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Old 04-11-2010, 11:27 PM   #1
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Fuel Tank Pressure

I noticed the fuel tank has pressure when I remove the gas cap (TPI). Does anyone elses do this?

Is it a problem with those two valves near the charcoal canister because when I was changing my vacuum lines I noticed one held pressure near the cansiter?
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Old 04-11-2010, 11:41 PM   #2
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Re: Fuel Tank Pressure

Quote:
Originally Posted by 88formula305tpi View Post
I noticed the fuel tank has pressure when I remove the gas cap (TPI). Does anyone elses do this?

Is it a problem with those two valves near the charcoal canister because when I was changing my vacuum lines I noticed one held pressure near the cansiter?
I have the same problem. Lots of pressure..also when the engine gets really hot, the tank starts humming. Also, strong fuel smell.
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Old 04-12-2010, 12:06 AM   #3
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Re: Fuel Tank Pressure

Mine has the same problem, sometimes when you sit in the car for a while when its off you can hear banging and popping sounds coming from the gas tank as well, but I never have wondered what it was enough to start looking into it...

Everytime I take the gas cap off it starts to blow air out, and the more you open it the faster it comes out and sometimes you can hear the tank pop when its released all the pressure.
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Old 04-12-2010, 08:31 PM   #4
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Re: Fuel Tank Pressure

the ecm is supposed to purge the tank when the engine warms up i think. I replaced my vacuum and evap lines going to the canister and those two valves near the canister and I am going to fill it up in a couple hours and see if it has pressure. I think if that doesnt purge.

I will check my canister filter to see if it is plugged and try blowing though the purge valve on the canister with and without vaccum supplyed to the top port. The haynes manual describes the system poorly but I am familiar with newer NVLD, LDP and ESIM systems.

Im wondering if the car runs lean or rich if it doesnt purge correctly?

IDK maybe its normal?
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Old 04-13-2010, 12:42 AM   #5
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Re: Fuel Tank Pressure

Quote:
Originally Posted by 88formula305tpi View Post
the ecm is supposed to purge the tank when the engine warms up i think. I replaced my vacuum and evap lines going to the canister and those two valves near the canister and I am going to fill it up in a couple hours and see if it has pressure. I think if that doesnt purge.

I will check my canister filter to see if it is plugged and try blowing though the purge valve on the canister with and without vaccum supplyed to the top port. The haynes manual describes the system poorly but I am familiar with newer NVLD, LDP and ESIM systems.

Im wondering if the car runs lean or rich if it doesnt purge correctly?

IDK maybe its normal?
Your guess is as good as mine, I have NO idea.
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Old 04-13-2010, 12:45 AM   #6
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Re: Fuel Tank Pressure

There are other threads about this problem but no certain fix yet. I will probably bring it in to chevrolet one of these days.
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Old 04-13-2010, 08:01 PM   #7
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Re: Fuel Tank Pressure

I have the same problem, but can't figure out how to fix it.

Is it just TPI owners facing this prob, or is there others?

Would it have anything to do with the gas cap? Vapor canister? The plug under the fuel tank (read that in another thread with similar issues).
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Old 04-13-2010, 10:13 PM   #8
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Re: Fuel Tank Pressure

Quote:
Originally Posted by Samurai_Of_Dawn View Post
Is it just TPI owners facing this prob, or is there others?
I have a V6
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Old 04-14-2010, 01:48 AM   #9
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Re: Fuel Tank Pressure

I replaced most of my vacuum lines and purge lines near the canistor that were very dry and cracked. Hasnt done it since and seems to run rich now but has more power? I am going to check tommorow if it is warm because I think it happened worse when hot and It has been cold the past couple days. Ill post back if it still does it or not.
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Old 04-15-2010, 02:00 AM   #10
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Re: Fuel Tank Pressure

Cool man, keep us informed.

