Camshaft

The camshaft is the precision-ground rotating shaft in the engine’s valve train that controls the opening and closing of the intake and exhaust valves at precisely the correct points in the four-stroke combustion cycle. Driven by the crankshaft via a timing belt, timing chain, or gear set at exactly half crankshaft speed, the camshaft carries a series of egg-shaped lobes — one per valve — whose profiles are precisely engineered to open each valve by a specific lift amount, hold it open for a specific duration, and close it at the correct moment relative to piston position. The relationship between the crankshaft and camshaft timing (valve timing) is critical: intake valves must open as the piston descends on the intake stroke to allow the air-fuel charge into the cylinder, and exhaust valves must open as the piston rises on the exhaust stroke to expel combustion gases. On dual overhead camshaft (DOHC) engines, separate intake and exhaust camshafts allow independent optimisation of intake and exhaust valve timing. Variable valve timing (VVT) systems — now near-universal on modern engines — use camshaft phasers driven by oil pressure to advance or retard camshaft timing relative to the crankshaft in real time, optimising power, torque, and fuel economy across the RPM range. The camshaft is lubricated by pressurised engine oil delivered to its bearing journals and, on DOHC engines, to the variable camshaft phaser actuators.

Each lobe on the camshaft acts as a rotating eccentric that lifts a follower (either directly via a bucket tappet, or through a rocker arm) as the high point of the lobe rotates past the follower. This lifting action compresses the valve spring and opens the valve. As the lobe rotates further and the follower descends from the high point, the valve spring forces the valve closed. The lobe profile — the shape of the transition between the base circle and the nose of the lobe — determines the rate of valve opening and closing, with aggressive profiles opening valves quickly for maximum airflow and gentle profiles reducing noise and mechanical stress. The duration the valve remains open (measured in crankshaft degrees) and the valve lift (the maximum distance the valve opens) together determine the engine’s breathing capacity at any given RPM. Performance engines use longer-duration, higher-lift camshaft profiles to allow more air-fuel mixture into the cylinder at high RPM, at the cost of idle smoothness and low-RPM torque. Variable valve timing systems vary the phase angle between the camshaft and crankshaft by rotating the camshaft phaser assembly — advancing timing improves low-RPM torque and idle stability; retarding it improves high-RPM power. The ECU continuously optimises this relationship based on throttle position, engine speed, load, and temperature.

Cam lobe wear causing loss of valve lift: Cam lobes wear progressively from the sliding contact with their followers, particularly in engines with flat-follower (sliding) designs. A worn lobe reduces valve lift and duration, causing the affected cylinder to breathe poorly, producing a rough idle, loss of power, and increased fuel consumption. Tapping or ticking noises from the valve train may indicate lobe wear. Diagnosis requires measuring valve lift with a dial indicator or removing the cam cover for visual inspection of lobe condition.

Variable valve timing actuator (phaser) failure: The VVT phaser is an oil-pressure-operated actuator that rotates the camshaft within its sprocket. A failed phaser — from sludged oil passages, a failed phaser solenoid, or mechanical wear — causes the camshaft to lock in its most advanced or retarded position. Symptoms include a rattling noise at idle (particularly at cold start), rough idle, poor fuel economy, reduced power, and a check engine light with a cam timing fault code. Maintaining clean, fresh oil is the primary prevention for phaser failure.

Timing belt or chain failure causing camshaft-crankshaft desynchronisation: Covered in detail under Timing Belt and Timing Chain entries — the result in an interference engine is catastrophic contact between the open valves and the pistons, destroying both components and requiring complete engine replacement or rebuild.

Level 1 — Valve Train Noise and Performance Monitoring: Listen for ticking or tapping from the top of the engine (the cam cover area) that changes with engine speed. A light ticking that disappears after warm-up may be normal hydraulic tappet behaviour at cold start. Persistent ticking or a rattle at idle that is accompanied by a check engine light with a variable valve timing code indicates a VVT phaser or actuator issue requiring prompt attention. Any noticeable loss of power, rough idle, or increased fuel consumption alongside valve train noise warrants professional diagnosis.

Level 2 — Oil Change Compliance for VVT System Protection: Variable valve timing systems are critically dependent on clean, low-viscosity oil delivered promptly to the phaser actuators on cold start. Extended oil change intervals allow oil to thicken and sludge, blocking the small oil passages that supply the phaser solenoids and actuators. Use the exact viscosity grade specified by the manufacturer — 0W-20 or 5W-30 specifications on modern VVT engines are not interchangeable with thicker grades. Change oil at the specified interval without extension.

Level 3 — Professional VVT and Timing Service: VVT phaser solenoid cleaning or replacement requires cam cover removal on most engines. A failed phaser unit requires camshaft removal on many designs. Camshaft replacement due to lobe wear requires valve train disassembly and, on DOHC engines, timing belt or chain removal. Any camshaft work requires precise re-timing of the valve train to the crankshaft — incorrect valve timing causes rough running and, in severe cases, valve-to-piston contact at high RPM.