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Lexus IS 250: How to Replace Thermostat

A stuck thermostat can force your Lexus IS 250 or IS 350 to overheat. Learn how to replace it yourself and keep your IS running cool.

By James Karuga - November 11, 2014

This article applies to the Lexus IS 250, IS 350 (2005-2014).

The thermostat in your Lexus IS blocks the flow of engine coolant to the radiator when the engine is cold in order to bring the engine up to operating temperature. Around 200 degrees, the thermostat opens to let the coolant in to the radiator. When a thermostat is worn out, the car's temperature gauge might read incorrectly, and there might be an overheating condition and possibly a leak or two. If it won't close, your car will take longer to get up to temperature, resulting in excessive engine ware.

Tools Needed

  • Socket
  • Screwdriver
  • Pliers
  • Torque wrench

Step 1 - Drain coolant from radiator

The engine doesn't need to be drained of coolant fluid, but it's still a good idea to do so before starting to avoid a mess. Draining the radiator should be enough as the rest of the coolant is stored in the engine block. Unscrew the radiator drain cock plug at the bottom rear of the radiator. Drain half or one gallon of the coolant and pour it into a clean bucket so that you can reuse it later.

Pro Tip

Keep the coolant away from pets; it smells sweet and is very poisonous.

Figure 1. The plug is located at the bottom of your radiator, highlighted above.

(Related Article: How to Flush Your Radiator - ClubLexus.com)

Step 2 - Unclip upper radiator hose from thermostat housing

The thermostat is placed towards the front section of the engine where the top radiator hose is connected to it. Use a set of pliers or some kind of clamp to squeeze the metal band. The black plastic that the upper radiator hose connects to is the thermostat housing. Coolant will likely drip out of here as you pull it off.

Figure 2. Use a clamp or something similar to free the coolant hose from the thermostat.

Step 3 - Uninstall thermostat

The thermostat housing is held in place by three bolts. It's shaped like a triangle with a nut at each point. Remove the bolts and extract the thermostat housing.

  • Figure 3. Location of thermostat housing bolts indicated by arrows.
  • Figure 4. Thermostat housing highlighted in red above.
  • Figure 5. Lexus IS 250/IS 350 thermostat.

Step 4 - Install a new thermostat

After pulling the old thermostat, align the thermostat valve and insert the new thermostat into the water inlet housing. If the gasket needs replacement, get a new one to take its place.

Pro Tip

Lexus owners recommend a thermostat be replaced once the vehicle clocks around 145,000 miles as a precaution to prevent the engine head from warping due to overheating. If the hose clamp uses screws, be careful when tightening it to avoid cutting the hose. As time passes, the coolant ages and it may lose additives that prevent corrosion, which causes a thermostat to malfunction and the engine to waste fuel. Hence, motoring experts advise coolant be replaced after a vehicle clocks 100,000 miles.

Featured Video: How to Replace Thermostat in Lexus IS 250

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