but have you checked your float level in your carb? Maybe your float seals bad and will cause you slight flooding in your carb when your tank is full due to slightly more pressure with a full tank.

Hope this helps
Good Luck!
 
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