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Old 02-08-2008, 07:38 AM   #1
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Fuel Tank/ Extreme Pressure after city driving

For about 2 months I've been having a problem with excessive pressure building up in my gas tank, it doesnt happen often, only after a 45min-an hour of street driving (problem has not occured on freeway). Once the pressure becomes great enough it feels as if fuel is cut off, the car will bog out as I step on the gas, the car has died on me while accelerating on green at intersections and wont start up for a few minutes, or if it does start I'll step on the gas pedal and get a knocking noise from the engine as if its getting no fuel, then back off. At first I had no idea why my car would suddenly turn off and act this way but then I realized how much pressure was building up in there. I know pressure in the tank is normal but this is something else. Any input on how to get rid of so much pressure?
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Old 02-08-2008, 04:45 PM   #2
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Re: Fuel Tank/ Extreme Pressure after city driving

The purge valve on the charcoal canister is supposed to open and and burn gas vapours stored in the canister when you are driving down the road. Wonder if maybe it is not opening sometimes and causing pressure to build in the tank?
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Old 02-08-2008, 08:46 PM   #3
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Re: Fuel Tank/ Extreme Pressure after city driving

Thanks for the reply. Can you give me info on where to find the charcoal canister?
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Old 02-09-2008, 01:24 AM   #4
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Re: Fuel Tank/ Extreme Pressure after city driving

I dont even use my charcoal canister... I dont have a single problem with mine. I have a 305 TPI as well... Do you have aftermarket headers and exhaust?? The reason I ask is because living in a warm climate with headers... (close to gas lines) may cause vapor lock. Vapor lock occurs when the gas gets too warm and starts to boil from the heat. Making the engine bog and die. It may also make added pressure in the tank. I dont have stock fuel lines, I changed them so I never ran into this problem. Mine are braided stainless steel lines that run on the drivers side but mounted on the tunnel and fire wall to keep them away from the heat from the headers.

The reason it may not happen on the highway is because you have more air flow going through the engine bay.

You may wanna try putting a fuel pressure guage on it then drive in the city for a while and see where its at.
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Old 02-09-2008, 05:09 AM   #5
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Re: Fuel Tank/ Extreme Pressure after city driving

The canister is under the hood, driver's side, behind the headlights. Small round black cylinder with hoses on it and an electrical connector.

I would disable the purge solenoid and bypass the canister to see if it fixes the problem. Un-plug solenoid, plug port on engine, loosen gas cap.

Do a search for "charcoal canister" and you will find lots on the subject.
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Old 02-09-2008, 07:39 AM   #6
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Re: Fuel Tank/ Extreme Pressure after city driving

What leads you to believe there is excessive pressure? Seems to me that pressure in the tank would NOT cause the symptoms you are describing. It does sound like the engine is being fuel starved, when was the last time you changed the filter? Have you checked fuel pressure when it is acting up? Also, as it appears you have a TPI car, fuel is constantly being circulated, so I don't think vapor lock is really an issue here. On carbed cars, running a 5-7 PSI, and no return fuel line, I could see it, but, not at 40+ PSI, with a return line.....

If it is building too much pressure, are you sure the problem is not too MUCH fuel?
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Old 02-09-2008, 08:50 PM   #7
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Re: Fuel Tank/ Extreme Pressure after city driving

His car would still run with too much fuel... It would get extreemly hot and wash the cylinders. maybe even burn a whole in a piston. its hard to tell. eliminate the canister and see if that helps. I maybe wrong on the vapor lock but you can still get vapor lock with a fuel injected car. it doesnt matter how much pressure there is.

Another thing I would do is to check ALL of the vaccum lines. make sure they are all connected and in working condition.

Honestly his problem can be a multiple of things that we have listed. Is the car a stock TPI car from the factory or has it had an engine swap? If it was a swap I could see him running into problems. Things that the person who did the swap looked over... but who's to say it was a swap?? we dont know... I think we need a little more information on the car.
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Old 02-11-2008, 08:31 PM   #8
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Re: Fuel Tank/ Extreme Pressure after city driving

Stock 305TPI Engine. The car smells of fuel after driving it. Today it turned off on me twice in highway driving, that was new. After driving for a while u can smell gas around the rear of the car. There is also moisture from the vapors around the gas cap from so much pressure. I switched the gas cap today. Could my problem be my fuel pump cutting off and turning back on somehow? I should mention that after a 30minute drive, I get a loud humming noise coming from the gas tank and within seconds the car decides to start loosing fuel pressure (the hotter the day is, the quicker the humming appears along with all its problems), then it either acts as if its going to bog out or actually boggs out. Once it dies out i crank and crank the ignition until it finally turns on after 4-5 cranks, when it finally stays on it idles REALLY high then slowly adjusts itself as I drive. When slowing down, I try to keep my foot slightly on the gas because it feels as if it might keep the fuel from totally cutting off and the car from dying out. The humming in the tank returns within minutes if I dont let the car cool down and relieve the pressure from the tank by removing the cap. Once it gets to driving, the car has a noticeable loss in performance. IM TOTALLY CLUELESS ABOUT THIS ONE...

