Hi all, wonder if someone could explain to me how the coolant reuptake works on an x308 XJR. I'm having a hard time wrapping my head around it.

From what I understand, there's one line that runs from the header/expansion tank into the surge tank. I would presume that the line runs to the bottom of the surge tank like a pickup in a sump. If I understand how the cap works, at a certain pressure, it "opens" to the higher position which allows the coolant to be routed into the surge tank.

I would guess that after this, the cap would "close" again once the pressure is equalized. When that closes, would that not also seal of the one line that it can regain coolant from? It's it even supposed to be able to reclaim coolant, or am I just making that up?

 

Yes, the line that is connected to the highest port on the header tank goes to the overflow (surge) tank. The pipe entering the overflow tank ends near its bottom so that, with certain minimum amount of coolant in this tank, the pipe is always immersed. When the coolant pressure exceeds the cap pressure, the cap's spring loaded disc moves up and allows the excess coolant to flow into the overflow tank.

Subsequently, while the coolant is still under pressure, the expelled coolant remains in the overflow tank. When the engine (and the coolant) are in the process of cooling down, at certain temperature point, the cooling system will, due to contraction, transition from being under pressure to being under vacuum which will grow as the coolant temperature goes further down. At the creation of certain small starting amount of vacuum, a small check valve (having a very soft spring), located somewhere in the centre of the cap's main disc, will open and the vacuum will pull the overflown coolant back into the header tank and into the cooling system.

 

Very interesting, so during outflow the full lower ring moves out of the way, but during reclamation, it is just the little spring loaded ball valve? I am genuinely wondering if I potentially got a bad cap with my new header tank.

 

Looking at the "classic" radiator cap, it is easier to see how it works - the big disc is pushed up against its big spring and excess coolant (pressure) is released into overflow tank; when vacuum is created in the cooling system on cool-down, the vacuum will pull the small brass disc in the centre down, against a soft spring behind it, and the expelled coolant will be sucked back pass the small brass valve.

The construction of the "pressure valve" and the "vacuum valve" can be very different in the modern rad caps but the principle of operation is the same as in the case of the "classic" cap. On many modern caps, the "pressure valve" is somewhere inside the plastic block of the cap so you don't actually see the big strong spring loaded disc. Same goes to the soft vacuum valve.

Basically, there are two one-way valves in the cap: one to let excess pressure (coolant) out when certain system pressure is exceeded and one, working in the opposite direction, that is opened by the vacuum of the cooling system.

If the "pressure valve" of the cap is faulty, it may release the coolant into the overflow at lower pressure but this will not result in coolant loss as it will still be sucked back on cool-down. In this case, the cooling system will not be at the required pressure so the coolant will boil earlier (at somewhat lower temperature) in the case of engine overheating.

If the "vacuum valve" is stuck closed, the hoses will, due to vacuum, collapse when the engine cools-down.

If there is still some residual pressure in the cooling system when the engine has cooled-down, this would mean that there are leaks in the head gaskets. In this case, the system will not be able to suck the expelled coolant back (no vacuum) so the system will require frequent top-ups. As the coolant continues to be just pushed out (due to expansion and due to being pressurised by the combustion gasses) into the overflow tank and not reclaimed, this will eventually overfill the overflow tank and the coolant will start spilling out.

 

Thank you, this helps a lot. I fear that I am essentially out of other possibilities at this point, and the diagnosis for my other thread of potential head lift is seeming the most likely. The good news is, I have learned an immense amount of how these related systems work and interact, the bad news is they all seem to be functioning fine and all seem to be pointing the same direction. Looks like I'll have to start sourcing the parts to do head gaskets.

Not super exciting considering I recently had the supercharger out to replace almost all of the cooling hoses, I was halfway torn down already at that point haha. I'm pretty certain that the previous owner did a poor job when he swapped the replacement cylinder head onto the car, which would also explain why the bypass valve was wide open when I purchased it. Now that I have put the pulley on the car, the increased boost appears to be showing these symptoms where they were hidden before.

 

I had the same symptoms with my x308. Persistent coolant loss despite a new header tank and cap. It seems that the head gasket does not "blow" catastrophically, but fails over time. Mine eventually did.