High pressure in the gas tank

sliverstorm · 09-15-2007, 07:06 PM

Hello, just got a quick question.

When I take off the gas cap to fuel up, there is a hiss and the smell of gasoline vapors escaping. Now, I am not completely surprised by this, as it's happened with other cars and hot days make vapors expand, but I read that the charcoal canister in the engine bay is supposed to collect the fumes for burning in the engine. Is this buildup of fumes in the tank an indication that my charcoal canister is not working, pipes are blocked or not connected, etc?

jkelly327 · 09-15-2007, 09:10 PM

I'm leaning in the direction, I'll be monitoring responses with the hope of solving the issue...

Mrdosedriver717 · 10-10-2007, 08:38 AM

there is some kind of clog in the fuel tank vent hoses. I am having the same problem in my 87 but it is simpaly because I have a cap on the line

Watch driving the car for long periods of time without removing the gas cap though as you are pressureizing the fuel tank which could become dangerous!

4v6 · 10-10-2007, 11:34 AM

Cant remember if theres a temperature operated vent valve for these like on the 3vzfe that opens up when the engine is warm and burns the excess fumes.
Its possible its not working if you have one fitted.
Might be worth checking the lines to and from the canister.

sliverstorm · 10-10-2007, 02:38 PM

so I should check canister AND fuel lines? ok. I'm not sure how I'll check the lines though. I can obviously access the one that goes into canister, but what about the one that goes to the fuel tank? I suppose the most obvious way would be pull the line from the tank and see if pressurized vapor comes out :P

Also mrdosedriver, shouldn't it be generally ok to have the pressure? I mean, I don't like having it and gas gets wasted when I take off the cap, but what I mean is it's not like it will explode from sheer pressure. right? :P

4v6 · 10-10-2007, 03:00 PM

If it gets too high it could split the tank...

sliverstorm · 10-11-2007, 12:48 AM

well the thing I don't get is why it would get that high. it is all dependent on the temprature- in a chamber the amount of gas that will vaporize is a function of pressure and temprature, and since the container is a fixed size pressure is dependent on temprature.
PV=nRT anyone?

MuMan · 10-11-2007, 03:29 AM

Is it pressure?...or just a release of built-up vacuum..
The gas cap is supposed to be vented to allow air to enter under vacuum in the tank, but not release fumes under pressure from heat/expansion. (one way valve)
Could be faulty..

sliverstorm · 10-11-2007, 03:32 AM

I am fairly sure it is pressure because if I remember correctly, there is a strong smell of fuel in the air after I open the cap.

Mrdosedriver717 · 10-11-2007, 10:31 AM

I know on my car it is positive pressure in the tank. And just like anything else under pressure it is dangerous.

I can always tell when the pressure in my tank is getting high because I can my hear my fuel pump starting to wig out! If I watch the fuel pressure gauge it starts to drop sometimes below 40 psi!

I just stop and release the fuel cap and its fixed.

I think in my setup I am NOT going to actually use the stock vent on the tank but rather find a vented gas cap that releases positive pressure or just drill a small whole through the center of my stock cap.

mr2willwin · 10-11-2007, 10:41 AM

My mk2 does the same thing, just unscrew it slowly is what i'm told.

PearlJammzz · 10-11-2007, 11:22 AM

Mine sounds liek a soda can when I pop it open...isn't this caused by the vacuum though?

MuMan · 10-11-2007, 09:12 PM

It may still be vacuum. You will still get fuel smells when the vapor is displaced by the inrushing air..and the fuel pump is being loaded up working against a vacuum, so it will draw more current and drop efficiency. I actually collapsed a tank one time because of a non-vented cap...but not an MR2.
Either way..try running the car with the cap released half a turn, and see what difference there is.
Or..better still, fit a vented cap..I won't tell anyone

09-15-2007, 07:06 PM	#1 (permalink)
sliverstorm Tinkerer extrordanaire Join Date: Sep 2007 Location: California Posts: 87 Thanks: 0 Thanked 3 Times in 3 Posts iTrader Rating: (0/0% )	High pressure in the gas tank Hello, just got a quick question. When I take off the gas cap to fuel up, there is a hiss and the smell of gasoline vapors escaping. Now, I am not completely surprised by this, as it's happened with other cars and hot days make vapors expand, but I read that the charcoal canister in the engine bay is supposed to collect the fumes for burning in the engine. Is this buildup of fumes in the tank an indication that my charcoal canister is not working, pipes are blocked or not connected, etc?

09-15-2007, 09:10 PM	#2 (permalink)
jkelly327 No Skills Join Date: Sep 2007 Location: Lawrenceburg, KY Posts: 1 Thanks: 0 Thanked 0 Times in 0 Posts iTrader Rating: (0/0% )	Same issue on a 88 I'm leaning in the direction, I'll be monitoring responses with the hope of solving the issue...

