check thermostat valve....same thing happened with me....the valve wasn't closing completely...
easiest way for me to check thermo valve is to start the cold car...radiator is cold...keep running it engine temp will rise but rad will still be cold. on reaching optimal temp valve will open n there wud b sudden increase in radiator temp.
caution shud b taken to safegauard ur hands from injury
BTW lean mixture=high engine temp
rich mixture=colder running engine