Ok, so I have just purchased my first s2000 a couple weeks ago.
I bought it with a blown headgasket, I first replaced the thermostat because the heat was ice cold, but it was the headgasket.
I replaced that the first week and it had been running good although because the car was sitting for a few months before I got it. The clips on the harness were easy to break. Soo I ended up breaking the clip to secure the ECT sensor together.
Now the problem is, while driving the coolant temp gauge will go all the way up to one bar before red, then go down/up all while driving but never below 4 bars.
At first I thought it was just the sensor BUT every so often the heat will go cold which makes me wonder what is wrong.
Maybe the ECT sensor somehow controls the heat?
I replaced the thermostat with an Autozone one so maybe it failed from the box.
A side note: the car doesnt go past 3 bars idling for over an hour.

 

uhh.. bump for you.. but far to blasted to honestly think about this lol

 

autozone thermostat.... really

 

are all your posts going to be this logical, clear and helpful?

 

you have air in your coolant. rebleed the system per honda's instructions and you will be set.

 

I've had similar issues with the autozone thermostat. Get an OEM Honda one, bleed the system extremely well and see what happens.

 

I am not sure, I know with civics there are 2 sensors, one for the ECT, more for computer then another sensor sending unit for the gauge, are you sure you didn't break the harness for the gauge sender unit?

 

problem still not fixed, but I know its not the ECT sensor now.
if I unplug it at idle, the gauge drops to one and the idle goes up and down like a vacuum leak.
It doesnt do this while driving so I assume the sensor is just fine.
next step is buying mishimoto hoses and thermostat.
I'll keep updated, if anyone else has some advice let me know

 

bleed the system per honda

 

so its actually overheating cause sometimes itll spray coolant from the overflow .
I think it may possibly be a fan connection cutting in and out while driving.