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

10-10-2007, 08:38 AM	#3 (permalink)
Mrdosedriver717 Matte Red 87 SC V6 Join Date: May 2006 Location: Columbus Ohio Posts: 21 Thanks: 0 Thanked 1 Time in 1 Post My Google Map iTrader Rating: (0/0% )	there is some kind of clog in the fuel tank vent hoses. I am having the same problem in my 87 but it is simpaly because I have a cap on the line Watch driving the car for long periods of time without removing the gas cap though as you are pressureizing the fuel tank which could become dangerous!

10-10-2007, 11:34 AM	#4 (permalink)
4v6 http://www.Typ-85.co.uk Join Date: Jun 2007 Posts: 184 Thanks: 2 Thanked 24 Times in 14 Posts iTrader Rating: (0/0% )	Cant remember if theres a temperature operated vent valve for these like on the 3vzfe that opens up when the engine is warm and burns the excess fumes. Its possible its not working if you have one fitted. Might be worth checking the lines to and from the canister.

10-10-2007, 02:38 PM	#5 (permalink)
sliverstorm Tinkerer extrordanaire Join Date: Sep 2007 Location: California Posts: 87 Thanks: 0 Thanked 3 Times in 3 Posts iTrader Rating: (0/0% )	so I should check canister AND fuel lines? ok. I'm not sure how I'll check the lines though. I can obviously access the one that goes into canister, but what about the one that goes to the fuel tank? I suppose the most obvious way would be pull the line from the tank and see if pressurized vapor comes out :P Also mrdosedriver, shouldn't it be generally ok to have the pressure? I mean, I don't like having it and gas gets wasted when I take off the cap, but what I mean is it's not like it will explode from sheer pressure. right? :P

10-10-2007, 03:00 PM	#6 (permalink)
4v6 http://www.Typ-85.co.uk Join Date: Jun 2007 Posts: 184 Thanks: 2 Thanked 24 Times in 14 Posts iTrader Rating: (0/0% )	If it gets too high it could split the tank...

10-11-2007, 12:48 AM	#7 (permalink)
sliverstorm Tinkerer extrordanaire Join Date: Sep 2007 Location: California Posts: 87 Thanks: 0 Thanked 3 Times in 3 Posts iTrader Rating: (0/0% )	well the thing I don't get is why it would get that high. it is all dependent on the temprature- in a chamber the amount of gas that will vaporize is a function of pressure and temprature, and since the container is a fixed size pressure is dependent on temprature. PV=nRT anyone?

10-11-2007, 03:29 AM	#8 (permalink)
MuMan Nothing is forever.. Join Date: Sep 2006 Posts: 348 Thanks: 0 Thanked 21 Times in 14 Posts iTrader Rating: (0/0% )	Is it pressure?...or just a release of built-up vacuum.. The gas cap is supposed to be vented to allow air to enter under vacuum in the tank, but not release fumes under pressure from heat/expansion. (one way valve) Could be faulty..

10-11-2007, 03:32 AM	#9 (permalink)
sliverstorm Tinkerer extrordanaire Join Date: Sep 2007 Location: California Posts: 87 Thanks: 0 Thanked 3 Times in 3 Posts iTrader Rating: (0/0% )	I am fairly sure it is pressure because if I remember correctly, there is a strong smell of fuel in the air after I open the cap.

10-11-2007, 10:31 AM	#10 (permalink)
Mrdosedriver717 Matte Red 87 SC V6 Join Date: May 2006 Location: Columbus Ohio Posts: 21 Thanks: 0 Thanked 1 Time in 1 Post My Google Map iTrader Rating: (0/0% )	I know on my car it is positive pressure in the tank. And just like anything else under pressure it is dangerous. I can always tell when the pressure in my tank is getting high because I can my hear my fuel pump starting to wig out! If I watch the fuel pressure gauge it starts to drop sometimes below 40 psi! I just stop and release the fuel cap and its fixed. I think in my setup I am NOT going to actually use the stock vent on the tank but rather find a vented gas cap that releases positive pressure or just drill a small whole through the center of my stock cap.

10-11-2007, 10:41 AM	#11 (permalink)
mr2willwin Cage Fighter Join Date: Nov 2006 Location: Holladay, UT Posts: 336 Thanks: 15 Thanked 6 Times in 6 Posts My Google Map iTrader Rating: (0/0% )	My mk2 does the same thing, just unscrew it slowly is what i'm told.

10-11-2007, 11:22 AM	#12 (permalink)
PearlJammzz Cage Fighter Join Date: Feb 2006 Posts: 2,115 Thanks: 9 Thanked 74 Times in 69 Posts My Google Map iTrader Rating: (7/100% )	Mine sounds liek a soda can when I pop it open...isn't this caused by the vacuum though?

10-11-2007, 09:12 PM	#13 (permalink)
MuMan Nothing is forever.. Join Date: Sep 2006 Posts: 348 Thanks: 0 Thanked 21 Times in 14 Posts iTrader Rating: (0/0% )	It may still be vacuum. You will still get fuel smells when the vapor is displaced by the inrushing air..and the fuel pump is being loaded up working against a vacuum, so it will draw more current and drop efficiency. I actually collapsed a tank one time because of a non-vented cap...but not an MR2. Either way..try running the car with the cap released half a turn, and see what difference there is. Or..better still, fit a vented cap..I won't tell anyone