Ah the joy of buying a used car lol.

My car would drive fine for 10-15 miles on fwy then it'll start to run warmer. If I exit the fwy or leave it idling, it'll come down and stay there. At stop and go conditions, it'll run fine. The problem is intermittent. It'll drive fine for awhile on fwy then temp needle will go up. Is this classic symptom of bad thermostat? Help me out guys. Other than this loving my LS!

 

Hi,
Things to check are the pressure cap on the expansion bottle, the thermostat and that there are no obstructions in the radiator.
Another is a the water pump on its way out.
Check the coolant level in the expansion bottle when cold then recheck when the engine has warmed up the level should have risen slightly indicating the system is pressurized correctly.
Also check the cooling fan for correct operation although this rarely comes on whilst driving at highway speeds.

 

while your checking everything, replace the fluid if you havent already too

 

Thanks for the replies. Mechanic said radiator is clogged, it looked like it's never been changed. Is it possible for radiator to go for 19yrs? Getting it changed along with new thermostat as of now. We'll see how it goes.

 

Got the car back. Drove all day and no overheating. Glad it's over. Looks like previous owner enjoyed the car too much and forgot about maintenance which explain why engine was replaced 30K miles ago. This is what came out of the car. Notice the gutted thermostat and clogged radiator.