Car overheats after driving on freeway for about a hour.

[SuprAllen]

I had this and mine was an air lock in the heater matrix, on a 5 mile run it would blow a lot of water out of the overflow I've cured it by making a bleed valve and fitting it in the top heater hose. I used standard 15mm plumbing fittings and a house radiator bleed valve. Fill the rad as normal and then refit the cap( I replaced mine because it let water out without being under pressure ) take out the bleed valve from its seat and you can get another 1/2 pint of water into the system through that hole. Put the bleed nipple back, run it till it's hot, and bleed it as if it was a house radiator. My mate had the same problem and has cured his the same way. If I can work out how to add pictures to this I'll post them