Jeep’s 4.0-liter inline six-cylinder engine is probably one of the best Jeep’s most recognizable power generators ever used. It was used in many different models, from the Wranglers to the Grand Cherokee. The design hasn’t changed since 1988, when the 4.0 liter was converted to fuel injected. The thermostat is easy to change and is located at the front of the cylinder head. The thermostat on the 4.0 liter engine will open at 195 degrees and replacement is available at any auto parts store.
Remove the radiator cap. Make sure the engine is cool before starting.
Place the drain pan under the radiator. You will find a drain cock on the passenger side of the bottom of the radiator. Open it and drain the radiator until the coolant level drops below the top radiator hose.
Locate the clamp on the upper radiator hose at the engine end where it attaches to the thermostat housing. Loosen the clamp and remove the hose from the housing.
Locate and remove the two screws on each side of the housing with a regular or socket wrench and ratchet. The housing can be removed when the screws are out, but you often have to tap it with a rubber mallet, because the gasket will cause it to stick.
Remove the housing and thermostat from the cylinder head. Clean any remaining gasket material from the housing and cylinder head with a putty knife.
Install the new thermostat and gasket, making sure the thermostat is oriented the same way the old one came out. If the thermostat is set back, it will not function properly and the engine will overheat.
Tighten the housing screws with a regular or socket wrench and ratchet. The screws should be tight, but don’t tighten too hard or the housing grips will break.
Replace the upper radiator hose and tighten the clamp with a screwdriver. Replace the coolant in the radiator and replace the cap. Test run the engine to verify that it is operating properly.
Warnings
Never open the radiator cap when the engine is hot. The system is under pressure when it heats up, and boiling coolant can splash out if the pressure is suddenly released.