By Jim Kerr
Routine vehicle maintenance is often thought of as simply changing the oil and checking the tires. Even annual tune-ups are often forgotten until the vehicle starts to act up. But there is one thing that is part of routine maintenance that should never be overlooked: it’s the engine timing belt. Forget about it, and it could cost you a new engine.
Not all engines use a timing belt. Some drive the camshaft with a chain. Chain drives are more expensive to build, but often last the life of the engine. They are also narrower than a timing belt, so there is a tendency to now use chains more than belts to enable more compact engine designs for tight engine compartments. How do you know if your engine has a belt or a chain? It may be difficult to tell by looking, so the best way is to check the owner’s manual maintenance schedule.
I have never seen a recommendation for changing a timing chain at specified intervals. A chain will start to make noise long before it will do any damage to the engine. Belts on the other hand, can be working merrily away and all of a sudden – bang! – you’re walking.
Engines with timing belts will have a scheduled change interval in the maintenance schedule. Most newer engines need them changed every 100,000 km. Some will recommend changing them at 160,000 km, but there are some that need inspection, adjustment and possibly changing as low as 50,000 km. Don’t guess. Check the owner’s manual or the with the service department of a local dealership.
Usually belts don’t break. The teeth on the inside of the belt that meshes with the sprockets come off. The camshaft stops turning but the crankshaft doesn’t. Pistons come up and hit the open valves, perhaps costing you an entire engine.
Most current engines are not free-spinning. The need to reduce exhaust emissions has forced engine designers to eliminate the recesses in pistons that would allow the piston to move to the top of its stroke without hitting an open valve. These recesses trapped pockets of unburned fuel, creating more emissions. Non free-spinning engines will bend the valves every time if the crankshaft is turned without turning the camshaft at the correct time.
Changing a timing belt usually isn’t hard but can be time consuming. However, do it wrong and you will be looking at major engine repairs. Usually there are timing marks on the crankshaft sprocket and the camshaft drive sprocket or sprockets. These marks are aligned in relation to the block and cylinder head. When the marks are lined up, number 1 piston will be at top dead centre of its stroke.
After removing parts such as the crankshaft pulley, engine covers and sometimes an engine mount, the timing belt tensioning mechanism is released so the belt can be slipped off the sprockets. A new belt is installed and the correct tension set on the belt. Too tight and it will “sing” or make a whining noise. Too loose and it may slip. Check that the timing marks are still aligned, and before putting it back together, rotate the crankshaft two full turns in the direction of engine rotation and check those marks again. If a sprocket is out of time by one tooth, the engine will run but be low on power. Two teeth out and it will usually run but roughly. Three teeth out or more and you may bend a valve.
After a belt has failed, usually a new belt is installed and then a compression test is done on the engine. Cylinders with low compression will likely have a bent valve, requiring the cylinder head to be removed and repaired. It is much easier to replace a timing belt before it fails than to replace the belt and valves after it fails.