Gas Tank Pressure Buildup

I have a '97 Toyota Carolla and just recently (the last two days), I’ve noticed that when I was driving, the gas meter would drop very rapidly (as in from half tank to empty line in less than 10 miles) and then the engine would start to make sounds consistent with running on fumes.

When I went to a gas station, I unscrewed the cap about a quarter turn and let the pent up air escape, the hissing sound would last about 100 seconds.

I’m wondering if it has something to do with the gas cap or something else might be happening?

This happened over the weekend while I was in Phoenix twice when temps rose above 40?C… but did not happen when I drove at night or if the tank was over half full.

Some friends suggest I should “burp” the tank before I drive in the day time, but I’d like to know the root cause.


You need to make it clear if you are searching for a answer to the erratic gague operation or the “running on fumes” condition or the long (100 seconds is long but maybe you are just venting at a low level) vent time. Is there a warning on the filler door about “open cap slowly” or something to that nature?

I want to search the answer to the pressure buildup. I listed observations which I believe are possible effects caused by this condition.

No, there is not a warning to “open cap slowly” as this is the first time I’ve experienced this before. I wouldn’t say I vented at a very low level either (but not too high as well), as the air coming out was quite cold and with quite a bit of pressure. I’m just being cautious.

My guess is that if there is that much compressed air in the fuel tank, some of it might get into the fuel line and cause the run on fumes sound that I heard.

I’m not too sure about the fuel gauge, as it went from empty line to about 1/3 full after all the air was let out of the tank. I then proceeded to fill the tank, and it was full at 7 gallons, or roughly 65% of the tank in a 11 gallon tank.

This event has happened twice, both on hot days ~ 105F outdoor temp and both times went from just under half tank line to empty line in less than 10 miles.

Am I just going in a circle once again and not providing any additional info?


The car should vent itself if needed. The gas cap is desinged to vent at a given pressure. If the tank is almost empty it will have more volume to vent even if it has no more pressure, that is normal. While it is hard to say, I would not consider your expereience to be an indication of a problem.

If you really want to try something try a new fuel cap. It is cheap and it is possible that it might make a change.

Possibly not? Here’s a description of the OBD-II compliant Evaporative Emissions Control system It is quite adamant that there is no pressure relief outlet vent in the gas cap on 1996 and later cars. Things might have changed since it was written of course, but probably not for a 1997 vehicle.

The idea seems to be that both inlet and outlet venting of the gas tank is through the EVAP canister.

And it sure does sound like SpewMuffin’s gas tank is not venting through the gas cap. But it also sounds like it is not venting through the EVAP canister either. Except that I’d think that if air couldn’t get into the tank as gas was burned that after a few miles, the fuel pump would start to bog down, so it would appear to be an intermittent problem? That only happens on hot days? (plausible) And only when the tank is half full or less. (not so plausible?).

So, maybe some sort of intermittent problem with the purge/whatever valves in the EVAP system.

I really can’t account for the the misreading fuel gauge. Conceptually, I suppose that high vapor pressure in the fuel tank could compress the float on the fuel level read out and cause it to sink to the bottom of the tank. But I can’t bring myself to believe that’s really happening.

Gasoline has a property called “Vapor Pressure”… The hotter it is, the higher the pressure of gasoline in a sealed container. Gasoline contains both Propane & Butane. These fractions tend to boil off. That’s why gas tanks are sealed and have vapor recovery systems. This time of year, left over “winter blend” gasoline (sales are slow) has more propane and butane than it should and when it’s 105 degrees, fuel tank pressure can reach the upper limits of automobile gas tanks…As summer progresses, the refiners will provide lower vapor pressure gasoline and tank pressure will not be so noticable…

Your gas tank should be able to bretah in through the activated charcoal bed in the charcoal canister. Hydrocarbon molecules (gas vapors) become entrapped in the charcoal when the tank breathe out. These molecules are purged into the engine’s intake for bruning upon startup and breathed back into the tank during operation.

One possible cause of this level of pressure buildup is a saturated charcoal bed. In some vehicles this can happen from “topping off” the tank. Another possible cause is a plugged or kinked vent line between the tank and the chanister.

I’d get ot checked of I were you. Insufficient breathing of the tank can stress a fuel pump from vacuum in the tank.

I agree with all but this OP is seeing excessive postive pressure in the tank. I thought Joseph had it handled until vtcodger pointed out the features of the system.

Is the amount of propane in gasoline there because it would require excessive handling to remove it. I saw a great History channel show on gasoline that explained the propane is removed by heating the crude,perhaps not all is removed so easily. Propane at one time was considered a “problem” part of the crude (like gasoline was) but now it is the most used alternative fuel.

Are you sure that air/vapor is escaping out of the tank when you loosen the gas gap? Might it be that air is rushing into the tank because a vacuum formed as the gas was being pumped out of the tank?

I guess my point is that the only way the OP could be seeing a significant positive pressure buildup is if the charcoal bed is saturated or the vent line between the gas tank and the canister was kinked.

The blend of heavier vs lighter fractions is adjusted to higher volatility for easy starting in the winter vs lower volatility to reduce evaporation losses and prevent vapor lock in the summer.

Ether is another low boiling point fraction in gasoline.

" I was in Phoenix twice when temps rose above 40?C… but did not happen when I drove at night or if the tank was over half full. "…40 degrees C is 105 degrees F. The tank was pressurized, perhaps as high as 10 or 12 PSI. Gas tanks are vented to prevent any vacuum from forming, which would quickly collapse the tank…

“Back in the old days…” hundreds of cars would be pulled off the side of the road because of vapor lock, gasoline boiling in the fuel lines, fuel pump and carburetor float bowl if a preseason heat wave arrived before the “winter gas” was sold…

Propane and Butane are deliberately added to gasoline to aid winter starting and to get rid of the propane and butane! Av-Gas has none and therefore has very low vapor pressure to prevent vapor lock. Carb heat must be used not to prevent icing, but to aid cold weather starting…

As far as the rapid change with fuel gauge is concerned I think there may be a sensor connection problem caused by the high heat, that is making the gauge change so fast. I don’t know if there are two sensors used or just one.

If the fuel pickup and level float are anchored to the top of the tank,
plus pressure buildup causes the tank to bulge at the top and bottom…
the pickup and float could be lifted from the fuel, changing the gauge and starving the engine.