Hey y'all,

Do gas caps go bad? My car started running really weak yesterday like it lost it's "umph" after a little bit of driving when it was 100degrees outside. When I stopped at a gas station to fill up, there was hella pressure in the tank, way more than ever! Now after this, it was fine as I drove home. Today the cap is loose for now just in case. Does this all sound legit? Can gas caps cause this?

Bugs

 

hmmmm.. weird, I have had the same thing happen once in a while, good question. Anyone have any input on this?

 

Gas caps are designed to hold pressre, for emissions reasons. In fact, alot of newer cars will set a code if there isn't enough pressure built up in the tank.

I'd be more inclined to attribute the difference in driveability to the new gas you got.

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"So many Mustangs, so little time..."
ICON Motorsports

 

It's never held this much pressure before. It was really really bad. Are these caps designed to hold a certain amount of pressure and then release above a certain point? If so, thats my problem. Thats why I'm asking.

 

The gas tanks are supposed to be pressurized to keep emissions down, as was previously stated. On hotter days, more pressure is going to build up. The increased pressure isn't going to decrease drivability any, and I would also think that the type of gas you put in might have something to do with it. (Was it crown? heh) Or, it could be related to a number of other random things. It's certainly not going to be the gas cap. If there wasn't any pressure under the cap, then I'd be concerned with replacing the cap.

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89 iroc-z 305 tbi
k&n filtercharger, open element air filter. nuffin' else

 

Here's the thing...there was TOO MUCH pressure in the tank. Are the caps supposed to release pressure after a certain point? I don't want it to explode on me!

 

For all practical purposes, there is no such thing as too much pressure. It doesn't hurt anything.

Out here in CA they actually use a machine to test gas caps for their pressure-holding ability... your car will flunk if it doesn't hold pressure.

Actually, it sounds like you might have had some gas that was formulated for cooler weather in there before, and the new gas was set up for hotter weather. They vary the volatility of gasoline quite a bit to adjust to the predicted weather.

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"So many Mustangs, so little time..."
ICON Motorsports

 

Here's the only change is gas I did. I usually always use Chevron and sometimes shell. Well this weekend I was traveling and put in Union 76 gas. I had already driven half a tank of it with no problem on Sunday. Then on Monday this happened when it was hot as hell outside. As for the pressure, yes there is always some pressure, but there was so much this time that when I loosened up the cap, it flowed out for like 5 seconds and a lot too like a hairdryer blowing. You guys sure that is safe????

 

Yes, that's safe. (Hey, at least it's not leaking gasoline out anywhere.)

Did you switch octanes, by any chance?

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89 iroc-z 305 tbi
k&n filtercharger, open element air filter. nuffin' else

 

Nope

 

IIRC there is a pressure release vent mounted on/near the panhard bar.



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"Question with boldness even the existence of a *** ; because, if there be one, he must more approve of the homage of reason, than that of blind-folded fear."

-Thomas Jefferson

 

Correct. The tank pressure valve is located above the liquid level of the tank and is supposed to seal completely in case the car inverts. Excessive pressure should be vented by this valve, but the relief setting is really quite high. The valve's main purpose is to admit air to the tank as the fuel level decreases to preven crushing the tank. It would be more likely to have the excessive pressure overcome the EVAP valves and flood the canister with vapor than to have this tank valve release pressure.

Hot days and an empty tank can create a high volume of pressurized air and fuel vapor - a nice, combustible mixture. It's really quite normal to even hear the tank "relax" a bit as the pressure is relieved. There shouldn't be more than 20 PSIG in the tank, but almost twenty gallons of air at that pressure makes for a nice breeze.

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Later,
Vader
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Justice and Freedom will Prevail
Adobe Acrobat Reader

 

Ok, I think the gas cap is fine. It's not happening again and that day I was really low in fuel and it was about 102deg's outside at that time (in regards to what vader said). It hasn't happened again, just coincidence I guess. Anyways, I noticed the car is bogging down when it's hot outside like above 95deg's. Just started happening 2 days ago. It feels like the hp has been cut by 30%. My first thought was the fuel filter but I don't see how it could be because it runs fine when it's cool. Any ideas?

Here are my mods:

91 Z28 305TPI 5spd
Hypertech Stage II, EB Intake & Runners, Ported Plenum, EB Headers, 3" Cat, 3" FM Catback, 1.6RR's, 3.42gears, B&M Ripper Shifter, MSD Wires/Cap/Rotor/6AL, AFPR, Air Foil, K&N's, Gutted Airbox, TB Bypass, platinum plugs, and 160deg Stat.

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[This message has been edited by Bugsbunny (edited October 03, 2001).]