when the thermostat and coolant/temp sensors are working properly within spec:
below 82c the thermostat should be fully closed and the coolant only flows through the block so it gets heated up asap.
the needle should rise upto 82c 1/2 way up the gauge as fast as possible (bout 5min driving down the road).
the thermostat should only begin to open above 82C, sending the excess heat energy over to the radiator to transfer heat from coolant to air. at this point the needle should stop rising and then stay exactly 1/2 way throughout any driving conditions as the thermostat automatically maintains an exact temperature equalibrium.
Sensor
the gauge is only as accurate as what the sensor was calibrated to.
when the gauge sensor (sensor with a single terminal pin) is at 82c it should develop approx 0.8kohm.
test by removing it, connect to a multimeter and dunk in boiled water with thermometer and check resistance at certain degrees.
I've done a test and graph to compare results to
http://micra.org.uk/threads/pollymobiles-rebuild.35251/page-119#post-704079
when the resistance is correct, the gauge itself should be reading at 1/2 way.
Thermostat
the wax inside should normally be able to control the valve
quickly,
accurately and able to operate through it's
fully stroke.
Response time - if the mechanism is sticking or can't move the valve fast enough to changing water temps from the engine, it'll cause the temps to oscillate up & down.
Accuracy - if the wax has degraded from its stamped rating and changed the temp at which it begins to change state n move the mechanism, it'll alter the temp range at which it tries to keep the engine at.
Stroke - if the valve is not fully closed when the thermostat is below 82c, it'll allow some heated coolant to leak past and unintentionally cooled. this will prolong the heatup period and also cause the thermostat to oscillate.
check by dunking the thermostat in a boiling pan of water, for few min. it should fully open quickly.
once it reaches the rated 82c, it should be fully closed by then.
below 82c it should be fully closed.
here's when I was testing my 92c thermostat
another note, when warming up a cold engine I always recommend turning the heater dial to full cold to shut off the coolant through the heater matrix.
this prevents any coolant that was just heated by the block from being cooled by the heater matrix, allowing the engine to simply heat the coolant up fast as possible. once up to temp, then I'll consider opening the heater.