I wanna say it's the gas cap, but I doubt it's the prob. I heard it's "normal" for cars like these to make that noise, but it does make me wonder about that thought.
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Old 04-15-2010, 10:42 AM   #11
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Re: Fuel Tank Pressure

I'm having this problem too. For me it is especially bad on hot days, like over 90. I put up a thread about this a week or so ago. I've also done lots of searching about this problem and it seems to be fairly common but nobody has been able to come up with a definate solution. Here is what I've done so far to try to correct the problem:

1. Replaced gas cap with vented cap
2. Replaced all lines and valves going to charcoal canister
3. Replaced purge solenoid on charcoal canister
4. Replaced white vent valve on drivers side near fuel tank
5. Disconnected vapor line at charcoal canister and tank and blew out with compressed air

I just did all this stuff last weekend but I can't really tell if anything has worked so far because it's been about 20 degrees colder this week. I may also try to put some kind of heat shield over my feul lines that run next to the header on the drivers side. As a last resort I will have to pull the fuel pump and make sure no lines are clogged inside the tank.

If anyone is able to solve this one PLEASE keep us posted. This problem is driving me crazy.
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Old 04-15-2010, 10:46 AM   #12
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Re: Fuel Tank Pressure

Here are some other threads about this problem in case they might help anyone.

http://www.thirdgen.org/techboard/tp...ump-whine.html

http://www.thirdgen.org/techboard/tp...iling-wtf.html
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Old 04-15-2010, 11:06 AM   #13
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Re: Fuel Tank Pressure

buy a vented gas cap or disconnect the end of the charcoal filter on the non-tank side this will allow the tank to vent but still might have a gas smell.
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Old 04-15-2010, 11:28 AM   #14
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Re: Fuel Tank Pressure

Even though it is an annoying problem, I've noticed that I get better gas mileage when theres pressure in my tank. Everytime that I take the gas cap off to refill and theres a pressure release, I got about 30-50 more miles out of my tank versus when I take it off and there isn't a pressure release, but then again I have a V6, and most of you guys consider this to be a TPI problem.
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Old 04-15-2010, 11:50 AM   #15
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Re: Fuel Tank Pressure

The problem is that the exessive pressure can vapor lock your fuel pump causing your car to stall wherever you happen to be at the time. I have yet to have that happen but I'm sure whatever it is about the pressure that causes my fuel pump to whine really loud isn't good for it. Also, I can actually hear the fuel boiling inside my tank. It's probably a good thing I don't smoke.

Hey Fireinme17: Out of curiosity what fuel pressure do the V6 engines run? Do they have a fuel pressure regulator similar to the ones on the tpi fuel rail? I haven't ruled out my fuel pressure regulator as the problem.
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Old 04-15-2010, 02:46 PM   #16
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Re: Fuel Tank Pressure

Quote:
Originally Posted by 88gta_hiflyer View Post
Hey Fireinme17: Out of curiosity what fuel pressure do the V6 engines run? Do they have a fuel pressure regulator similar to the ones on the tpi fuel rail? I haven't ruled out my fuel pressure regulator as the problem.
Actually I haven't looked into it, nor have I had mine tested. I could find out for you though.
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Old 04-15-2010, 10:51 PM   #17
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Re: Fuel Tank Pressure

88formula: Keep us updated man. Hopefully all that replacing did something for your ride.

It's interesting that nobody has the definite cure for these cars.

I just purchased a venting gas cap for the car. Gonna drive it this weekend and see if it makes a difference. 57kid said they are illegal, but I think it might be where he lives, cali, and some other restrictive states. I'm not 100% on if it's legal/illegal in AZ, but out here it's not too restricted like cali is.
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Old 04-16-2010, 07:54 AM   #18
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Re: Fuel Tank Pressure

Quote:
Originally Posted by FireInMe17 View Post
Actually I haven't looked into it, nor have I had mine tested. I could find out for you though.
No worries I was just curious because I read that this problem can be caused by a bad pressure regulator or clogged return line and I didn't know if V6 cars ran as high of fuel pressure as tpi's.

Anyway, I took the car for a long drive yesterday and it was pretty hot. I never heard the fuel pump whine and the gas wasn't boiling when I stopped but there was still a lot of pressure in the tank when I loosened the gas cap. I think the vented cap helped more than anything and like I said before, none of the stuff I replaced appeared to be bad.

Even though the vented cap seems to help I want to continue to try to figure this problem out. The car did not come from the factory with a vented cap and it ran fine. I assume the fuel was not boiling in the tank when the original buyer brought it back from a test drive. That tells me something is not right if it's doing that now. I need to get a pressure gauge to check my fpr and check for a clogged return line. If that checks out I'll pull the pump.

