I bought this car a few months ago (200k miles), supposed to have a new timing belt and water pump, etc.

A few weeks back it overheated while driving and spewed some coolant over by/underneath the dist cap by the t-stat housing and the temp gauge was at full H. I started the car back up and noticed the radiator fans were not working (I had been driving around with the AC off because of the nice weather). I then turned the AC on and the fans immediately fired up. I topped off with coolant and drove the car home with AC on and had no problem. I then replaced the fan switch and all was good.

About 2 weeks later I pulled up to work and notice the car is spewing coolant on the same spot. I did not notice the temp gauge when I pulled up but then turned it back on and the temp gauge started rising immediately. After leaving work I turned the AC on to make sure the fans were running and the temp immediately shot up to almost H. I limped the car home with the HEAT on full blast and the temp gauge hovering at the 3/4 spot.

As a premature measure I went ahead and replaced the thermostat with a Honda OEM unit. I filled up with coolant and now here is my problem - I can let the car idle with AC off and the fans will never turn on. BUT the temp gauge will stay at 1/4 and act like the car is fine. The top radiator hose gets way too hot to the touch at this time. It was 90+ degrees outside so there is no way that the car did not reach the temp needed for the fans to turn on. I let it idle for about 10 minutes this way. I've also driven around without the AC on and the temp gauge stays at 1/4 and the fans off - this is all in 85-90+ degree humid TN weather. Now, if I turn the AC on the fans immediately kick on.

1) Which sensor does the temp gauge on the dash run off of - ECT or fan switch sensor?
2) Is it possible my brand new fan switch is junk? I bought a Beck Arnley unit off Amazon, not an OEM one.
3) Is it possible it's junk not reaching full temp even in this weather? Seems impossible.

Basically, I feel like the car may be overheating but my gauges and fan switch are not telling me so. I think I will know in the next few days when/if it overheats again and starts spitting coolant.

 

For your 99, the coolant temperature sending unit controls the temp gauge . See pic.

Check to make sure your heater hose does not have a leak. IIRC, one end of the hose connects under the area of the distributor.

The radiator fan switch is a normally open type of switch, when the coolant temperature is above 199 degrees F, it should close to turn on the fans. Disconnect the connector at the radiator fan switch and jump the connector with a wire or small paper clip, turn the car to ON, does the fans run?

Feel the lower radiator hose to see if it get almost as hot as the upper radiator hose. If it is, then the thermostat has opened.

 

I jumped the radiator fan switch connector with a paper clip, turned the car on, and both fans turned on.

I checked the lower radiator hose and it took a while but it finally warmed up significantly. I believe the new OEM thermostat is working fine.

The only time I could detect a leak in that area is when the car overheated and it was dripping on I believe the heater hose and then evaporating and causing the smoke. I have not seen a leak when it has not overheated in this area.