If the engine is slooow to come up to temperature, fails to reach temperature, or drops in temperature while driving, the thermostst is not doing its job. It would seem that yours is stuck partially open. It is not unknown for thermostats to be bad straight out of the box, but before buying a new one, test the one you have. Simple enough... just get a pot of water near boiling (90-100 deg) on the stove, tie a thin piece of wire to the thermostat and suspend it in the water.
The thing should go from fully closed to wide open in 20 or 30 seconds. If it doesn't close fully it is bad. If it doesn't move at all... it is very bad
Cheers... jondee86
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