Keep us posted if anyone else has any luck with this.
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Old 04-17-2010, 10:04 PM   #19
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Re: Fuel Tank Pressure

Hello.my 91 Camaro RS does the same thing.major pressure in the tank on warm/hot days.I had a chevy celebrity acted same way,till the fuel tank cracked from the pressure.
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Old 04-18-2010, 01:16 AM   #20
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Re: Fuel Tank Pressure

FYI - Modern fuel is formulated to raise it's vapor point.
Fuel injected cars do not vapor lock, only carburetors with mechanical fuel pumps suffer from vapor lock. Modern carb'd cars do not suffer from it either because the fuel pump is in the fuel tank where it's nice and cool and keep the line pressurized even at idle unlike mechanical pumps.

Having said that getting the fuel lines in the engine compartment next to the headers so that they get hot enough to boil the fuel will cause vapor lock, but why would you want to do that?
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Old 04-18-2010, 05:05 PM   #21
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Re: Fuel Tank Pressure

It's not a TPI problem, My carbed camaro does the same thing.
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Old 04-18-2010, 09:42 PM   #22
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Re: Fuel Tank Pressure

It is normal for a lot of "air" to be released upon opening the gas cap. On a hot summer day after driving for while, removing the cap for refueling will vent quite a bit of volume. Figure if the tank is near empty that is 12 - 13 gallons of "air" at a little under 1 psi of pressure.

Here is info on tank pressure that I copy & pasted from another post I made:

The vapor pressure control valve (CCP system) prevents the tank from pushing vapors into the canister until the tank pressure exceeds 5 KPa (0.72 psi).

The tank vent (honk valve) is there as a safety for excess pressure, and to allow air ingress as the fuel is consumed (2-way valve).

With tank pressure it will vent at 5.5 - 7.6 KPa (0.8 - 1.1 psi).

Under vacuum it will allow air ingress between 0.88" - 1.9" Hg.

There are two types of canisters in use. A solid bottom and an open (filtered) bottom. The solid bottom types have a vent on top marked AIR. This is to be open to the atmosphere.

Note that while in a purge cycle the ECM checks the BLM to be sure that it doesn't drop too low. If it does the ECM will back off the purge volume. The purge is controlled via PWM of the solenoid.

The pressure information listed above is from the '92 FSM.

:end of copy from other post:

Note that the charcoal canister is open to the atmosphere. Either through the bottom filter on the older ones, or the air vent on the newer ones. The canister will never have any pressure in it.

If the vapor pressure control valve (CCP system) is installed backwards then the tank can not vent to the canister. The tank vent valve will then open to the atmosphere once the pressure builds past it's limit.

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Old 04-18-2010, 11:25 PM   #23
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Re: Fuel Tank Pressure

That didnt fix anything besides a vacuum leak the tank still has pressure when the cap is removed even when it is cooler.

RBob's post makes sense, 1 psi of pressure probally sounds like more when the tank is near empty and there is more air to put under pressure and release when removing the cap.

I guess it sounds like more pressure than there actually is in other words.

Never had this happen to my other vehicles but they are different evap systems.

By the way which valve is the vapor pressure controll valve, I see two near the canister that look identical and the purge is on the canister if im correct?

Last edited by 88formula305tpi; 04-18-2010 at 11:29 PM.
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Old 04-19-2010, 12:10 PM   #24
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Re: Fuel Tank Pressure

RBob thanks for the info. I didn't realize that the charcoal canister vents all the time through the air vent but you are right. I checked my canister and it is not clogged. I can easily blow air through it and feel it coming out the air vent. That means my tank is venting, I've checked that entire line and even blew it out with an air compressor.

So this brings me back to the question, why is the fuel boiling inside my tank on hot days?

I was hoping it was something as simple as a bad valve or a clog in the vent line but all those things check out. I still need to check my fpr with a gauge to make sure it is working properly and also to check for a clog in the fuel return line. Other than that I'm about out of ideas unless the fuel really is just being heated too much from the headers. Seems like if that was the case everyone would have the problem though.
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Old 04-19-2010, 01:31 PM   #25
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Re: Fuel Tank Pressure

Picture is from '92 FSM for TPI. The vapor pressure control valve being the round one near the canister with the Y shaped inlets/outlets. The purge solenoid being mounted on the canister itself.

