Why does my diesel fuel tank have an EVAP return line? Where does it go

[D22Hazza]

Okay so I know that I can't hold pressure from the lift primer pump back towards the tank. So I've pulled my tank out to inspect it etc. I was surprised to find three lines on my tank; two are the normal feed and return going into the electronic fuel sensor/pump the third looks like it goes into my tank directly and has a one way valve or check valve. Not sure why it is here and if there is even an EVAP system on my vehicle?

Photo Attached

I have a 2010 YD25DDTi common rail, 2WD built in Thailand.

 

[tweak'e]

thats valve is the check valve for the vent line. it allows air to go into the tank to replace the fuel that gets sucked out. they will let air out if under pressure.
you won't ever hold pressure from lift pump to tank due to the tank venting to air.

 

[D22Hazza]

you won't ever hold pressure from lift pump to tank due to the tank venting to air.

Okay thanks for that, that makes sense about venting to air, where about does the line that this valve is on terminate to get the air on the vehicle? Also I don't understand why I won't every hold pressure between lift pump and tank because in the normal scenario where everything is working correct, you shouldn't have to use the hand pump to prime ? So if that's the case some sort of pressure or vacuum needs to keep the lines from the tank to lift pump primed. Unless the pump in the tank pushes fuel up the line in the first few seconds when the ignition is turned to on ?

 

[D22Hazza]

How do I check if I have a bad fuel check valve?

 

[D22Hazza]

Okay, so I've traced the check valve line and found where it vents and it is in the engine bay. See a photo. I also just did a pressure test on the tank feedline and I think I have algae in it. See attached video. can anyone confirm Is this a bit of algae or just gunk?

Would algae cause no start?

 

[tweak'e]

look like bug. just put a dose of biocide in its cheap. just be aware most bottles are for big storage tanks not ute tanks. you will need something to measure the amount with.
it will come out with normal running.

handy to know where the vent is as you can put small amounts of air pressure in to push fuel up to the filter. i've had to do that in the field.

pretty sure there is no in tank pump, injection pump sucks fuel up from the tank. if there is an air leak in the line it will allow fuel to drain back to the tank.
some people fit one way check valves on the fuel line, i've used a diesel rated boat style primer before. much easier to push fuel up from the tank when doing a filter change than to suck it up with the factory primer.

if you chasing a fuel issue, put a clear fuel line after the fuel filter and look for bubbles.