Type R Club of America

After it’s completely warmed up , grab your upper and lower radiator hoses. If there’s a drastic difference in temps that new thermostat may have failed.

Sorry if I missed this-was the new thermostat OEM?

If not, consider replacing it with one from Honda. I did some back-to-back testing on D16 thermostats last year and there was a wide variance between OE and aftermarket.

Beyond the thermostat I would start looking at the temperature sensors. Air pockets in the cooling system usually create overheating and/or lumpy idles, not over-cooling.

Lastly, where are the temps falling from, and to? It's possible that your cooling system isn't operating optimally right now because of issue with the fan or radiator, and the system is returning to "normal" when it's coasting down a hill.

Disclaimer: I don't own an ITR yet, but have been playing with Hondas for a while =)

The car acts perfectly normal.... no issues.

Both hoses heat as they should, and everything holds pressure.

I did use a Honda OEM thermo$tat, no change after replacing it.

I'm only using the factory gauge, not looking at the coolant temps directly with a hondata or something similar.

The gauge is falling from "normal" (just to the left of the water on the gauge ~40%) to maybe 20-30% on the gauge. Never all the way to the C line or below.

Not much to add I'm afraid. The symptoms really do match a t-stat that is sticking open, but it's unlikely a new OE has failed and shows the same symptoms as the previous.

Might want to post to Honda-tech in the GSR section to confirm that this sort of thing isn't normal for that model.

IF it's abnormal then time to break open the FSM to see if it lists a range of operating temps. I don't know if you have an OBD1/OBD2/Chipped ECU, but there may be a way to see what signals the temp sensors (there may be more than one) are sending to the engine. It's possible that the temp is fine but there is a sensor issue or a gage issue. Good luck!

Just wanted to post an update... it turned out to be the temp sending unit. The little one wire deal under the distributor.
After replacing it, the gauge reads normal as it should.