For your heat exchanger, is there a petcock or bleeder bolt on the top end of it? If not we're going to have to add one or some other way to bleed out air. The trapped air won't flow back to the reservoir. Some will but not all. The pump is electric and a constant rpm. Higher engine revs won't purge out air in the intercooler system. I see from an earlier pic you posted, both incoming and outgoing lines enter from the bottom. This might be the issue and I think your heat exchanger could be half full of air. I think the only way to purge it fully, without a air bleed, is to unmount it from the radiator, turn it over and lossen the line going back to intercooler to let air out and coolant to fill the void until it starts coming out the lossened connection. Making a mess of course. Ultimately a bleed bolt or petcock added to the top of the H.E. is needed to make it simple to bleed. Or a different H.E. with coolant inlet entering at the bottom, coolant exit at the top. It would be better but would still have air trapped in the outgoing hose since it still has to be routed down below the height of the H.E. But then you could leave that upper line disconnected and have a helper fill the intercooler system with coolant until it gets the top of the H.E. and the coolant line. I know, I'm long winded. That's what my wife tells me. Hope it helps. I've got a ways to go before I have that road to cross.