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Gas tank has alot of vaccum????

shady

shady

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Is it normal for a gas tank to have ALOT of vaccum? when I pull the cap on my burb it sucks in so hard it opens the cover flap for at least a full second sometimes 2. I've had cars with a little pressure or so before(assumed it was pressure), but by the time the cap was unscrewed it was all over. I have time to unscrew it, and set it on top of the pump before the tank evens out:dunno: It always runs fine and I put a new cap on already with no change.

I just didnt know if this is normal so figured I'd ask.
 
it should have a vent tube to help with that. Mine on the other hand has zero vacume when I open the cap...
 
EVAP canister is your tank vent, assuming this is either one of your '88's. Try sucking in on your gas cap (the underside) I can't recall if the non-emissions caps allow air to be drawn in through there, or if EVAP handles all tank breathing.
 
Doesn't the evap canister have a filter in it that can plug up?
 
Mine sucks air for a lot longer than yours if I've been driving a long time and the tank is close to empty. Mine goes for several seconds......:dunno:

I just assumed this was normal for the TBI engine.
 
It's got a charcoal "filter" at the bottom that is supposed to be changed, so I assume yes it's possible to plug up the canister, whether via the charcoal piece, or even mud, wasps, etc.

Unfortunately when I put my truck together I didn't install an EVAP canister, so I don't have intimate knowledge of how they work internally. I think they are a simple hollow container inside, the charcoal to catch liquid fuel that might make its way up there.

Since I don't have an EVAP, I installed a breather. A wasp made a nest in my front diff breather, which I failed to put some sort of valve/filter on. If the tank vent is simply an open line, I'd make sure the end of it is not plugged. If EVAP, I'd make sure the line to the tank was clear.

If someone knows better than I, wish they'd post up. :(

I would think though, that if EVAP was purging correctly, even with a plugged canister the line to the carb/tbi would provide the necessary relief of pressure/vacuum.
 
dyeager535 said:
EVAP canister is your tank vent, assuming this is either one of your '88's. Try sucking in on your gas cap (the underside) I can't recall if the non-emissions caps allow air to be drawn in through there, or if EVAP handles all tank breathing.
Click to expand...

the system has a non vented cap as part of the emissions evap. in Ca they test the to make sure you have a non vented cap and that the tank will hold 10 lbs of pressure. They clamp the line at the canister and pressure the tank to test. If it does not hold pressure you fail smog.:eek1:
 
But do they test for vacuum? I knew the non-vented truck caps were rated for pretty high pressure, believe it was 15 or 16PSI that I saw somewhere.
 
I really feel for you guys in California… what a complete hell hole that state is.
 
yeah this is on my burb


I have the canister. it went through the fire but looked fine. just burned the hoses off basically. I hooked up the lines to the only way seemed possible. maybe i got them wrong:dunno: i'll try to get a pic but my back kinda decided to start spazzing on me so......... its possible there plugged too do to the fire and melted chunks / hose getting in there.
 
dyeager535 said:
But do they test for vacuum? I knew the non-vented truck caps were rated for pretty high pressure, believe it was 15 or 16PSI that I saw somewhere.
Click to expand...


They just put it on a machine that attaches in place of the gas cap and clamp the vent at the canister. Its pressures and has to hold it for a specified time.
The smog year trucks are not suppose to have a VENTED CAP. The first year (2008) my blazer failed smog for the first time ever because of this new test. This test in OBD2 vehicles the computer does this test. they also charge extra to preform this test on older vehicles.
 
Yes, but vent and vacuum are two separate functions. Vented the fumes would be able to escape the tank through the cap, and those haven't been used for quite some time, at least on light duty cars and trucks. If the cap allows air to enter the tank to it would not allow a vacuum to form.

The stant locking cap on my '87 (I didn't install it!) is actually vented and allows fresh air to be drawn through. The non-locking on my '85 is not vented, but does allow fresh air in.

If the cap isn't vented AND doesn't allow air back into the tank, then the restriction to airflow is somewhere else, or its the wrong cap.

