Gas tank pressure?

[cygnusx1]

I posted this several days ago on the fuel delivery threads and got nothing. I wonder if anyone here can answer my question. I hate to be a cross poster but this one should be an easy yes or no question.

 

http://forums.hybridz.org/showthread.php?t=104310

 

.....Surely you guys gas up your Z's once and a while and can tell me if this happens to your Z as well.

 

Thanks

[Guest Loose_Screws]

If the tank has pressure, then the vent line is clogged. Connect a clean fuel hose to the vent line where your R/C filter is and try to blow through it with your mouth (DO NOT USE AIR HOSE!). If you can't blow through it, then it will need to be replaced.

 

For what it is worth, most fuel tanks are not designed to handle very much positive pressure and can rupture. This is why you do not want to try and blow through the line with an air hose (unless you have gas cap off and don't mind the crud backwashing into your tank).

[cygnusx1]

OK good then at least I know there should not be pressure in the tank. I thought that maybe the fuel injection system maintained some kind of pressure in the tank and it was normal. However, it just doesn't seem right to me so I posed the question and I was right to be suspect. I will check that vent line with the gas cap off so it won't build pressure. I know that the original carbon cannister had a diaphram valve to open the vent line in the presence of engine vacuum only. Is that right?

 

Thanks Loose_Screws

[Guest Loose_Screws]

I know that the original carbon cannister had a diaphram valve to open the vent line in the presence of engine vacuum only. Is that right?

 

Thanks Loose_Screws

 

That I don't know. I 'disposed' of my carbon canister sometime ago and vented to the atmosphere and now I am running a fuel cell.

[Guest bastaad525]

I've always wondered about this myself.

 

My Z NEVER does this, and I think it's because I have some leak somewhere, not leaking fluid fuel, but vapors. Any time I take a left turn while applying throttle I get fuel fumes in the car... very annoying.

 

However, my '91 Sentra, and my wife's '94 Geo Prizm, when you open the gas filler cap there is a rush of pressurized air releasing. I'm not sure if it only does it when the tank is low, but that is the only time I ever check I was told this was normal on EFI cars.

 

So with two out of our three cars doing it, I am left wondering, should my Z be doing it as well??

[cygnusx1]

Now I have no idea weather the Z tank should let out a rush of air when opening the filler cap with a low tank... hmmmm

 

Back to square one. Normal or not?

 

Anyone....Anyone.....Bueller?

[spotfitz]

My '77 280Z does it and as far as I can tell it's all stock, except for the locking gas cap I put on shortly after getting it.

My '00 Blazer did it and my '97 Silverado did it.

I thought that because it is not open to atmosphere that as the gas is used that there is a slight vacuum created.

[cygnusx1]

You would think if it were a sealed system, as the gas went down, you would get vacuum....but the fuel pump adds pressure to the rail and the extra pressure bleeds back to the tank where it would build up if it had no opening to atmosphere. I am not sure what the pressure is in the return line after the regulator but I know it's positive pressure and it goes into the tank. So THAT could be the cause of the pressure build up in the tank. The reason the fuel pressure gauge is not affected by tank pressure is that BOTH the return and the supply see the pressure in the tank so they cancel out.

 

Is it SUPPOSED to build and hold pressure or is it SUPPOSED to vent to the great outdoors? This has become a great mystery.

 

Oh another point. Fuel is incompressible so the pressure from the fuel pump does not get to the tank UNLESS there are air bubbles in the fuel lines such as in a low fuel situation.....aha....That may be why you only get the tank pressure on a nearly empty tank!

 

It is also worth noting that the stock fuel cap on my 76 looks like it is designed to let air in to prevent vacuum in the tank.

[Guest bastaad525]

I always figured it was NOT supposed to build up pressure. First reason is, well it's vented. those 3-4 hoses that go from the fuel tank to the vapor tank, and then from there one hose goes up front to the carbs/intake (in stock configuration). So I thought there's no way pressure should be able to build up in the system.

 

Second reason is, I've heard a few times that pressure in the tank can actually cause running problems with the car, which sorta makes sense... wouldn't it be increasing fuel pressure throughout the entire system?

