Aux Coolant Tank Plumbing

[FlightRisk]

Without getting too far into the weeds,  I'm trying to replicate the WRX aux tank design into a car that doesn't have one.  From what I could find on my own, the purple hose goes to the turbo, green goes to the overflow tank, yellow goes to the radiator, and there's another on the bottom of the aux tank that goes to the cylinder head that you can't see in this photo.  My question is what is the purpose of the yellow hose?  I'm thinking that coolant will flow from the head to the radiator both through the upper coolant hose and the yellow hose although I don't see why it's necessary for some of it to flow through the aux tank just to go to the radiator anyway so I must be missing something.. TIA.

[nvu]

The yellow line is needed for constant flow from the upper tank to the radiator.  The green line is just for moving coolant in and out of the reservoir tank.

The yellow hose connects directly to the radiator.  It allows the hot coolant to flow back into the radiator once the thermostat opens.  The purple hose is heated coolant flowing out from the turbo.  Also internally at the turbo tank, the yellow nipple extends to the bottom of that tank to prevent from pulling any bubbles.

The green lines are for over pressure purging when the car coolant expands, and also sucking back coolant when the car cools.  In normal operation it sees no flow. In that picture, the coolant caps are installed wrong.  The tabbed cap should be up top, the circle one should be on the bottom.  If you keep in mind, any coolant lines coming out from the heads is positive flow, air bubbles can collect at the highest point where they can be purged to the overflow.  The tabbed cap opens first and is a 2way cap that also allows coolant to be sucked back in once the engine cools.  The circle cap is higher pressured and is there as a fuse of sorts to prevent blowing out radiators should you get a bad headgasket blowing high pressures into the cooling system..

 

[FlightRisk]

Thanks that helps.  I misunderstood the flow direction on the turbo line (purple).  I thought it flowed to the turbo from the aux tank.  So where is the other turbo line connected, somewhere else on the cylinder head?  Now I’m having trouble understanding why coolant would flow from the turbo to that tank if the tank has a line directly to the head (positive flow) when the thermostat is closed or does it just not flow through the turbo until the thermostat opens?  

Edited June 26 by FlightRisk

[nvu]

There's a line that goes out from the head and directly in the turbo.  There are two coolant ports on the turbo, lower one is in, top one is out.  The purple one shown is out from the turbo. 

If you push it hard enough, the turbo can get red hot.  The tank is there to make sure the turbo is always covered in coolant even if the engine is off.  It's also the at the highest point of the car, making it easier to purge air pockets.  If you run your own tank, place it as higher than the turbo and make sure the line flows back into the turbo without any weird bends or loops that can trap air.

[el_freddo]

The line under the tank you can’t see is back to the water pump much like the heater line does, it plumbs in at the same spot on the water pump on its own port. 

The yellow line is to purge air from the radiator. The standard turbo radiator doesn’t have a rad cap and relies on this line to help fill the radiator when replacing coolant/initial fill after rebuild etc. 

Green as stated is the radiator overflow. 

Think of the cooling system as being upside down. The heater, oil cooler and turbo circuits (including the remote reservoir) come from the block to the turbo, oil cooler and heater independently and return to the waterpump independently. This coolant comes in at the back of the thermostat which meters how much cold coolant from the radiator is allowed into the engine. The outflow to the radiator is simply an overflow of hot coolant and is why Subarus both NA and Turbo warm up so quickly even in cold weather. 

Looking at a diagram of the coolant flow in the FSM is helpful with these things too. 

Cheers 

Bennie

[FlightRisk]

Thank you both that helps a lot.

[nvu]

Oh right, el freddo reminded me about the heater core circuit.  When the thermostat is closed, a majority of the flow in the upper coolant tank gets pulled back thru the bottom tank port.  It's taking whatever heat from the turbo and recirculating it through the engine.  Pretty well thought out design.  So that bottom port goes back directly to the waterpump suction side, or tee's into the heater core's suction side back to the water pump.  It doesn't go to an output port from the head.