So How Much Vacuum Is Normal When Removing A Gas Cap?

I didn't have any vacuum before I replaced all the fuel lines and gas cap, now when I open the tank after its been sitting for a month or so, the sound is astounding! I don't know if its vacuum or pressure (never paid any real attention to it) but it can go on for a full 2 seconds sometimes! FWIW, others at my local C&C have commented that it is normal.

 

Yes it should vent somehow.

Ernie had a problem with fuel tank venting.

http://ferrarichat.com/forum/showthread.php?t=241642&highlight=fuel+tank

 

40 psi neg pressure (vacuum) according to google but that means nothing to an untrained ear. Found this information in the 1984 OM:

When the engine is running, the fuel vapors previously stored in the carbon canister (B) are released by engine heat and sucked into the intake manifold (G). When engine is shut down, last minute vacuum sucks fuel vapors from the right fuel tank (I) into a liquid/vapor separator (D). Vapors are sucked into an activated charcoal canister (B) where they are absorbed and stored.

Between the liquid/vapor separator (D) and activated carbon canister (B) are two valves in the tubing (H). One is a two way valve (C), and a normally open 'roll-over' valve (E) which closes only in case of inverted car syndrome. This keeps gasoline from running back from the gas tank feeding a fire.

Considerations: The system is run only by vacuum from the intake plenum...there are no electrical or other controls. The fuel cap (A) is NON venting but several parts should be capable controlling the vacuum. The 'air purge inlet' valve (G) at the carbon canister should vent the system to air once the vacuum is bled down and may be doing that. Then there is a mysterious 'two-way' valve. One-way valves I understand but why 'two-way'? Different flow rates each way? Start with the valve at air vent valve G.

Related aside, I had a CEL on my GM. Apparently sensing no vacuum, readout said fuel cap was not tight. Sure enuf. I have no 'roll-over' valve tho.
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Considering that atmospheric pressure at sea-level is 14.7 psi and a perfect vacuum is zero psi, -40 psi is obviously a bogus number. Vacuum in typically measured in inches of water or mercury.

 

Hi Tim,
The 14.7 and 0 PSI you refer to are in absolute pressure, PSIA.

The 40 PSI number that Paul cites, must be gauge (gage) pressure, which uses nominal atmospheric pressure as 0.

BUT... -40 PSIG is still a bit excessive, might be difficult to generate and I would also think it might do a nice job of collapsing fuel tanks.

About the lowest manifold vacuum (which generates the vacuum for the vapor recovery system) is about 30 inches in mercury, which is approximately -14.7 PSIG or 0 PSIA.

Regards,
Bill

 

I like this but have two comments:

on the 456 I have excessive vacuum. Once, I even needed a wrench to unscrew the cap at the filling station. I've been told there might be a couple factors involved, but probably will involve taking gas tank out to correctly diagnose/fix. I've let the cap sit in a VERY untight manner ever since...but, still get an great whoosh!

On the Mondial T/348, the objection I have is not having that one-way valves [there are two exits] between the canister and the air intake manifold: when that canister breaks down and it will, the bits get deposited into the #3/#6 valve intakes causing much carbon buildup in those cylinders not to mention intake valves. Of course that should be checked on an engine out service as it's very difficult IIRC to get to normally.

my 1.1 cents.

 

Manifold vacuum would never get to 30 inches. -40 psig would not be possible under any normal atmospheric conditions.

 

Pretty much what I was saying

Regards,
Bill

 

As long as I have owned my 308 I hear woosh when the cap is removed. Why is this an issue?

 

My '82 308 has always had pressure when I open the gas cap....... the one way valve on the vent side inline with the char/canister is in working order and level in its mount.

 

MY 81 NA is not bashful about making a he11 of lot of vacum noise when I remove he cap.

IIRC the 2Vi cars did not have a vent. Starting with the QVs they did on the metal filler neck.

PROs is that correct??????

 

Sorry guys, the 40 psi is obviously bogus and I can't find the source.

On recollection, the only time one opens the gas cap is immediately after shutting down. An interesting experiment would be to wait 1 minute, 2 or 5 after engine off before opening the cap for a perceivable difference. And do this on several cars.

 

Paul,

Mine makes a GOOD hissing sound several seconds MANY MONTHS after the last drive (I Know; I Know: DRIVE IT!!!!)

 

Tommy - I think the excess vacuum is being caused by the fuel pump drawing the fuel out of the tank rather than the vacuum from the intake manifold. I had a plugged vent line on an Audi Coupe I owned, and the electric fuel pump had enough suction to significantly collapse the tank, which had a broad and shallow shape. It didn't take much - I expanded the tank by attaching my tire compressor to the tank's vent fitting, and it only took a few psi to pop the tank back out again.

