WHOOOSH when unscrewing gas cap -- normal or not normal?

[Wade Nelson]

Hey, ZX owners, do you get a "whoosh" when you unscrew your gas cap?

Mine is pretty substantial, I'd say a couple of psi.

I haven't owned any other cars that kept the tank this strongly pressurized unless something was amiss.

I'm wondering if I have a smashed / clogged vent line from the tank to the charcoal cannister. My car was obviously driven over something hard at some point in time. Frame rails are...ugly. Vent line might have gotten flattened. I also had trouble fueling at certain vapor-recovery pumps out in CA - but not all.

Normal or not normal, in your opinion?

[Kurbycar32]

I would assume that your fuel tank is actually under a vacuum, not pressure due to a clogged vent line as you already suspected. No its not normal. It sounds like you already have it figured out but check your vent lines. If the tank was venting properly it wouldn't be possible to create a vacuum or pressurize the tank.

[black gold man]

I have a 83 that is like new cond 77000 mi i have had this happen before.

If you put cool or cold gas in the tank and it is much warmer outside the fuel will expand.

I would not worry about it

[RedBird64]

Hi,

I've run into this in other vehicles and usually goes back to venting, or the lack of. Are these caps vented, in themselves?

[black gold man]

2nd opinion "There SHOULD be enough pressure to "pop the cap" on a ZX. Not enough to blow it out of your hand but definitely enough to whoosh out when you open it up. That's a design feature to keep some pressure on top of the fuel."

http://forums.hybridz.org/index.php/topic/80388-280zx-fuel-tank-vacuum-and-fuel-check-valve/

If you look at the cap on zx i would say it is designed to hold pressure.

[Wade Nelson]

That's a good thread. FSM says the cap is designed to allow air in to prevent a vacuum from forming in the tank as fuel is pumped out.

FSM calls for pressurizing against the check valve to test the system for leakdown, and sticking the gas cap in your mouth to see if it admits air against a vacuum. Mmmm, tasty!

I filled up yesterday, very hot day today, and I'm smelling strong gas fumes. I've clearly got a venting problem, or else a leak.

[zbane]

I've always had that happen with my ZX, as well as with my 01 infi. It's not something I'd be concerned with. As to your problems filling with certain gas delivery systems, that boils down to the angle/depth you're inserting the nozzle. The (for lack of the proper term) fuel relief tube (runs mostly parallel with the fill tube) is meant to signal the station pump when to stop pumping. If the nozzle is in the wrong way, it'll splash back fuel in such a way as to provide a false positive on tank level. Trial and error, and you'll soon know just how to seat that thing in there.

[Wade Nelson]

Just to follow up, it's definitely pressure, not vacuum. I can feel the fumes blow past my fingers when I unscrew the cap.

From the FSM it APPEARS that the tank is SUPPOSED to be kept under a SLIGHT amount of pressure. The one-way valve leading to the charcoal cannister, I probably need a new one, some rust or corrosion is probably why it's taking MORE pressure than it should to overcome.

[Wade Nelson]

More follow-up. By only filling my tank 3/4 or 5/6 I'm having a lot fewer problems, fumes, vapor locking, etc.

I suspect the fuel expansion in hot weather is CONSISTENTLY filling the vapor recovery line with liquid fuel, so then it can't do it's job properly of venting the tank.

With fuel leaks fixed, never filling it to 100%, a toggle switch for the auxiliary fan, and alcohol-free fuel, my vapor locking problems have almost completely gone away.