As usual, excuses for my spanglish.

A third-world recipe:

Some T gauges are inaccurate (mine, a Japan-made round one, read 6 degress under de real T). I suggest to borrow a "turkey" termomether (the metalic one used on baking and cooking on furnaces, a long metalic with a dial on the top, ACE and Sears have a good ones - and probably you can find one on a drawer on your kitchen), open the radiator and put the thermomether on it. Start the car and register the temperature. You would be see a increment on the amount of water returning from the engine on the instant you thermostat are open.

Register the temperature and run some minutes more, the temperature will stabilize after some time, if not, your radiator (or the hoses, upper or lower) are clogged.

Of course, if you don't have a increment on the flow of water, then your thermostat are clogged or damaged.

Also, If you (at same time) make a registry of the T on the car thermometer (build into the control panel), you can estimate any difference between the two sources of data and use it the future for make mental corrections to the day-by-day readings.

I take the idea of using a cooking thermometer from this board, you can see it on old postings.

Sincerely,

Joaquin Novara
Rojo 121 Amazon
Lima - Peru