I got a new transmission in my '89 and I noticed after driving the temp gauge starts to go past the 210 mark. I flushed the coolant and there is brand new coolant I changed the oil (was low and heard low oil can cause overheating) and still overheating I bought a thermostat and checking the old one seeing if that is the problem. I heard of different things I was told to check the hoses see if they close when it overheats. I noticed under the hood there is new wiring which I assume is for the new transmission.

If the thermostat is ok what else will I need to inspect (besides water pump) for the overheating issue?

 

Radiator, fan clutch, fan shroud, efan, rad cap, dirty a/c condenser, etc.

 

210 and a little over is not over heating, wait until you hit the 240 mark before freaking out. Replace the radiator cap for good measure.

 

its like in between the 210 mark and the next white mark.

 

Thats fine,

Where is it at idle and where is it at highway speed?

 

Same spot and once it started heading towards the red but that was before I flushed it out.

I sometimes think the temp gauge is messed up cause after the new tranny it said my volts were under 14 like almost 9 volts. I checked with my voltmeter and it said 14 volts which leads me to believe that maybe temp gauge is wrong too. also I noticed a hissing sound when it started to overheat and TBH I tried spraying with soapy water but no bubbles and it evaporates