Thermostat Inspection

WARNING: This page is about a different car, the 2003 Suzuki Vitara. However, it is still accessible from the selected car via links, so may be relevant.

Make sure that air bleed valve (1) of thermostat is clean. Should this valve be clogged, engine would tend to overheat.
Check to make sure that valve seat is free from foreign matters which would prevent valve from seating tight.
Check thermostat seal (2) and O-ring for breakage, deterioration or any other damage.

Fig 1: Identifying Thermostat Air Bleed Valve & Thermostat Seal
Courtesy of SUZUKI OF AMERICA CORP.
Check thermostatic movement of wax pellet as follows:
1. Immerse thermostat (1) in water, and heat water gradually.
  
  Fig 2: Checking Thermostatic Movement Of Wax Pellet
  Courtesy of SUZUKI OF AMERICA CORP.
2. Check that valve starts to open at specific temperature. See THERMOSTAT DESCRIPTION .
3. If valve starts to open at a temperature substantially below or above specific temperature, thermostat unit should be replaced with a new one. Such a unit, if reused, will bring about over cooling or overheating tendency.