Pressure build up in gas tank? - Toyota Nation Forum : Toyota Car and Truck Forums
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#1 Old 12-24-2009, 01:10 AM
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Pressure build up in gas tank?

Hey guys...

My car is 1997 Toyota Corolla and I just recently bought it off my brother in-law. Right off that bat I toke it in to get a leaking steering pump replaced, motor mounts replaced, and front brake pads and rotors replaced.

The day I got it back from the shop I drove for about 20 minutes and my check engine light came on. The car was running low on gas and I never filled it up since I got it so I thought it may just be old or dirty gas that was causing the light to come on since the car was sitting for quite some time and was never driven.

I went to go fill up and when I unscrewed the gas cap quite a lot of pressure was released. I understand that it is normal to have some pressure but this was more pressure then I have ever heard come from a gas tank. When I got home I disconnected the battery to get rid of the check engine light to see if it would come back again.

Its been about 5-6 more 20 minute drives and it hasn't come back yet but there is still a lot of pressure being released every time I unscrew the gas cap. Should I be replacing my gas cap or something?

My last car was a 2008 Pontiac Wave new off the lot and I can't recall ever hearing pressure being released when unscrewing the gas cap.

Found this on another post:
Quote:
my buddy had his gas cap go bad on his 1970 mustang and it ended up imploding the fuel tank because of the suction from the motor and the cap failed to relieve the pressure.

Last edited by danpluso; 12-24-2009 at 01:13 AM.
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#2 Old 12-24-2009, 09:42 AM
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its possible you have a bad EVAP solenoid in the emissions control systems. it would have been pretty useful to know that code.
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#3 Old 12-24-2009, 02:30 PM
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I thought disconnecting the battery didn't completely erase the code from the system.. If it is a problem shouldn't the error code and check engine light come back in time?

Last edited by danpluso; 12-24-2009 at 02:31 PM.
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#4 Old 12-24-2009, 05:34 PM
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your charcoal canister needs to be replaced...it's stopped up. And, if you wait to long, it could literally burst the tank causing a leak. I know this from experience. My friend has replaced two tanks already before we figured it out.

I suggest you leave your cap slightly loose for now until you can afford the charcoal canister. They're not cheap either. Or disconnect the evap line coming to the charcoal canister. It's the bigger of the two hoses.

And last, try not to top off your gas tank in the future. Usually this is the cause. Fuel will travel up the evap line and into the charcoal canister causing it to harden, basically clogging it.

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#5 Old 12-25-2009, 02:53 AM
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Hey thanks for the advice...

I just did the longest drive I have done so far in the car (about 1 hour) and to my surprise when I unscrewed the gas cap there was pretty much no pressure release at all. Not sure why...

Are you sure it is a charcoal canister and how much do they usually cost?

Should I just try out a new gas cap first because I have heard of similar problems caused by a faulty gas cap..
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#6 Old 12-25-2009, 02:30 PM
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Well, the gas cap is supposed to make a perfect seal... not let out pressure.


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#7 Old 12-25-2009, 05:38 PM
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Instead of assuming the MIL came on for an EVAP problem- why not just have the code read and find out for sure where the problem lies? You've had several suggestions (some expensive ones) based on an event that very well maybe normal (pressure release when opening the cap). How can you fix something if you don't truly know if its broken?

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#8 Old 12-26-2009, 06:45 PM
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Thanks for the advice guys!

I actually went on a 1-2 hour drive the other day and when I opened the gas tank up there was pretty much no pressure released at all. Went on a little 20 minute trip the next day and also hardly any pressure was released again.. Is it possible for this charcoal filter to get unclogged over time with driving? Like I said the car was sitting and was never used for probably a good year before I bought it and started driving it.

I guess I will just keep driving it for now and see if the CEL comes back on and continue to check the pressure in the gas tank after each drive.
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#9 Old 12-26-2009, 09:01 PM
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My '94 ALWAYS had tons of pressure released after short drives, and little or none after longer drives, usually.

The charcoal canister only releases vapors into the engine under certain circumstances. It's possible that those conditions aren't met on shorter drives with these cars, but on a longer drive, pressure is eventually released.

In the 4 years I had the car it was always like this, and the previous owner said it was like that for the 8 years he had the car (he got it at 2 years old). No problems ever with the charcoal canister or tank.

Since the pressure is sometimes released, that indicates that your charcoal canister is working properly. Maybe it is not working at it's best, but like I said, my old Corolla did just like that for many years with no problem. I wouldn't be worried.

Just change the oil, and in a few years time you'll be cursing yourself for buying that car...you'll be looking at it in the driveway saying 'why won't you die! I want a new one!'

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#10 Old 12-26-2009, 09:59 PM
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Quote:
Originally Posted by danpluso View Post
Thanks for the advice guys!

I actually went on a 1-2 hour drive the other day and when I opened the gas tank up there was pretty much no pressure released at all. Went on a little 20 minute trip the next day and also hardly any pressure was released again.. Is it possible for this charcoal filter to get unclogged over time with driving? Like I said the car was sitting and was never used for probably a good year before I bought it and started driving it.

I guess I will just keep driving it for now and see if the CEL comes back on and continue to check the pressure in the gas tank after each drive.
You're EVAP system is monitored closely every time you drive your car. If you had an actual problem- it would trigger the light upon the very first drive cycle within minutes. The fact that your light hasn't been triggered is an indications that everything is functioning properly.

JJ
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#11 Old 12-27-2009, 03:38 PM
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Okay thanks for the advice and reliving some pressure off me..

