It's a '97 DX HB with 135k mi. on it. I noticed the last couple of days that when I go 75mph (indicated) or higher the temp. gauge starts climbing to 2/3 or 3/4 of the way up. It NEVER went passed the thermometer picture before. It was always under that, and steady. I tried turning the A/C off, thinking it was that, no good. The FAN works fine, as does the AC fan. But I 've noticed the fan comes on a lot more often now and stays on longer (maybe because of the 90deg. weather we 've been having?).
The dealer replaced the radiator fluid at 91k mi. when they did the timing belt and water pump. I hope it's just the thermostat, and not a sign that the head gasket is going or the radiator.
I puzzled why it only does it at 75mph & up. I guess because the engine gets hotter and more fluid flows through the thermostat? Not sure. Does anyone have any ideas? I 'm almost ready to go out and buy a thermostat but I want to make sure. I think it's a little too soon for the head gasket to go, but you never know. I 'm not ruling out a water pump or radiator either but I have NO leaks. Thanks for your help guys!

 

first do a proper mixture for summer 70% h2O and 30% coolant is good for cooling....

next replace the thermo.....

get some Redline water wetter,

next turn the dial on heat turn off, the fan....no A/C for u sir....

otherwise upgrade to Del Sol VTEC dual core rad....it bolts up without any problems....or go with a LS/GSR/ITR rad...will need the mounts to be welded in but it is defiantly worth it...

 

Water pump perhaps? If you keep putting fluid into the radiator, you might have head gasket issues. Otherwise, what eg6ajk said. Redline Water Wetter works.

 

I have to disagree. Coolant does not help the heat capacity of the water, it actually hurts it. Racers do not use coolant, they generally run straight distilled water with possibly some Redline Water Wetter.

I run 70/30 water/coolant with Redline Water Wetter. I have zero cooling problems in 95+ degree weather. I use this mixture on the race track. The only reason I use some coolant is because I run the car on the street and it will see temperatures occasionally below freezing.

 

thanks guys.
Nope, I don't have to add any antifreeze. The reservoir level stays at FULL and there are no leaks, and NO white smoke blowing out of the exhaust.

I suspect like eg6ajk said, it might be the mixture needing replacement which is about 44k mi. old now. The dealer replaced it as well as the water pump when they did the timing belt at 91k mi. Car has 135k now. I believe they said they did a 50-50 mixture..
I also think the radiator could be on its way out but I 'll replace the thermostat just in case and then probably wait for the radiator to go. Don't want to spend too much money because I 'm thinking of selling the car before year's end to get a new '02 Si possibly.