Fuel tank breather

Yakuza is right there is a breather pipe connects from the top of the tank to the fuel inlet, but it’s not vented to the outside air as it is within the fuel cap. Further it is correct, it’s only there to allow air to escape when refilling.

My understanding is that negative pressure (vacuum) caused by drawing fuel opens a one way valve to let air in to the tank, but doesn’t allow fuel vapour to escape. As such a certain amount of negative pressure is required to open the valve, that’s why you hear the woosh when you open the filler cap.

I think the original posters inlet valve is not working correctly, so the pressure drops too low in the tank and fuel won’t flow.

You could test for this, just drive the car until you get the fuel flow issue and then try opening the fuel filler, if it is really hard to open you have too much negative pressure. If it opens relatively easily then you have another problem.

Other problems might be a blocked or partially blocked fuel filter or a weak fuel pump, both of which would work better if you ease or remove the cap whilst driving.

As said, the Jimny has a sealed tank and an evap system, with a charcoal cannister under the bonnet to absorb any fumes.

There are two pressure valves inside the fuel pump.
An inlet valve, and an outlet valve. Both join to the vent line that goes to the charcoal cannister under the bonnet.


With the tank half full, and the car standing in the sunshine the fuel heats up and some vaporises, raising the tank pressure. The outlet valve opens and the excess vapour flows into the charcoal cannister, where it is absorbed (and any air just passes out of the short vent pipe on the cannister) the valve closes so the tank stays at a slight positive pressure.

If you undo the fuel cap now, it goes Pssshhh! as vapour comes out and you smell fuel. The MOT man opens the cap and listens for a Pssshhh! to check the cap seals are working.

When you start the engine, there is an electrically controlled purge valve that opens and connects a port on the cannister to the inlet manifold, so fresh air is drawn through the charcoal from outside through that short cannister vent pipe, the absorbed fuel vaporises into the air and is burnt in the engine, meanwhile the charcoal is cleaned out.

As fuel is drawn out of the tank (or if the temperature drops when standing) air has to be drawn into the tank to replace it. This is through the inlet valve on the pump along the vent line to the evap cannister- air is drawn in via the vent hose on the cannister.

The valve regulates so there will be a slight negative pressure in the tank.  If you stop the engine and open the fuel cap, it now goes hhhhsssP! as air is drawn in (chances are you can't tell whether it goes Pssshhh! or hhhsssP! apart from one smells, the other doesn't) , again the MOT man is listening for this.


Long explanation over- if you are experiencing fuel starvation, and curing it by opening the fuel cap then

1) The vent line from tank to cannister is blocked/squashed (if you take the fuel cap off you can blow down this pipe with your mouth- you will feel the pressure needed to open that valve, if you put the fuel cap back on & quickly blow down it again until you turn blue then stop and  run and open the fuel cap it goes Pssshhhh! from the air you blew in  (as described earlier) This will also test the inlet valve in the pump at the same time. 

If the vent pipe appears blocked, remove the fuel cap and use a large syringe to draw air through the vent pipe, the outlet valve should open and allow air to flow into the fuel filler and out of the vent pipe.

2) The short vent pipe on the cannister is blocked/squashed/connected to the inlet manifold or something.  (you can take it off to blow through it is just a short loop of pipe.)
or
3) The cannister is bunged up or full of water/oil or something. (if you pull the purge pipe off the cannister you can blow in freely through the port- DO NOT SUCK!!!) Actually you can blow into any port and air will come out of the other 2.



NB: if you overfill the tank by rocking the air out of the space at the top and adding more and more fuel after the pump cuts off, when the fuel expands liquid fuel goes along the vent line & fills up the evap cannister, it may even come out of the little vent tube. When you start the engine, neat fuel is purged into the inlet, and the car will run really rough until it clears.

After all that explanation, there is also the possibility that
4) the fuel pump is worn out- I'd check the fuel pressure with the tank cap open, it should be over 30psi at idle.



 

Buy a new fuel cap.  Only get a genuine Suzuki.one.  The Chinese knock-offs on eBay aren't a one-way seal and the car will just stink of petrol.

I always thought the fuel cap had the one way (in) vent valve in it, but the manual shows two valves (in & out) in the top of the fuel pump connected to the line to the evap cannister, so the cap is just a sealed bung. (I recall the one on my 200TDi having a complicated lever & weight affair to allow air in but seal in case of a roll-over)

I suppose someone ought to take a cap apart and have a look inside it! (My Gen 3 is long gone)

There is also a float valve to prevent fuel going back along the line if the vehicle is at an angle, it doesn't prevent it when the tank is overfilled and warms up though (from personal experience!)