I have an 84 as well. So I have to say that yours is running just a tad too hot. I'd put a 160 degree thermostat in and as far as the check engine light, Check to see if a code was set at the ECM, if so I belive it will show your Oxygen sensor is burnt out or just needs a new one(They wear out) All of this I assume that the car runs OK in all other regards? If not I'd be looking for a vacume leak, which will set off a check engine light as well. you give very little background to go on, but I believe this is your problem.