okfoz

No, I am not talking about Megamaid.

So not to mess with the actual intent of the question, I want ONLY CARS WITH TPI information.

What I am talking about is the gas cap... Specifically when you release the cap, does it let air in, or let air out, even after sitting for a time? This came up because I have 3 cars, one seemed to release air, the other two were definitely sucking as I watched the fuel tank filler move, and I heard the tank pop like it was buckled when I released the cap...

Scott Moyer said his released air even after not being run for a few weeks. I know one of mine actually sucked air after an entire year of not being driven (long story don't hate me)

Thanks...

John

jimdutro

iTrader: (1)

Blow yes suck no. My `88 Iroc was having issues with a very strong gas smell outside the car after a long drive on a hot day. I would remove the gas cap and gas would spray out under high pressure all over me when I pushed in on the restrictor plate. I replaced the sending unit in the tank which has a gas tank pressure valve/switch and also replaced the vacuum canister because it literally fell apart. Problem solved. Now when I remove the cap there is no pressure or sucking, nothing. Car runs perfect.

lonestar7

iTrader: (1)

Mine builds some pressure in the tank if you drive for a while, especially on hot days. No pressure or vacuum after the car sits for a while or over night.

okfoz

Thank you,
What I am going to start with (the cheep stuff) is the fuel cap.

Went to Autozone and they had this cheep looking cap, I actually purchased a car with one of these on it when I got the car. It does not screw in correctly, it was like the threads were the wrong size. I found some real GM caps for $7.10 each (the kicker was the shipping of $12.95) So I got 3, so I will be paying about $12.00 for them each...

one thing I noticed is the one original GM cap I have (at least I think it is) was basically a cap, there was nothing internal, no valves, no nothing. Two of the caps I have (on the problem cars) have some sort of valve system on them... Not sure if this was the problem or not, but I figure I can go less wrong with OE GM...

NOW, when you buy a new cap for your car, do not let the parts guy @ Autozone tell you that different 3rd gen engines had different caps, that is bogus information, I did a check in the parts catalogs and they ALL used the same cap 10164142.

John

T/Aperformance

The fuel tank should not have a vacuum in it at all. if anything there should be pressure. This problem is not the gas cap, There should be a one way vent on your fuel tank, It will be located on the driver side in front of the tank, This valve lets air into the the tank when fuel level goes down.

92GTA

iTrader: (11)

Mine has only ever blown it's entire life. Still does a little. On hot summer days after a long drive it's allot.

How's this History? lol, just sayin :P

82tarecaro

iTrader: (4)

My tank used to build up pressure. I took out the vent valve located where the rubber lines get connected to the hard lines on the body of the car. and cleaned it out with some brake cleaner spray after trying to find a replacement for months. It no longer builds up pressure and seems to be working properly. Just an FYI - that valve is a 2 way vent. So if you are collapsing your tank there is a possibility that it is malfunctioning and causing this problem as well.

okfoz

I actually thought of that, or the Charcoal canister may be clogged. I am not sure exactly how it works, but someone told me that the Charcoal canister takes the fumes from the tank and cycles them into the intake, the valve will purge the system and push air back into the tank, this does a few things, 1) it cleans the charcoal, and 2) it allows the tank not to create a vacuum.

Had a friend with a plastic tank on an early Pontiac Trans Sport (The ones that looked like a dustbuster) and as he would drive it would just turn off, stop running, let it sit for a short time and it would fire right back up. One day he got gas and when he released his filler the tank popped and it got him thinking, so the next time the car stopped he looked at the tank and sure enough it was collapsed, it had collapsed so much it stopped the pump from working. I forget what he did to fix it tho...

Thanks

okfoz

Are you referring to the little valve by the rear axle? I want to think they are white or black in color, and they clip onto the same bracket that the fuel lines do over on the Drivers side as T/A performance reminded me.

rusty vango

mine blows, I actually have to leave mine loose at the moment.as im chasing canister/purge valve issues as well

okfoz

Really as I think about this, obviously as you remove fuel from the tank, in a sealed system it would slowly evacuate the tank creating a vacuum to get the condition on my two cars. The return line from the unused fuel in from the fuel rail should be sucked back to the tank (from the vacuum) not sure how much force would be placed on it. The engine creates vacuum, obviously as the pistons suck air into the chambers thru the plenum, Add to that the added ventury effects that take place within the system.

