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Timing chains connect an engines pistons with the intake and exhaust valves. The intervals of opening and closing are very short -- from one-fifth of a second when idling to two hundredths of a second at 5000 rpm -- so small problems can seriously affect engine performance. These problems occur as the timing chain wears down. Several symptoms arise when this happens on a Chevy.
Engine Start Problems
If the timing chain is sufficiently worn to jump off the the gears that connect it to the engine cams, the engine will not be able to start. This can be diagnosed by opening the hood, removing the distributor cap and cranking the engine by hand. If the cams dont move, this indicates that the timing chain has broken or has slipped its gears.
Rough Running
A worn timing chain that is still on its gears may cause the valves to open when the pistons are at the wrong point in their stroke. The result will be the engine running rough as unburnt fuel is pushed through the engine. Rough-running engines will make a grinding or growling noise while idling and stutter instead of speeding up smoothly when you step on the gas pedal.
Backfiring
Since worn timing chains may admit unburned fuel into the engine, there is always a risk of backfire as that fuel is heated and explodes in the exhaust system. This produces a loud "bang" and a puff of smoke and can damage your exhaust system.
Noise from the Casing
Timing chains are protected by a casing inside the engine compartment. A worn timing chain may make noise inside the casing, especially if it is sitting loosely enough on its gears to rattle. To hear this, you will need to open the hood with the engine running and listen closely since it may be inaudible above the general engine noise while you sit in the car.