I remember searching for vented caps when I put my truck together without the EVAP canister. I couldn't find any applications with a vented cap, even any of the trucks in '73, so I think there are some mis-applied gas caps for these trucks.

FWIW, the '91 R/V service manual says a collapsed tank (extreme vacuum) will be caused by a faulty gas cap, or a plugged vent line.
 
Collapsed tank has been my main concern. I dont think its got THAT much vaccum, but I've seen at work that it doesnt take too much to suck in a large tank.
I'll have to get pics of my canister to see if the lines are right. I may just see if I can find another canister at the parts store. I dont have any way of checking to see if the ventline is plugged or not:dunno: I think I'd have to drop the tank to get to the other end of it.
 
dyeager535 said:
Yes, but vent and vacuum are two separate functions. Vented the fumes would be able to escape the tank through the cap, and those haven't been used for quite some time, at least on light duty cars and trucks. If the cap allows air to enter the tank to it would not allow a vacuum to form.

The stant locking cap on my '87 (I didn't install it!) is actually vented and allows fresh air to be drawn through. The non-locking on my '85 is not vented, but does allow fresh air in.

If the cap isn't vented AND doesn't allow air back into the tank, then the restriction to airflow is somewhere else, or its the wrong cap.

I remember searching for vented caps when I put my truck together without the EVAP canister. I couldn't find any applications with a vented cap, even any of the trucks in '73, so I think there are some mis-applied gas caps for these trucks.

FWIW, the '91 R/V service manual says a collapsed tank (extreme vacuum) will be caused by a faulty gas cap, or a plugged vent line.
Click to expand...

The cap may have a one way valve to let air in but not out. The venting is thru the charcoal canister and the purge valve. i could be wrong.
 
shady said:
Collapsed tank has been my main concern. I dont think its got THAT much vaccum, but I've seen at work that it doesnt take too much to suck in a large tank.
I'll have to get pics of my canister to see if the lines are right. I may just see if I can find another canister at the parts store. I dont have any way of checking to see if the ventline is plugged or not:dunno: I think I'd have to drop the tank to get to the other end of it.
Click to expand...


The 1990 canister i have has three ports.
Purge , to the throttle body
Air filter, tho it has no hose to the air clearer. Just has a plastic deflector that looks like a plug from a distance.
Tank to fuel tank.
 
shady said:
I dont have any way of checking to see if the ventline is plugged or not:dunno: I think I'd have to drop the tank to get to the other end of it.
Click to expand...

Remove the tank fill cap, remove hose from evap canister to tank, blow through hose.
 
If it's not causing any problems, then no harm no fuss?

Both my diesel rigs have a small vacuum. Nothing too dramatic. My aux tubes are capped off.
 
dyeager535 said:
Remove the tank fill cap, remove hose from evap canister to tank, blow through hose.
Click to expand...
I thought of that but didnt want to blow stuff into the tank if it lets go. I could do it the other way though:thinking:

2INSANE said:
If it's not causing any problems, then no harm no fuss?

Both my diesel rigs have a small vacuum. Nothing too dramatic. My aux tubes are capped off.
Click to expand...

I'm worried about premature pump failure, power loss, or tank issues. Its a lot of vaccum. By the sounds of it it isnt normal, so I'll be digging into it now. its REAL bad if I fill it up and run to empty. I normally cant afford to fill up a 40gal tank though:haha:
 
shady said:
I thought of that but didnt want to blow stuff into the tank if it lets go. I could do it the other way though:thinking:
Click to expand...

Either way. I didn't mean put any real pressure to it, just blow on the end of the hose and see if it flows freely. If it doesn't, you know you've got an obstruction. If it does, must be something in the evap or the gas cap valve.
 
dyeager535 said:
Either way. I didn't mean put any real pressure to it, just blow on the end of the hose and see if it flows freely. If it doesn't, you know you've got an obstruction. If it does, must be something in the evap or the gas cap valve.
Click to expand...

OK, so lets say there is a blockage.....what is this effecting on the truck's performance? Starving for gas, poor fuel economy, failed emissions?? :dunno:
 
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