My 1990 Honda accord heats up after about 15 to 30 min after driving it in the High Way. It usually starts heating after I hit the 60mph range. Once the temperature is going up I put in neutral and it cools back down. When I go a little far I have to be putting it in neutral constantly to let it cool down. It doesn't really heat up when i'm driving in the city at lower speeds. By the way after I drive it for about 5min the check engine light comes on. I checked the coolant and everything seems to be fine with radiator. Could anyone please tell me what it could be..Thank you.