D4D troubles

[sae70]

Found this in this Toyota forum http://www.toyotanation.com/forum/80-89-95-toyota-pickup-hilux/366553-gas-tank-under-pressure.html

It absolutely normal for the gas tank to have pressure in it. The tank needs pressure in it for the EVAP system to work properly. The tank will either have negative or positive pressure in it or none at all. The reason for this is to prevent gas vapors from escaping the gas tank.

Below is a reply i posted to TN member who was having fuel issues:

The pump does not create nor need to create any type of pressure in the tank to get fuel. All the pump does is suck fuel from the tank and pressurizes it in the fuel lines and fuel rail. However the tank always has some type of pressure in it and this is where your EVAP (Fuel Evaporative Emission Control) system comes into play. Have you ever went to fuel up and when you unscrew the gas cap you hear a gush of pressure come out, well that's the pressure, but it has nothing to do with the way the fuel pump operates, its totally separate from the EVAP system. When you shut the engine off high pressure starts to build in the tank from fuel vapors. That pressure pushes the vapors into the charcoal canister where they are stored until the engine is started. Only until the engine has reached operating temperature does the check valve open on top of the canister to allow the vapors to travel via vacuum hose to the intake manifold to be burned. The tank can also create high vacuum caused buy the fuel pump SUCKING fuel out of the tank. When that happens the check valve in the canister reverses so that the stored vapors do not get drawn back into the tank, but when the vacuum increases to a certain point there is a second check valve integrated into the gas cap that opens to allow outside air to enter the tank and therefor stabilizes the pressure. So you can say the system will allow fresh air in, but will not allow vapors out.

It's all a tad more complex than I ever gave it credit for

 

[Chris]

I can only say what I read in the manual Steve. The tank is designed to run under slight positive pressure. It's what it said. But due to ecomentalist laws, you can't just shove that oily vapour out into the atmos. Hence the filter thing on the end of the breather pipe. Don't think that there should be a vacuum in there. By logic, if it were sealed when you pulled a whole tank of fuel out of there the tank would virtually implode. It only makes sense for it to be neutral or slightly raised. If when you release the cap air rushes in then it's not working properly. I get this a lot on the swamp monster. It's fine for ages and then psst. I go underneath and clean the end of the tube shoved into the chassis rail and it's all cured again. But I am pretty sure I have tank air rushing out, not in.

Be good to get the definitive answer. I don't KNOW that I am right. I only THINK that I am.

 

[sae70]

And then further down there is this;

there should not be vacuum or pressure in the tank. It's open the the charcoal canister at all times so pressure or vacuum will be relieved through it. no valves. If there is pressure or vacuum the line orcharcoal canister is clogged.

this is a case where "normal" may not be the same as correct.

Correct operation of the system is as follows: any vacuum in the tank is relieved by air coming in the vent port of the charcoal canister. Pressure in the system is vented through the same port. Between 100-130f the BVSV opens connecting the line from the throttle to the charcoal canister. the port in the throttle body is positioned so that when the throttle is slightly open, vacuum is applied to the line. fresh air will then be drawn through the charcoal canister to pick up the fumed and carried to the throttle body. Yes it is essentially a vacuum leak but it only happens with the throttle open enough for cruising speed and is small enough to not be a problem.

is it a problem? not unless your gas tank starts to deform but the valve in the gas cap should open before it's noticeable. should...

Isn't the internet a great place

 

[jbecks]

Diesel engines do not have EVAP though steve thats for petrols! both articles! charcoal canisters are part of the evap system.


If we had a vacuum/sealed tank...the fuel pump would try to suck the fuel tank down the fuel lines Some hissing is normal especially when low on the juice....I just looked at the fuel cap and what looks to be a PRV built into the fuel cap,mike if driving the car with a loose fuel cap has solved your problems, you could clean/check the breather like chris says failing that buy a new fuel cap.

 

[sae70]

I did start to think that I'd never seen a charcoal filter on either of the trucks

I had one on one of my Vitaras that used to leak fuel from the bottom!!! It was way to much money to replace so I just removed it

 

[silvercruiser]

I always get a rush of air out when i undo my cap and its always done this. my collie runs and starts on the button so has to be normal.

 

[mike hatton]

Good Morning chaps,

Took the cap apart, there's a non return valve there that lets air in. It took quite a bit of effort to open so removed a coil from the spring. All good now and no hiss either way.