The serpentine belt drives the accessory components of a vehicle's engine and keeps things like the air conditioner and the power steering pump operating properly. The timing belt on an engine controls the timing of the valves on the actual motor. The timing belt needs to be changed periodically to keep your engine running properly, and the Lexus RX 300 is no different. While your exact model of RX 300 will have different requirements in terms of time between changes, the timing belt must be replaced when it is worn out or irrevocable motor damage can occur.
Park the Lexus in a flat, level area and set the parking brake. Let the engine cool down and chock the wheels for additional safety. Open the hood to expose the engine and disconnect the battery leads with a wrench. Slide the car lift under the front side of the vehicle and place it according to the manufacturer's instruction; most car lifts sit under the axle of the vehicle, but different models of car lifts will have different requirements. Lift the vehicle off of the ground/floor.
Locate the belt diagram on the hood or shroud cover of the car and use it to find the automatic belt tensioner for the serpentine belt. Place a socket and ratchet over the bolt on the tensioner pulley and release tension by leveraging it in the direction indicated on the diagram. Slide the belt off of the pulley and let it close again.
Remove the cover over the timing belt with an appropriate-size wrench or socket. Loosen the crankshaft pulley bolt with the crankshaft pulley wrench; this will require getting under the vehicle. Turn the crankshaft until the timing marks line up with the camshafts. Turn them together with the crankshaft sprocket until the pulley timing mark on the crankshaft lines up with the guide pulley bolt, and remove the crankshaft pulley bolts.
Manually adjust the tensioner to release tension on the timing belt. Remove the timing belt by lifting it off the sprockets and pulleys. Install the new timing belt by wrapping it around the crankshaft sprocket and lining up the marks on the belt with those on the shaft. Slide it over the tensioner pulley when finished and let it automatically adjust. Reverse the process to reassemble the various parts of the engine.