Last edited by Black IROC 86; 02-12-2008 at 07:28 AM.
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Old 02-12-2008, 10:32 PM   #9
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Re: Fuel Tank/ Extreme Pressure after city driving

check your ground on the fuel pump. and also check the connections underneath. Replace the fuel filter. You pump maybe bad... That or your computer is making it act that way. Any more things you have tried? If your smelling gas fromt h rear of the car it sounds like you have gas comming out somewhere... check the lines for leaks
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Old 04-07-2008, 06:22 PM   #10
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Re: Fuel Tank/ Extreme Pressure after city driving

Did you ever find the cause? My firebird is doing the same thing except it happens after 15 minutes of driving. If I keep going gas starts coming out of the carbon canister.
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Old 04-07-2008, 06:46 PM   #11
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Re: Fuel Tank/ Extreme Pressure after city driving

It's the pressure relief valve back by the fuel tank causing this.
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Old 04-10-2008, 04:18 PM   #12
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Re: Fuel Tank/ Extreme Pressure after city driving

The relief valve is the white thing above the axle? I know that is not causing this. I removed it and gas still come out of there and the other vent line.
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Old 04-11-2008, 12:28 AM   #13
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Re: Fuel Tank/ Extreme Pressure after city driving

Did you remove it then drive the car?? or removed it after you drove the car...?? when you put gas in the tank does the pump frequently stop because of the pressure?? Give me a little info about your car... year, engine, model...
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Old 04-11-2008, 05:51 AM   #14
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Re: Fuel Tank/ Extreme Pressure after city driving

Its a 88 5.0 TBI. I removed the valve then drove the car and it starts to buck like it running out of fuel. When I stop and look you can see gas coming out of the vent line under the hood and the relief valve. The pump does stop frequently when putting gas in the tank.
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Old 04-11-2008, 06:41 AM   #15
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Re: Fuel Tank/ Extreme Pressure after city driving

if its not those two things then check grounds on back of heads if they are tight then your fuel pump is dying and causing this.
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Old 04-14-2008, 02:33 PM   #16
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Re: Fuel Tank/ Extreme Pressure after city driving

I replaced the fuel pump and checked my grounds on the motor and its still doing it. When I replaced the pump I noticed the line to the carbon canister went to the bottom of the tank. The return line is at the top of the tank. Is that the way it should be?
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Old 04-14-2008, 02:36 PM   #17
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Re: Fuel Tank/ Extreme Pressure after city driving

try flipping them around. and also... try putting a new valve on the tank. I had the same problems and it turned out to be that stupid valve. except my motor never sputtered... yours did because you took that valve off which i wouldnt have ever done.
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Old 04-19-2008, 05:45 PM   #18
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Re: Fuel Tank/ Extreme Pressure after city driving

had the same thing try to leave your gas cap loose i did that car runs fine know...305/carb was a tpi.
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Old 04-19-2008, 06:55 PM   #19
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Re: Fuel Tank/ Extreme Pressure after city driving

I fixed mine. It was the fuel pump.
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Old 05-10-2008, 08:31 PM   #20
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Re: Fuel Tank/ Extreme Pressure after city driving

mine jad excessive pressure but never leaked or cut out and then i replaced the fuel pump and broke the relief valve by the tank and left the part that goes into the hose in it and now i dont have that problem. i gotta 92 rs 305 tbi and she runs shell chirp tires in 2 wit stock motor wit a chip and auto
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Old 09-26-2008, 09:48 PM   #21
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Re: Fuel Tank/ Extreme Pressure after city driving

Ok how bout this, mine does it only when the fuel level is close to 1/4 anything much more than 1/4 no problem at all. happened for the third time today i couldnt get it to stay running once i got it started. I dumped bout two gallons of cool fuel at the gas pump and ran fine. I dont have the fuel smell at the rear of the car but there is a hell of alot of pressure in the tank. my car is a 1991 z i did a motor swap it had the tpi 305 now its a 388 stroker, edelbrock tes exhaust exhaust is wrapped fuel lines are heat sheilded, custom burned chip, just replaced the fuel pump had the problem the first time about two months before i changed the pump and the second time bout a month after which was a week ago and then today
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Old 10-07-2008, 06:39 PM   #22
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Re: Fuel Tank/ Extreme Pressure after city driving

I have a 91 350 TPI with the exact same problem. This only happens on hot days while in a stop and go traffic. When on the Freeway i rarely ever noticed this as a problem. I was thinking the problem may be from the muffler and exhaust pipes around the gas tank heating it up too much since I don't have this problem in cooler weather. I plan to have the pipes and muffler near the gas tank wrapped in heat shield material.

Also the charcoal Canister for my vehicle has this little vacuum valve attached to the line coming from the tank. I have modified the valve so that the vapors just go straight into the canister instead of waiting for the solenoid to active it. That has helped.

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Old 10-07-2008, 06:42 PM   #23
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Re: Fuel Tank/ Extreme Pressure after city driving

awesome i will have to try that
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Old 10-08-2008, 09:32 AM   #24
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Re: Fuel Tank/ Extreme Pressure after city driving

Buy a ventilated gas cap and disconnect the solenoid that goes to the charcoal canister. I had the same problem of pressure building up. This seemed to fix. I have had no problems since.
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Old 10-08-2008, 09:32 AM
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