So we know there is plenty of vacuum, one of the things that strikes me is both of these cars with the sucking problem have a "rough" idle. They idle ok, but not smooth like I would want, One car has a previous owner 350 installed, I just figured it was an unbalanced engine or something... I am really starting to think it is almost like there is a vacuum leak or something, PCV valve or a broken line...

Furthermore, in order for pressure to be forced into the tank there almost has to be some sort of a pump somewhere pumping air into the tank...

Also I think if you remove the valve from the gas tank (by the rear axle) it will leak fuel if you fill the tank...

lakeport

I have a 88 305 TPI, and after a long drive on a hot day if i open the gas cap i get a swoosh of air coming out, even on short trips i get a little air blowing out

If i park it after driving for a bit, i can hear the air coming out of the release valve, had to clean it as it was tarred up, but all clean and works now

I am not sure if this air blowing out is normal or not, but happens to me

lonestar7

iTrader: (1)

I think some pressure build up is normal. Every vehicle I have owned whether gas or diesel has had pressure in the tank after it has run for a while. My Formula 350 was building a lot of pressure in the tank when I first got it, so much that you could hear it hissing out of the gas cap after a long drive in hot weather. I went through all the old postings about excess pressure in the tank. Seems like the problem is most likely related to the tank vapor/vapor canister system. I went through the vapor canister system testing, replaced suspect parts, still had the problem. After some more research, I decided that the line from the tank to the canister was clogged or something was plugged up in the sender /pump holder assembly that goes in the tank. Just as an experiment, I pulled the hose from the tank off the canister an feed 2~3oz of sea foam in the hose and and forced it back with compressed air. After driving it for a while, I noticed the pressure build up was much reduced on hot days, but it will still build up some pressure. I'm going to replace my old fuel pump soon, and I'm going to replace the the sender/holder assembly with new pipes at the same time. I think that may fix my problem.

BADNBLK

iTrader: (11)

Blows... Both of them

82tarecaro

iTrader: (4)

This little bugger. It seems to create issues in 4th gen cars as well because I found this over on an LS1 website.
It is located to the left of the gas tank behind the wheel.
Try taking it out and cleaning it up a bit. It worked for me. No guarantees, but it is a cheap fix.
The system is designed to be a sealed system so the problem should not involve your gas cap.

82tarecaro

iTrader: (4)

BTW - be careful if you decide to take it off. The prongs on it break easily. Slip a socket over it to compress the prongs so you don't break one of them.

82tarecaro

iTrader: (4)

That valve is 2 way so not only is it a vent for pressure build up, it is also supposed to let air into the tank as it is displaced of fuel. Not sure if the tank could collapse if the valve was not working, but it could probably create a vacuum in the system.

lonestar7

iTrader: (1)

I tracked down a new, old stock AC Delco vent valve and replaced the old one on mine when I was first working on my pressure problem. It made no difference for my car, which make sense to me now because the valve is only supposed to let air in the tank if it develops a vacuum. It's not supposed to let gas fumes out if the tank is pressurized.

82tarecaro

iTrader: (4)

No - it is a 2 way valve. It also lets excess pressure out of the tank.
Here is a quote from RBob concerning the issue in this thread:
https://www.thirdgen.org/forums/tech...-pressure.html


__________________

okfoz

Actually that was my thinking, it would not make sense for it to allow pressure and vacuum to balance the system, that would basically make it an open tube... possibly the charcoal canister allows for pressure release?

But as per the post above, apparently not

John

82tarecaro

iTrader: (4)

It is not an open tube. It has 2 valves built into it. If you take it off and try to blow through it you will see what I mean. It takes some effort to overcome the valve either way you try to get air through it.

As you stated - it is normal for some pressure to build up. I had excess pressure. Once I cleaned that valve up I did not have the same build up of pressure any more. Just a slight hiss when I fuel up on a hot day after driving her around a bit. I addressed the canister issues as well. I replaced all fuel lines, the canister filter and made sure the 2 one way valves by the canister were working properly. But i believe once you shut the car off, the tank will only vent through that 2 way valve by the tank. It is normal to hear the valve honking on a hot day after driving the car around a bit and then letting the car sit.

okfoz

The Valve part number

87 LG4, L69 & L98, 10029752
85-87 LB9 10033125

88 - 92 All V8 10033125
89 LC2 (TTA) 10033125

85-92 V6 LB8 & LH0 10033124

82tarecaro

iTrader: (4)

That valve is tough to find. I tried with no luck. I had resigned myself to finding a used one. It is worth a try to clean it out and see if it works for you. You could also totally remove it and see if you have any vacuum sucking back into the tank with it off. Your car will stink like gas all the time without it though.

