HELP!!!! air block!!!!!! overheating

[striper]

hi guys

i've just had a 500sec engine 1990 fitted
before i had a 380 the thing is i've had a compression test and the co is 1.6 and the hm (cant remember what its called) is 470

so the engine is great
when i left i ended up haveing a play on the dual carrigeway and the heat shot up to nearlly max when i pulled over i had what i think is an air block
what do i do

do you know what causes this

my heater is always blowing in warm air!!!!!!!!

scott