Another Over Heating Engine

[Scorps]

Hi. i know there are lots of posts on the forum regarding this subject, but having read through loads not found anything exactly the same so far, anyway here`s the problem. Travelling down the motorway around 70mph temp. started rising fast before i knew it it was in the red ..... got back home checked thermo stat, shut tight so i thought with it being summer i will leave it out, couple of days later on motorway same again, so this time flushed all system (thought jernals in rad. must be blocked) out several times because noticed water very rusty, used rad flush and other types, checked viscouse fan plus extra fan all ok, since doing this noticed now that when on motorway and temp gets up water boils over into header tank, also it only appears to get very hot when turbo kicking in alot, not actually loosing water at all, no missing, no white smoke, water pump ok, water flowing throughout engine and rad ok. any ideas please...........

[Koi]

If its only doing it a high speed, (engine under heavy load) then it could be blocked rad (internally). Maybe Rad cap is holding to much pressure before it opens.
Since you flushed it you may now have an air lock, were both heaters set to hot befroe you drained, flushed and refilled. Are both heaters now getting hot, if cooolant is expanding into expansion bottle but not overflowing it then thats what its there for. As long as when the engine cools the level goes back down as its sucked back in.

[Scorps]

Thanks for the reply, I will try a new rad cap as the one i have was seized closed and had to tap it to get it free, regarding the heaters they both get warm also filled system back up with them set to the hot position.

[TonyN]

Check all the obvious things first, viscous fan, water pump, blocked rad, new stat, airlocks, etc. Before worrying about the head.

It will overheat easily with no 'stat, especially if its hot, without the stat in, water flows to fast though the rad and doesn't have time to cool down properly.

Combustion gases in the cooling system are very corrosive, bad rust around the hose fitting are usually a sure sign the head has been cracked for a while.

A small crack won't leak alot of pressure into the cooling system, especially if the motor isn't getting hot running around town, but on the motorway when the turbo is spooled up the increased combustion will push into the cooling system and empty the coolent out, then it will overheat.