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Gas Tank Gasp -- Tank Pressure

Gas Tank Gasp -- Tank Pressure

Jan Wikstrom

Dennis wrote:

When I pop the tops to add gas, air rushes in like crazy!

Fred wrote:

I noticed with my '86 XJ6 that I cannot "top off" the tanks when fueling. Whenever I have done that, the car will stall after several miles, and not start again until I have opened one of the filler caps and allowed the air to rush in. Is this normal?

Alex Lynch wrote:

I read a post a few digests back concerning fuel tank pressure,although I did not notice any follow up. My tanks seem to have some pressure(about 2 seconds) when opened but not to the extent that I hear metal expanding.The pressure seems to subside on its own after a while if I don't open the cap.

Should there be any pressure at all in the tanks ?If not what is the fix? Does it have anything to do with the charcoal cannister?

No, there should be no pressure. The tanks are vented through a pipe system in the C pillars (!) and under the rear shelf to the charcoal canister in front of the left front wheel. The canister is supposed to vent to the atmosphere, and when the engine is running, engine vacuum should draw fresh air back in through the canister to regenerate the charcoal.

That's the theory; the reality is that the fresh air backdraft also draws in dust (the filterless intake is in the wheel well) and the charcoal pad eventually gets partly blocked. When it does, part of the engine vacuum is applied to the tank vents. If the tank is too full, this draws neat petrol through the vent piping and you can watch it dribble under the left front wing after you stop.

Note that the tank doesn't have to be overfilled to begin with; if it's a hot day and you fill it with cool petrol from an underground tank, the level may come up to the vent pipes all on its own...

The way I found out about this was that the partial vacuum made it hard for the pump to draw petrol from the tank. On a hot day, the circulation through the engine bay would heat the petrol enough to make the tank uncomfortably hot to the touch; this, together with the vacuum and a low static head as the tank nears empty, would bring the fuel to a boil at the pump inlet and the resulting cavitation would lead to lost suction and the engine would die. Nothing would get it going again, except waiting for half an hour for everything to cool down...

The simple cure: disconnect the vacuum line.


If the charcoal bed is clogged with dirt (there's no filter on the air intake of the canister), you tend to get vacuum in the tank while the engine runs. Pressure in the tanks leads me to suspect that there's a blockage in the pipes leading to the canister.

- - Jan

** Never let a machine know you're in a hurry **

Tom Russ Adds for the Series III

The pressure in the fuel tanks is actually a normal condition. The fuel tanks are vented to the engine's purge system via the charcoal cannister in front of the RF wheel arch. But, installed inline just before the cannister is what is called a "two pound check valve". This keeps all the vapor contained in the tanks and lines until it reaches about two pounds of pressure, then it releases it through the cannister and into the engine.

The valve is used so that when parked the car does not fill the cannister with vapors, thus causing a very hard hot start. One of the adverse side affects is that the tanks are constantly being subjected to expansion and contraction, which weakens them, usually at the seams and weld points (for the baffles), creating fuel/vapor leaks. If you reach in front of the rf tire and disconnect the tank vent line where it connects to the pressure valve, the tanks will vent direct to the atmosphere. Not what the epa intended, but it stopped the problem on my car. Don't worry about the purge system, the pressure valve is normally closed so nothing can get in.

You'd think you'd have a fuel smell in the rf of the car, but I've never noticed any smell since doing this. Some states are now testing the evaporation system as part of the smog test, if so you'll have to connect in order to pass!


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