Quote:
Originally Posted by 88gta_hiflyer View Post
So this brings me back to the question, why is the fuel boiling inside my tank on hot days?
There are several threads on this where others have had the same issue. IIRC, some of the problems were due to the muffler heat shield missing. The fuel lines running close to the headers can also be an issue. Sometimes I wonder if the problem is from the fuel. That it has a too low of a boiling point.

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Old 05-06-2010, 12:53 PM   #26
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Re: Fuel Tank Pressure

the other day was around 70 out and I took the cap off and there was very little pressure. I think it happens worse at less than 1/2 tank.
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Old 06-11-2010, 09:56 AM   #27
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Re: Fuel Tank Pressure

Have the same problem on my 87 Camaro. I have a 305 TPI as well but I converted it from LG4 carb to TPI last winter.

The car is running fine but have this pressure problems in the tank like you do.
I don't even have the charcoal canister function so the hose that goes from the gas tank goes directly out in the atmosphere.

Still I get this pressure inside the tank..
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Old 06-11-2010, 10:54 AM   #28
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Re: Fuel Tank Pressure

when i had my intake apart I noticed gas was dripping out quite a lot and the tank had a lot of pressure I changed the valve at the canister and I get no pressure when i open the tank even on real hot days.
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Old 09-10-2010, 11:55 PM   #29
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Re: Fuel Tank Pressure

I Have an 1988 gta changed the pressure valve in front of canister and the one near the TB. There is some pressure in tank the valve opens two ways 1 when the canister purge valve opens and opens it. 2 when tank pressure gets high gas fumes are sent to canister. Brob gave the numbers above in a post. My tank would vent forever when cap removed. I replaced both valves everything seem good!!
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Old 09-11-2010, 04:44 AM   #30
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Re: Fuel Tank Pressure

I solved this problem a while back, the gas tank popping and such, by replacing the white valve up near the tank in the rear. For a 91-92, the part number is 10033125. It wasn't the cheapest thing in the world, but it solved my particular problem. The part number may be the same for previous years, but I wouldn't really know.
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Old 09-17-2010, 12:43 PM   #31
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Re: Fuel Tank Pressure

What does it mean if gas is pouring out the bottom of my vapor canister through the filter?
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Old 01-19-2012, 10:53 AM   #32
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Re: Fuel Tank Pressure

Quote:
Originally Posted by RBob View Post
It is normal for a lot of "air" to be released upon opening the gas cap. On a hot summer day after driving for while, removing the cap for refueling will vent quite a bit of volume. Figure if the tank is near empty that is 12 - 13 gallons of "air" at a little under 1 psi of pressure.

Here is info on tank pressure that I copy & pasted from another post I made:

The vapor pressure control valve (CCP system) prevents the tank from pushing vapors into the canister until the tank pressure exceeds 5 KPa (0.72 psi).

The tank vent (honk valve) is there as a safety for excess pressure, and to allow air ingress as the fuel is consumed (2-way valve).

With tank pressure it will vent at 5.5 - 7.6 KPa (0.8 - 1.1 psi).

Under vacuum it will allow air ingress between 0.88" - 1.9" Hg.

There are two types of canisters in use. A solid bottom and an open (filtered) bottom. The solid bottom types have a vent on top marked AIR. This is to be open to the atmosphere.

Note that while in a purge cycle the ECM checks the BLM to be sure that it doesn't drop too low. If it does the ECM will back off the purge volume. The purge is controlled via PWM of the solenoid.

The pressure information listed above is from the '92 FSM.

:end of copy from other post:

Note that the charcoal canister is open to the atmosphere. Either through the bottom filter on the older ones, or the air vent on the newer ones. The canister will never have any pressure in it.

If the vapor pressure control valve (CCP system) is installed backwards then the tank can not vent to the canister. The tank vent valve will then open to the atmosphere once the pressure builds past it's limit.

RBob.
So, why do i feel more torque in my car with using the regular gas cap? Mine is a 91' Tpi car, but last night i tried a vented gas cap and while it did accelerate smoother and more consistently , it had more torque with the regular locking gas cap i had on there before.

