I have a 1990 Winnebago Warrior built on a Toyota pickup chassis powered by the 3.0 engine. When I stop for gas the tank is noticeably under pressure indicated as I remove the fuel cap, is this a problem? This may be a dumb question, but I willing to risk my ego here. The truck runs fine, otherwise.

 

Check that the vapor and fuel return lines at the tank are not reversed. I replaced my tank and reversed these by accident. The tank would be under considerable pressure and fuel would actually come out of my charcoal canister!

 

Other than the kind of problem noted by AdmiralYoda, the tank is supposed to be under pressure under many conditions. Mainly, a warm day when you have less than a full tank (in other words, when you stop for gas!)

When it cools, the vapors will re-condense, lowering the pressure in the tank. If it lowers the pressure below atmospheric (this will happen all the time; the fuel pump is removing fuel from the tank) the vented cap will allow air in. But won't allow any out (to contain the vapor).

So take a drive on a cold morning, and then remove the tank cap. If the "whoosh" is now gone, you're probably okay. It will come back as it warms up.

 

The tank on Toys is normally vented out through the Evap canister in the engine compartment.
From the Evap a vacuum line goes to the Bimetal valve and then to the throttlebody.
There is a continuous 3lb vacuum to the tank as long as the engine is running and above 95 deg F.
If this is working correctly there should never be pressure in the tank.

 

Are you sure it's pressure and not vacuum?
The noise you hear could be air going IN to the tank.
If that's the case, it's normal.

 

That's not my experience.

 

Then your Evap canister, Bi-Metal valve or the TPS port is plugged.

 

Resurrection! I believe I have too much positive pressure in the fuel tank. I do not have the Bi-metal valve (vacuum line routed directly to TB), it has good vacuum. Can I bypass the charcoal canister in order to determine if it is actually the culprit? How would I go about doing so?

Here is a video of the pressure I have. This is the worst I've seen it but I was also at 9000 ft... but normally here in town at 2600 ft I still have quite a bit of pressure.
(click on image)

 

I have seen several Return Lines clogged, not sure if that would cause the problem. Just recently had to replace a charcoal canister and on another truck I have seen the line on the bottom of the canister clogged. Would be worth checking.

 

I'm just wondering why you think this might be an issue. Every one of my last 4 Toyota trucks did the same thing and they all ran fine. Pull up for gas open the gas cap and hear air entering/leaving the system. So what?

 

It does seem a little extreme to me, but the altitude might be the cause. The tank is supposed to end up with some positive pressure when warm (due to the increased vapor pressure of the fuel), but never negative pressure (the vented cap lets air in, but nothing out, to prevent the tank from being crushed as fuel is pumped out).

That positive pressure of fuel vapors should be adsorbed by the activated charcoal in the evap canister, then sucked out by the throttle body once the engine is warm enough to run smoothly with the slight enrichment.

You could wait for a moderately warm day (so that the fuel is producing vapors, but not so fast that they would overrun a properly working evap cannister) and disconnect the tank-evap line at the cannister. If you get nothing out (and opening the tank cap produces a whoosh), you probably have a blocked line from the tank.

Another thing to look for is the line from the bottom of the evap canister; it wanders down into an opening in the frame (just to keep dirt out of it), but is otherwise open to the atmosphere. If that's blocked the engine vacuum can't pull air through the canister emptying it.

 

On my 83 2wd, I almost always have air pressure exiting the fuel tank when I take off the cap to fuel up. I take it as a sign that my gas cap may be only taking air in but not venting, but it could be something else. I figure that the fuel pump can't be complaining about the extra pressure it's receiving, but it's not normal, however I don't sweat it as long as the truck has been getting 24mpg highway since I got it, and I did take it all the way from Portland OR to NC, and have been getting the same thing this whole time. I would say run it as it is, heed the warnings at the gas pump that the cap may be pressurized, and keep moving on as you are unless you get fuel system problems that may be related, then do something about it.