This will cost money, but will tell you what is going on.
Look at the items here:
http://www.inotek.com/Catalog/fluke1th.html
Search for the best prices online,
Get a model 52 with 2 temperature imputs.
Get the Extension Wire Kits for whatever TC (J or K) comes with the meter, unless you are willing to make up your own long probes, not really hard, get J or K connectors to go with J or K wire. All needed parts are available at
http://www.omega.com/prodinfo/thermocouples.html
Tape one probe tight to the radiator outlet hose or tube. Tape a little insualtion over this probe, you dont want air over the surface of the hose to mess up you reading. Put the other probe in the airstream in from of your bumper. When you do tests, look at the temp diff between the airstream probe and the radiator outlet hose probe. That way air temp changes during the day will not affect your results as much.
Now you will know exactly what the temps are, and you can get a better idea how close you are to the point where the thermostat in the car is FULLY open and you have no more safety margin to play with.
You can test your thermostat in a pot of hot water on the stove, and characterize the amount it is open at different water temperatures.
Decide the max summer temp you will need your car to run in, and then you can calculate how high you can let the water temp get on your test days when you block off the grill.
This is all less hard than it sounds.
