A charge cooling circuit is a small closed loop: a pump, a core in the intake path, a heat exchanger at the front, a reservoir, and the hose between them. It holds far less fluid than the engine cooling system, which is why a small air pocket matters so much more in it.
Treat it as its own system. Fluid drained from the charge circuit does not mix with engine coolant, and the two reservoirs are separate even when they sit close together under the bonnet. Confusing them is the most common way a refill goes wrong.
The low point is rarely the lowest hose. It is wherever the maker put the drain, and if there is no drain, it is the lowest joint that can be released without pouring fluid onto something sensitive. Look before you loosen: on a charge cooler installation the pump and the reservoir are often the lowest items, and both are usually easy to reach.
Have a container that will take the full volume and a cloth under the joint, because the fluid comes out faster than expected once air enters. A tray placed before the joint is opened saves the cleanup that otherwise follows.
Air in a charge circuit does not simply rise to the top and stay there. The pump moves it round the loop, and it collects at the highest point in the core, where it stops fluid circulating through that part of the core. The result is a core that is partly air and an intake temperature that climbs under load.
That is why a circuit can look full at the reservoir and still be air-bound. The reservoir level tells you the volume in the loop, not whether the loop is full of fluid, and the two are different things once the pump has run for a while.
Fill slowly, then run the pump with the reservoir cap off and watch the level. As air leaves the loop it appears at the reservoir as bubbles and the level drops, and it will keep dropping for a while after the first drop. Top up until the level stops falling with the pump running.
If the circuit has a bleed screw at the core, open it while the pump runs and close it as soon as fluid appears rather than air. Working from the highest point downwards is the only sequence that removes air rather than moving it around the loop.
Use the fluid the system was designed for. A charge circuit contains aluminium core and tank, rubber hose and often a brass or stainless fitting, and the fluid has to be compatible with all of them. A fluid chosen for one metal at the expense of another causes the corrosion that later blocks the core.
Where the reservoir is a fabricated aluminium tank, check the filler neck and cap seal before filling. A cap that does not seal lets the circuit breathe, and a circuit that breathes pulls in air every time it cools.
The day after the refill, check the level cold and check it again after the car has run. A level that has dropped means air has moved to the reservoir, not that fluid has been lost, and it needs topping up rather than investigating at length.
Look for seepage at every joint you opened, and look at the pump body. A weep at a hose joint usually shows as a stain rather than a drip, and a stain found early is a clamp to tighten rather than a hose to replace.
