Yeah, I have the ELM327 and several apps, Torque is my choice because it has the free racing dashboard.  
 
This is ok, only problem is the signal and app response time is a little laggy.  Although this wouldn't be a real drama for water temp but it's just a bandaid, I want to keep my car looking as stock as possible and a couple of pillar mounted gauges are less noticeable than an LCD display.
 
When I track the car I will use the App, but would really like to just mount a couple of analogue gauges.

Getting back to your original question about coolant temp.
 
Could the thermostat be dodgy? Does it have an OEM Honda one fitted?
 
I know using an aftermarket thermostat on the 3sgte sometimes leads to fluctuating temperatures.
 
Edit: seems you can buy 60 something degree C F20C thermostats. Maybe that's what you have fitted? Hopefully not a band aid for a cooling issue. You mention it gets to 110 at idle which seems a bit high.

I have put a brand new thermostat in it suitable for the F20C that opens fully at 70C as opposed to the factory 85C due to the fact that there are no heat soak issues with the radiator.
 
The 110C is high but the radiator is dropping the temp down to 40C reliably and efficiently.
 
This is what I now know:
 
-  Coolant flows into the thermostat and has a similar temp regulation system as the SW20 MR2.
-  Water temp returning from the radiator is a constant 40C.
-  Water temp leaving the engine while car is being driven at speeds around 60km/h or higher is around 60C (low) and at idle slowly creeps up to approximately 110C (high).
-  After market water temp sensors are located incorrectly.
 
I think my next step is I have to tap into the ECU temp reading and see what the ECU is seeing.  The ECU has OBD2 wiring but it has not been installed so I will need to do this first, then I will use my ELM327 and App to check.
 
I'm really getting a feeling that there is something wrong with this system for the output temp to get so high.
 
 

What controls your thermofans?

There is a temp gauge mounted on the radiator that triggers the thermofans.

Does the idle temp get to 110'c if you increase the idle speed to 1200rpm?
 
My 1MZ on the factory ecu would end up idling at 550rpm when hot and the water pump was actually spinning too slow to pump enough coolant around the system to keep the temps down and would then slowly overheat. Now that I have the aftermarket ecu running and have a closed loop idle set for 850rpm, I have not seen the issue since.

Hey Knightrous, I tried that briefly but didn't see any result.  I'm thinking that you are on the right track though, I will give it another go.  The S2000 water pump was never designed to pressurise such a large system and move that much coolant, so kinda makes sense.  I have been pricing the davies craig electric water pump kit for $350, I definitely wouldn't have this issue if I installed that and removed the thermostat.
 
I would prefer to avoid electric though, if they fail it is sudden.

I would have brought up the same point as Knightrous.
 
My Sw20, before and after new thermostat, i had always got to around 90*C on idle, however as soon as i had started moving my temperature would drop to 60*C. My theory is that whilst at idle it would not push the water enough to actually reach and continue the flow of the cool water at the radiator end, however under rev's it would push it around an continue a circle with cool water.

It does sound feasible doesn't it?

You might need to clip the impeller on the mechanical water pump， else the electrical davies craig will be pushing against the slower mechanical pump.
This is the setup on the modified sw20 i bought. With no davies craig i needed to hold idle around 1800， or be moving to get airflow. Note I don't always drive like this， but seen this on couple occasions where i needed to remove electrical pump and drive the car a short distance.