I agree with proverbial_slang. u need to check the thermostat valve. to be sure, replace it with a new one. get one from a good spare parts shop. secondly, I am also suspecting a faulty auto fan switch. get it checked too. and its a good step you took to place ur fan on auto. my mehran is a 1996 model and is on auto fan since. the engine has never been overhauled. u just need to spend some effort on troubleshooting and you will be successful.