 

So... I always figured NOT having pressure was normal as it made the most sense... then when I got my sentra and it always has pressure when I go to fill the tank, I thought there was something wrong with THAT car, then I find my wife's car does it to... so like you, I'm left not knowing what to think

[Tony D]

My link isn't working but I am questioning if the cap BLOWS OFF, or if you simply HEAR the sound.

Most common is that you hear a sound like escaping pressure. In fact, what you have is vacuum in the tank. If you have hoses that are marginal, it is possible that the car, on long high speed drives, starts bucking and surging, and running lean because of a vacuum buildup in the tank from the fuel being sucked out. On short trips, you have no problems. But on those longe trips it may completely stop the car. I have seen the filler neck COLLAPSED from this phenomenon.

On earlier cars, people usually plug the crankcase accumulation line from the vapor cannister. This is the only way for makeup air to get into the tank.

On the later 260 and 280Z, removal of the carbon cannister and plugging the line is the culprit.

Opening up the line is all that is needed to resstore proper fuel feeding.

 

Pressure in the tank, that is a different story. And can be caused tby the same things....

 

Good Luck. WIsh this damn computer was working so I could get to the symptoms. I have seen this problem three times this summer (and that's saying something since I have only spent 19 Z-Related Days since May!)

 

BTW, the cap is designed to be SEALED. The venting is controlled completely through the line to the crankcase, or through the vacuum valve on the top of the carbon cannister.

 

The fuel pump does NOT add pressure to the system, it only imparts FLOW. The restriction by components in the system downstraam build pressure, as soon as the fuel is allowed to flow unimpeded, the thing will revert to flow, and not cause pressure.

 

To build pressure, you must increase the VOLUME of a given fluid within a space---this is decidedly not the case with fuel in a running car, it DECREASES in volume!

 

now heating of the fuel will cause an increase in pressure, but the aforementioned valves will vent this pressure (some 3 to 11mm of pressure water column I believe) to either the crankcase or carbon cannister when the car is sitting idle and not running, or to the engine for combustion through the air cleaner or other port if it's running.

Hpe this helps, you can e-mail me directly if youwant. This keyboard is being a beyotch right now! LOL

[cygnusx1]

I see. But yet it remains that I get a large rush of air when I open the gas cap on a low tank. I will double check to make sure it is pressure and not vacuum. Now you made me double think my memory. Either way it would be a bad thing and it's most likely a dirty or kinked tank vent line. If I remember correctly, the gas cap has a pin hole in it with a rubber "reed-valve" type arrangement...or at least that what it looks like. I always thought it was there to allow air into the tank to prevent vacuum build up. It's a '76 BTW.

 

With the tank mostly full of air, the fuel will heat up quickly and heat the large volume of air in the tank. That would definately cause pressure with a plugged up vent.

 

Thanks.

[phil280zxt]

My 81zxt has the same venting when the gas cap is removed. I always thought this was normal for cars 20+ years ago.

[bradz240]

I havethe same experience with my '83. Also the fuel pump which is new makes a loud noise on long drives. When i take the fuel cap off, pressure is released and the noise stops. Guess I need to check the purge valve on the carbon canister. If anyone figures this pressure build up out let us know.

[nienberg.11]

Cygnus, my '77 does the exact same thing, and it is definitely NOT vacuum! When the filler cap is removed, it literally rearranges my hair! I have not had any issues with my fuel pump or the car running badly however. My fuel tank vent line is set up just like yours. I have the carbon canister removed and a filter on the end of the vent line. The car did the same thing with all of its lines in a stock configuration also.

[cygnusx1]

Well, there you have it. Even though our vent lines are "open", there is still alot of air/fuel vapors rushing out when you open the gas cap. Apparently, the "vent" lines are not really wide open vents.

[nienberg.11]

I'm looking into buying a small check valve to weld or tap into the gas cap to release this pressure (this idea might have been mentioned earlier in this thread). I'm worried that there might be effects on the car's performance or gas mileage from the pressure buildup.

[cygnusx1]

A check valve in the gas cap would work. You need to find a valve with a very low cracking pressure though. I bet it's a fraction of a psi. Not easy to find a valve for pressures that low. A tiny pinhole drilled into the cap might work even better.