Gary

 

I'm beginning to agree and think that one of the 3 valves must be stuck or simply a hose has become clogged or sucked shut by the vacuum. There is also the canister air-vent valve whose function is letting air into but not out of the system. Removing and sucking on the hose (H) might provide some insight.

 

After seeing Paul's graphic yesterday (BTW, Thanks!), I decided to spend a few minutes at the end of my day over at the shop working on the car looking at this. I've recently replaced all that hosing and cleaned the separator can. Since my plenum is not yet on the car the hose from the carbon filter was still hanging so I grabbed the parker and started pumping down the system. I didn't want to go beyond a few inches of Hg as I had some concern collapsing the empty tanks. I took the system down to about 5 inches and it took about 30-40 seconds to drain off when I released the vacuum. I had a brand new carbon cannister (Ferrparts got their hands on a few new ones and I just happened to be there when they came in) sitting on the lift, so I swapped it in and this time when I let the vacuum off it dissipated immediately... 2-3 seconds. So something is not "open" in the carbon can or its purge valve.

Ideally in this system the fuel pump should not have any impact as there is plenty of interconnect between the two tanks and with the system vented to the plenum you should never see anything beyond the intake vacuum when the motor is running. If the two way valve is working (essentially just a flow modifier), once the motor stops, any vacuum in the gas tanks should bleed off to the manifold through the carbon can and the two valves... given a few seconds to stabilize.

In my case I think I'm seeing the carbon can starting to block up. Once I get the new one fully mounted, I'll cut the old one up and see whats in there. But a blocked "turnover" or "two way" valve would also keep the stabilizing bleed from occurring.

For those having excessive vacuum at the cap, I'd start by bypassing the two valves and run a line from the plenum directly to the carbon can. Run it a while (making sure not to have a roll-over incident...) and recheck your system. If its still holding vacuum, you carbon can is probably loaded up. If that clears the problem, you need to look at those two valves.

added thought:
In the grand scheme of things... other than any splatter(?), there really isn't an issue with vacuum in the system, its there when running. Compensating by leaving the cap loose is inducing a vacuum leak and will affect how the motor runs. Is this something than needs to be fixed? yes. Is it something detrimental? only to your senses.

Rick

 

Something to keep in mind here when dealing with tanks and negative pressure. Tanks can withstand positive internal pressure to a considerably greater extent than they can withstand positive external pressure which is what a vaccum creates. Accidental internal vacuum is always a design consideration in chemical plants. The vessel may be designed to withstand 100 psig internal pressure but will buckle at only a few pounds external pressure. I have seen liquid filled tanks do this when someone opens a drain valve without opening the vent. The larger the tank, the more susceptible it is.

 

I misstated this!! CORRECTION: I had plenum on my mind and should have typed Vapor Separator. There is already a direct line from the plenum to the carbon can.

For those having excessive vacuum at the cap, I'd start by bypassing the two valves and run a line from the VAPOR SEPARATOR directly to the carbon can. Run it a while (making sure not to have a roll-over incident...) and recheck your system. If its still holding vacuum, your carbon can is probably loaded up. If that clears the problem, you need to look at those two valves.

Sorry for any confusion,
Rick

 

Thank you for posting this. I'm driving a F355 GTS and since it arrived in August, each time I get gas, the vaccum is so strong that I can't get the cap off for at least 15 minutes.

I was advised that it is the Roll-over valve and to purchase from Bosch. Problem is, I can't seem to find the part. Any ideas ? thank you for your time.

 

What was the other car?

 

After giving more thought, the engine, when off, should be at atmospheric pressure. The venting would be through the air intake via throttle body, air sensor and the air filter. The TB and sensor should have small gaps to facilitate initial idle prior to aux air valve being used (IOW air should be in the intake with no vacuum and engine off). No vacuum should be present in an engine at rest.

 

On my 348 it's located up under the left rear wheel well. You will need to remove the wheel, then take out the front and top part of the wheel well liner (aka the splash guard) and you should see it right around the filler neck. Or to find it easier just follow the small vapor lines coming off the top of the tank, back to the roll over valve. You'll still need to remove the splash guards to get to it though.

Actually, if you look at the 4th & 5th pic, in the link Tim posted, to the purge valve thread, you can see the bottom of it. There is a line running from the rollover valve to the second nipple on the charcoal canister.