Quote:
Just change the oil, and in a few years time you'll be cursing yourself for buying that car...you'll be looking at it in the driveway saying 'why won't you die! I want a new one!'
So true... I am already thinking that way, lol.. My last car (and first car) was a brand new 2008 Pontiac Wave (Standard) so I am finding it hard to get used to this old gutless car. I'm never buying an automatic again I can tell you, I hate Autos! Even now I sometimes still try to down shift when coming to a stop. I don't think I will ever get used to driving an auto.
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#12 Old 02-23-2011, 09:00 PM
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Quote:
Originally Posted by trife96 View Post
your charcoal canister needs to be replaced...it's stopped up. And, if you wait to long, it could literally burst the tank causing a leak. I know this from experience. My friend has replaced two tanks already before we figured it out.

I suggest you leave your cap slightly loose for now until you can afford the charcoal canister. They're not cheap either. Or disconnect the evap line coming to the charcoal canister. It's the bigger of the two hoses.

And last, try not to top off your gas tank in the future. Usually this is the cause. Fuel will travel up the evap line and into the charcoal canister causing it to harden, basically clogging it.
This turned out to be the cause of an apparent leak in my 94 Prizm gas tank. Thanks for the tip!

I bypassed the irredeemably blocked charcoal canister by extending and attaching the tank vent hose directly to the bottom of the charcoal canister, where a little gas is designed to leak if necessary. The tank leak immediately stopped dead, but I feared the tank had developed a micro crack from the pressure that simply did not leak noticeably under normal pressure. After coating the tank with talc, tying plastic wrapped rags under the tank, and tracing the leak stain marks, I am convinced the vent hose connection was leaking at the tank and running down the bottom of the tank, making it look like the tank itself was leaking. I can not now produce a detectable leak.

Thread is old, but so is the car :-)
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#13 Old 02-24-2011, 09:32 AM
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pressure?

Hey all,

I know this thread just got drug up, but it is timely for me as I just tried to repair and use the previously bypassed Thermal Vacuum Valve. I will monitor the situation when I remove my fuel cap. The term "pressure" is being used here which would make one expect a rush of air OUT of the tank. My experience with fuel tanks is there is typically a "vacuum" created as the fuel is pumped out IF no air is allowed IN. I know older Ford tractors are vented through the fuel cap and when replacement caps aren't vented and monitored, you can collapse a fuel tank.

The only time I would expect "pressure" would be if there was a significant temperature increase that caused the fuel to expand. Am I missing something?

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#14 Old 02-24-2011, 04:37 PM
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I'm looking at this largely by logic instead of actual testing and experience, but here are my thoughts. The charcoal canister is designed to trap gas fumes so that they can later be burned, correct? This shows that the design expects there to be a positive pressure in the tank at some point. The gas would evaporate faster at a higher temperature, but I am guessing it does evaporate even at lower temperatures. I guess it is the evaporation more than the expansion of the liquid that causes the positive pressure.

What you say about a vacuum being created in the tank as fuel is pumped out makes sense. My 94 Prizm has a valve in the gas cap to allow for negative pressure to be equalized. But even if air always needed to be sucked in while fuel was being pumped, as soon as the fuel stops flowing, evaporation can start to create positive pressure. There could also be a difference between engine designs in the amount of vacuum created in the tank by fuel flow.

On the practical observation side, I am hard pressed to think of how a vacuum could be causing the problems with blocked Corolla/Prizm canisters. It appears there is a positive pressure problem when the canister is stopped up.
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#15 Old 02-25-2011, 01:10 AM
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Quote:
Originally Posted by pulse6 View Post
I'm looking at this largely by logic instead of actual testing and experience, but here are my thoughts. The charcoal canister is designed to trap gas fumes so that they can later be burned, correct? This shows that the design expects there to be a positive pressure in the tank at some point. The gas would evaporate faster at a higher temperature, but I am guessing it does evaporate even at lower temperatures. I guess it is the evaporation more than the expansion of the liquid that causes the positive pressure.

What you say about a vacuum being created in the tank as fuel is pumped out makes sense. My 94 Prizm has a valve in the gas cap to allow for negative pressure to be equalized. But even if air always needed to be sucked in while fuel was being pumped, as soon as the fuel stops flowing, evaporation can start to create positive pressure. There could also be a difference between engine designs in the amount of vacuum created in the tank by fuel flow.

On the practical observation side, I am hard pressed to think of how a vacuum could be causing the problems with blocked Corolla/Prizm canisters. It appears there is a positive pressure problem when the canister is stopped up.
Correct. Petrol is a very volatile liquid and readily evaporates, which means that you will quickly get a build-up of vapour in the tank. Since the vapours are a gas, their density is affected by changes in temperature, but a change from 20 to 30 degrees would only see an increase of 3% in volume (and thus a 3% increase in pressure since volume is fixed) - still, this is much more than the liquid would expand (liquid densities are much less sensitive to temperature changes). The charcoal canister basically acts as a one-way breather for the fuel tank - it collects the tank vapours and temporarily stores them in the charcoal until a time that the engine is operating under vacuum and low load (and a few other variables), at which point a VSV opens and allows the vapours to be drawn into the intake and burnt. If the charcoal canister was to block then you would get pressurisation of the fuel tank and a hiss when you open the cap as the vapours escape and the pressure equalises.

In addition, the fuel cap should contain a one-way breather valve that allows atmospheric air to enter the tank to equalise pressure but not allow vapours to leak into the atmosphere, but this frequently blocks or gums up. Since you have a net decrease in the contents of the fuel tank over time, the pressure will decrease below that of atmospheric (ie a "vacuum"), which will also cause a hiss when you open the cap as air rushes in to equalise the pressure.


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