I have mentioned before that one of my goals for maintaining lean burn in the Insight is to keep the water temp in the 180 to 190 degree range at all times. Even with a 100% blocked grill (manually adjustable) I was not able to meet this goal in winter, especially while using EOC for as much as 20% of the miles driven.
So I designed a system that uses a 3 way "T" valve to modulate the amount of coolant flowing through the radiator from 100 percent to as little as 0 percent that can be adjusted from inside the car.
For safety, I also have a digital temp sensor where the coolant exits the upper radiator hose and another where the coolant re-enters the engine in the lower radiator hose.
The 3 way valve never blocks the flow of coolant out of the engine, it just diverts as much as 100 percent of the flow around the radiator.
Here are some pictures
The T valve is attached in line with the upper radiator hose between the engine and radiator. The bypass hose attaches inline with the lower radiator hose between the radiator and the engine,
Attached to the T valve is a push-pull cable with 3 inches of "throw". This cable extends into the passenger side of the firewall.
Here is the valve being test fitted in the car.
Here is the in-car adjustment handle. It is the lowest cable with the long lever handle. Above it are the knobs for adjusting the warm air intake and the adjustable grill block detailed previously in this thread.
This system worked flawlessly all winter. One morning with an outside temp of18 degrees, I was able to make my 54 mile commute to the shop while diverting 100% of the coolant past the radiator. The highest water temp was 190 degrees.