engine management light on

[ThyRosh]

whenever i go down a steep hill, hit a bump to fast or under harsh breaking my engine management light comes on, also when im going down hill my tempreture creeps up to about 100 and the heating goes really cold, but when i go up a steep hill, the tempreture drops and the heating gets hot again, i recently f**k up the engine by runnin it out of water and then i had to replace the thermostat and rad, but it all started after a while after these were refitted, any suggestions please?

[GrimeTiing]

Sounds like the head gasket is going mate, is there a white gungy liquid under the filler cap?? also check dipstick for the same.

[ThyRosh]

the oil filler cap?, the dipstick was clean, apart from oil obviously, and it had enough oil in it but ill check the cap later, i thought it could be some sort of sensor? beacause when im facing downhill it all goes wrong so im thinkin a loose connection somewhere. but i dont know whats loose or what could be loose