Camshaft

Each cam lobe pushes a valve open as the shaft turns, and a spring closes it again. Most modern petrol engines have two camshafts per cylinder bank (DOHC): one for the intake valves and one for the exhaust valves. The camshaft turns at half crankshaft speed. The camshaft must stay precisely in step with the crankshaft. If the chain or belt stretches or jumps a tooth, the control unit sees the mismatch through the camshaft and crankshaft position sensors and sets a correlation code.

Engines with one camshaft per cylinder bank are called SOHC (single overhead camshaft); those with two are DOHC (double overhead camshaft). Older engines had the camshaft in the block and opened the valves through pushrods.

The crankshaft drives the camshaft through a chain, belt or gears. On engines with variable valve timing, a phaser on the end of the camshaft can rotate it slightly relative to its sprocket, using oil pressure. The shape of each lobe decides how far a valve opens (lift) and for how long (duration). The camshaft turns at half the speed of the crankshaft, because each valve opens once every two crankshaft revolutions.

Camshaft timing errors cause rough running, hard starting and codes like P0016 or P0011. On interference engines, a timing drive that slips far enough can let valves hit pistons. Worn cam lobes or followers can cause ticking noises and loss of power. Regular oil changes with the specified oil are the best protection, because the camshaft relies on a constant oil film.