In the Volvo 2003T, there are two lines coming our of the engine, going to the heat exchanger. One from the block (aft of the thermostat) and one from the turbocharger. They meet in a T

There is a plastic plug in the line from the turbocharger, with a hole in it about 3mm diameter. This is to balance the flow.

The plug is a poor press-fit into the copper pipe. It can move along the pipe and partially block the T, restircting flow.

Our engine ran fine like this at low revs, but overheated when under load.

It is easy to push the plug back up to the turbocharger end, then nip the pipe slightly with vice grips to avoid the plug moving again.

All 2003T engines shoud probably have this treatment.