If the timing belt is worn out or damaged, a vehicle that's running just fine may suddenly stop. You may hear a clunking noise and may not be able to restart the vehicle anymore. The broken belt must be replaced as soon as possible. This should also be properly maintained according to recommended service intervals.
How to replace a timing belt
Step 1: Disengage the negative battery cable.
Step 2: Pull out the distributor cap (if the vehicle is equipped with one). For electronic ignition engines without a distributor (but a cam and crankshaft position sensor instead), look for the top dead center (TDC) on the #1 cylinder. Consult the engine repair manual for this.
Step 3: Turn the engine until you reach the point wherein the crankshaft pulley's timing mark is lined up with 0 degree on the timing scale. Use a wrench or socket for rotating the engine. (Note: Do this only when the timing belt isn't torn or broken yet.)
Step 4: Check the alignment of the distributor rotor with the distributor housing's index mark. The rotor should be positioned to fire the #1 cylinder. If the alignment is off, you'll have to turn the engine. (Note: Read the engine repair manual as a guide.)
Step 5: Disconnect whatever components get in the way of the timing belt cover removal. You may have to remove accessory drive belts or the crankshaft pulley. (Note: Check the service manual to find out which parts have to be disconnected or pulled out.)
Step 6: Undo the bolts or screws securing the cover. After these have been removed, you can pull the cover off the engine. (Note: Some engines are equipped with a two-piece timing cover.)
Step 7: See if the crank is properly aligned with the camshaft timing marks. (Note: Check your service manual for the alignment procedure and necessary adjustment prior to belt installation. This varies for different types of engine.)
Step 8: Look for signs of oil leak around the belt. Oil leaks may be traced to the crank and cam seals, oil pan, and valve cover or the water pump and its by-pass hose. Fix the leaks before installing a new belt.
Step 9: Use the right tools for loosening the tensioner of the timing belt. For spring-loaded tensioners, a socket and wrench will do (some would require a male hex/Allen wrench). In some vehicles, tensioner mounting bolts are hidden by motor mounts or other components. You need a special tool to reach through the tensioner bolts in this case or to work on releasing other spring-loaded tensioner types.
Step 10: Hold the tensioner while loosening the mounting bolts. Don't remove the loose bolts yet.
Step 11: Rotate the spring-loaded tensioner away from the belt. To keep the tensioner in loose position, you have to retighten the mounting bolts.
Step 12: Inspect the tensioner pulley for dents, cracks, and other signs of damage. To check for loose or worn-out bearings, one good technique is to turn the tensioner pulley to spot any rattling or humming sounds. The tension pulley could be misaligned with the belt due to worn-out bearings, which can be best determined through uneven wear spotted at the timing belt's rear. You have to replace the pulley and other broken parts in case of bearing damage or wear.
Step 13: Slide off the belt from the sprockets after relieving tension. If the belt is stuck in the grooves, use a screwdriver to pry it gently.
Step 14: Reassemble everything that was replaced and removed. Make sure that the bolts are torqued according to manufacturer specifications or the engine manual. For hydraulic timing belt tensioners, you may have to remove the tensioner first in order to squeeze the piston back into the cylinder. Use a vise for this. Once all holes have been aligned, insert the holding pin and then put back the tensioner.