Last edited by ninetyone; 03-03-2012 at 02:02 PM.
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Old 03-03-2012, 11:21 AM   #33
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Re: Fuel Tank Pressure

I've been looking for that white valve near the gas tank and gm parts store is no help would you have the part numbers and or where you got that valve ? i tried the part number below but GM didn't know what it was. please help
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Old 03-03-2012, 04:41 PM   #34
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Re: Fuel Tank Pressure

Try gmpartsdirect or newgmparts. Toss the number in Google and hit every promising link. I just found a similar one on ebay under "camaro fuel valve" but it's the wrong color and may be for a 2nd gen. Could try it it cause it's just a vent valve. I can't imagine that it'd be different, but who knows.
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Old 03-03-2012, 06:37 PM   #35
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Re: Fuel Tank Pressure

watching thread
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Old 03-12-2012, 10:59 PM   #36
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Re: Fuel Tank Pressure

I had the same problem with my 87 camaro (LG4) about 5 years ago. Turns out a dirt dauber had decided to build it's nest in my vent line. Cleaned that out and I haven't had any problems with the tank building up excess pressure.
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Old 03-21-2012, 01:31 AM   #37
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Re: Fuel Tank Pressure

My vent was bad , went to wreaking yard and within 10 min had five from camaros and firebirds for $5 three worked two were bad. It seems to have taken care of the gas smell . I put mine above the tank next to the filler neck.
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Old 09-04-2013, 10:18 AM   #38
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Re: Fuel Tank Pressure

I have a 92 trans am and it won't start without the fuel cap loose
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Old 09-07-2013, 01:02 AM   #39
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Re: Fuel Tank Pressure

my 91 rs all it dose is when it gets to 3/4 tank full it wont let me fill it up no more its air locking the tank who has a fix
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Old 09-08-2013, 11:19 PM   #40
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Re: Fuel Tank Pressure

Quote:
Originally Posted by 91rs-t-tops View Post
my 91 rs all it dose is when it gets to 3/4 tank full it wont let me fill it up no more its air locking the tank who has a fix
Are you sure your fuel gauge is right or your sending unit is registering correctly. Sometimes after years of people trying to over fill the fuel tank the sending unit starts to incorrectly read the fuel. I can't see air being able to be trapped with only 3/4 of the tank filled. The filler neck is close to the top of the tank, So I would guess the sending unit.
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Old 09-08-2013, 11:38 PM   #41
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Re: Fuel Tank Pressure

The fuel system from tank to injectors is new its trapping air it will bible down after a few miniutes
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Old 09-09-2013, 09:46 AM   #42
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Re: Fuel Tank Pressure

Quote:
Originally Posted by 91rs-t-tops View Post
my 91 rs all it dose is when it gets to 3/4 tank full it wont let me fill it up no more its air locking the tank who has a fix
Check the length of the filler neck, see if it has been cut and spliced back together with a piece of hose. If so need a new tank with a stock un-cut filler neck.

RBob.
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Old 09-09-2013, 10:40 AM   #43
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Re: Fuel Tank Pressure

Tank is like new. It's got me beat
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Old 09-10-2013, 12:40 AM   #44
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Re: Fuel Tank Pressure

Quote:
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The fuel system from tank to injectors is new its trapping air it will bible down after a few miniutes
I don't quiet remember but I believe there should be a vent tube in the filler neck that allows air to be vented up the filler neck when being filled. Some aftermarket fuel tanks did not but these in the tanks. and since you have a new tank I'm thinking maybe this is the problem.
When this happens, How much more fuel can you add to make a full tank?
And how many gallon of fuel does your tank hold? Yes I do know how many it should, I'm just trying to figure out the problem and find a solution.
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Old 09-10-2013, 07:58 AM   #45
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Re: Fuel Tank Pressure

I can add about 2 to 3 more gallons very slowly and it will go full I'm talking gas can and tranny funnel at that point
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Old 09-10-2013, 01:33 PM   #46
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Re: Fuel Tank Pressure

Well its one of two things at this point, Either there is no vent tube or clogged vent tube in the filler neck, Or the sending unit may be bad or got bent causing the fuel gauge to be off, 2 to 3 gallons of fuel really isn't that much,
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Old 09-10-2013, 01:33 PM
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