T/Aperformance

A couple of things. First your fuel tank is supposed to have pressure. It is designed to hold pressure. Second if you cleaned the one way vent like you said it is no longer working as it is deigned to. This valve is only to allow air into the tank You FYI is wrong. One of the emission test that I used to do was a pressure test on the fuel tank . The fuel tank must hold pressure. If there was a drop in pressure You car would fail the emission test, The valve is a one way vent not a two way vent..
What keeps the fuel tank from collapsing is the purge valve on the charcoal canister, Depending on the year of the car it is controlled by engine vacuum or engine vacuum and the computer.

T/Aperformance

Pressure build up easily in our tanks from heat. remember the biggest heat source right next to our fuel tank (our exhaust, muffler) . And it is very easy to build pressure with gasoline by its own nature.. Also you have fuel being sent back to the tank from the engine. This fuel is heated sitting on top of the engine in the fuel line before it makes its return to the fuel tank.
There is no air being forced into the tank or any of pump beide the fuel pump. Pressure is good to have in our tanks, It helps with the fuel pump as it is designed to work with pressure, And if the fuel tank does not have some pressure in it the pump can fail before sooner. There is also a point that pressure in the fuel tank helps the gasoline keeps it from boiling at a certain temp. If a vacuum is created in the tank It lowers the temp at witch gas or any fluid will boil.

Yes if the valve is removed it can leak fuel.With it removed you have opened up a sealed fuel system.

T/Aperformance

The only problem I have with this is that when I was doing emission inspection back in 1987 threw 94 in Both the state of GA and in MO. We had to do a pressure test on the fuel tank. We had to pressurize the fuel tank to 15 psi and it had to hold that pressure, I had to leave the gauge on while I did the other visual inspection to make sure all emission equipment was on the car. If the pressure dropped at all the car would fail its inspection.

82tarecaro

iTrader: (4)

It's a 2 way valve as stated in the GM repair manual. My valve works fine now. It builds up pressure - just not excessive pressure.
The purge valve opens to allow unburned hydrocarbons to vent into the intake manifold. This happens when you are driving. The purge valve is not there to vent pressure in the fuel tank.

JT

iTrader: (1)

The GM factory repair manual used by the dealerships does state that valve has two functions:

82tarecaro

iTrader: (4)

Thanks JT.

okfoz

Oddly enough, I have determined that all of my cars actually create a vacuum, not the 2 & 1 as I thought originally... How I determined vaccuum or pressure was pretty scientific, it was after years of research that I came up with my sniffer test (ok, fine it was 5 seconds). My theory was, if the tank had Pressure, when the pressure was released at the filler it would give off a fuel smell. If it was under vacuum it would not give off smells. So Scientifically, and methodically I opened the first gas cap, and gave it a whiff, nothing, same with second, and third... I could hear the hiss, so I know air was moving.

1) I heard a hiss
2) I did not smell fuel
3) I did not get high.

Conclusion, they all suck, not blow (in my case)

All of these have been sitting for around an equal amount of time, they were all cold at the time of the experiment, it was in the evening to where the cars were cooling from the night air. Two I could hear the tank thump and expand.

One thing to consider, 15psi on 15 gallons of volume.

A Gallon has 231 cu in of volume,
The total volume of a 15 gallon tank is 3,465 cu in.

Because I do not know the exact dimensions off the top of my head, The smallest amount of area that 15 gallons, or 3465 cu in would be in a shape of a sphere.

First I have to calculate the radius from Volume ( Volume = ⁴⁄₃πr³ )

So 3,465 = ⁴⁄₃ * 3.14 * r³
3,465 = 4.189 * r³
827.2 = r³
r = 9.38 (Seemed awful small to me too, checked the math)

The calculation for a sphere is Area =4π(r^2)

or A = 4 * 3.1415927 (9.38 squared)
or Area = 1,106 squared inches

So by putting 15 psi, you essentially put 15* 1106 or 16,584.6 total pounds of pressure on the tank

Think about that for a while...

John

navy02ws6

SHE'S GONE FROM SUCK TO BLOW!!

Sorry, couldn't resist.

scottmoyer

No. Based on what John is saying, his cars still suck! He should buy a Chevy!

okfoz

I am not sure that the alternative of having my car "Blow" is much better...

82tarecaro

iTrader: (4)

LOL

okfoz

LOL, I can tell that some of us are more cultured than others, I was hoping someone would pick up on that. I was going to watch that movie with my 9 year old, then I caught it on TV... and thought again, he can wait a few more years.