 

I wonder what keeps that vent line to the engine bay from breathing??

[Tony D]

You guys are reinventing the wheel, have you read the applicable FSM portions regarding the pressure limit on the fuel tank? When it's all set up like it's supposed to be, there IS a manometer reading that is applicable to the tank BEFORE the carbon cannister admits fuel, or the crankcase lets those vapors release for storage!

 

By relieveing this pressure, you are DECREASING the ability of your fuel pump to operate at DESIGNED FLOWRATES! The fuel system is designed to take into account the head produced at maximum pressure before venting, and maximum vacuum before letting air be readmitted to the tank.

 

If you alter this in any way, chances are good if you have a functional check of your evaporative emissions system for any sort of emissions compliance, you will fail.

 

You are making a MOUNTIAN out of a Molehill. This is a non-issue!

 

All EVERYONE in this post has done is give SUBJECTIVE OPINION!

 

Put a GAUGE on the gas tank, and MEASURE this "massive pressure" and QUANTIFY it before you start reinventing the wheel!

 

You guys are literally messing with fire? Wh is this such a big issue?

[nienberg.11]

You guys are reinventing the wheel' date=' have you read the applicable FSM portions regarding the pressure limit on the fuel tank? When it's all set up like it's supposed to be, there IS a manometer reading that is applicable to the tank BEFORE the carbon cannister admits fuel, or the crankcase lets those vapors release for storage!

 

By relieveing this pressure, you are DECREASING the ability of your fuel pump to operate at DESIGNED FLOWRATES! The fuel system is designed to take into account the head produced at maximum pressure before venting, and maximum vacuum before letting air be readmitted to the tank.

 

If you alter this in any way, chances are good if you have a functional check of your evaporative emissions system for any sort of emissions compliance, you will fail.

 

You are making a MOUNTIAN out of a Molehill. This is a non-issue!

 

All EVERYONE in this post has done is give SUBJECTIVE OPINION!

 

Put a GAUGE on the gas tank, and MEASURE this "massive pressure" and QUANTIFY it before you start reinventing the wheel!

 

You guys are literally messing with fire? Wh is this such a big issue?[/quote']

 

Easy, killer.

 

First, I have no idea what you meant in that first paragraph. What is a manometer? Since when does the carbon canister "admit fuel?" The last time I checked, all it did was regulate distributor vacuum and feed fuel vapor to the manifold, not that I run mine anymore. What does crankcase pressure have to do with gas tank pressure?

Secondly, I'm not too concerned about a gravity-fed fuel pump's ability to function without positive tank pressure. What concerns me more are inconsistencies in fuel delivery brought on by fluctuations in the tank pressure. If a vent somewhere is blocked, I don't think installing a small check valve to replace it would be "re-inventing the wheel."

Also, there is no E-check in my county, and if there was I'd have serious doubts about the value of leaving 30 yr old emissions controls in place. In my view, it's more worthwhile to reduce emissions by simply tuning the car properly using updated methods (ie Megasquirt) than to attempt to struggle with the ancient jumble of stock emissions controls.

I was concerned about this because it's not normal to be greeted by a blast of compressed gas vapors every time you unscrew your gas cap. I think I will take your advice and measure this pressure when I get the chance. Last I checked, though, the purpose of this forum is to discuss issues like this and their possible repercussions, not to write them off as "molehills."

[zguy36]

A manometer is a pressure sensing device, in which pressure pushes a fluid up a tube (ie, inches of mercury).

 

I also have had issues with pressure in my tank. None of the stock charcoal canister parts were there and many of the vent tubes were gone. With no money at first, I plugged everything off and left the tank sealed. Worked great in the winter, but in the summer, I noticed my tank bulging one day. I vented my gas cap to compensate for this, which worked great unless you corner hard with above 3/4 full tank. I would get tons of gas sloshing out the vent (very bad). So, I unvented the cap and unpluged the line that originally went to the charcoal canister. I do not have e-check here so I don't have to worry about emmisions. I have not had any trouble with this current setup and I get very consistent gas tank pressures. My molehill was a mountain, with gas spewing on the ground, or bulging tanks due to pressure.