Car Camshaft Guide: Sensors, Breaks, Power Gains & Degreeing

The car camshaft is one of the engine's most critical components — a precision-machined rotating shaft that controls the opening and closing of intake and exhaust valves. A car can sometimes start with a bad camshaft position sensor, but will run poorly or not at all depending on severity. A broken camshaft causes immediate and catastrophic engine damage. Performance camshafts do make cars faster by increasing airflow, and degreeing a cam in the car is possible but significantly more difficult than on an engine stand.

Can a car start with a bad camshaft sensor?

Sometimes — but it depends on the fault type and how the ECU responds. The camshaft position sensor (CMP sensor) tells the engine control unit the exact rotational position of the camshaft so it can time fuel injection and ignition precisely. When it fails, the ECU loses one layer of timing reference but may still be able to operate using the crankshaft position sensor (CKP) as a fallback.

In practice, outcomes vary by failure mode:

• Intermittent signal loss: The engine starts and runs, but may hesitate, misfire at idle, or exhibit rough acceleration. The ECU logs a P0340–P0349 fault code and illuminates the check engine light. Fuel economy typically drops 10–15% as injection timing becomes less precise.
• Complete sensor failure (no signal): Many modern engines will still start using CKP data alone, but will run in a degraded "limp mode" — reduced power, rough idle, and poor throttle response. Some engines, particularly those with variable valve timing (VVT) systems like Honda's i-VTEC or BMW's VANOS, cannot optimise cam phasing without CMP data and may stall under load.
• Failure on a distributor-based engine: Older vehicles where the CMP sensor also triggers the ignition module directly may fail to start entirely — the spark signal depends on the sensor output.

Common symptoms of a failing camshaft position sensor

• Check engine light with fault codes P0340, P0341, P0342, P0343, or P0344 (intake cam) / P0365–P0369 (exhaust cam on dual-cam engines)
• Hard starting — engine cranks longer than usual before firing
• Rough idle and intermittent stalling, particularly when warm
• Noticeable hesitation or stumble during acceleration above 2,500 rpm
• Reduced fuel economy — typically 5–15% worse than baseline
• Failed emissions test due to incomplete readiness monitors

A CMP sensor is an inexpensive repair — typically £15–£60 for the sensor itself and 30–60 minutes labour on most engines. Delaying replacement risks eventual no-start conditions and, on VVT-equipped engines, incorrect cam phasing that accelerates wear on the timing chain and phaser unit.

What happens if a camshaft breaks?

A broken camshaft is a catastrophic failure that causes immediate engine damage and in most cases requires a full engine rebuild or replacement. Unlike a sensor failure, a physically broken camshaft shaft or severely damaged lobe does not produce warning lights and gradual symptoms — it typically causes sudden, severe mechanical failure.

Sequence of damage when a camshaft breaks

• Immediate valve timing loss: The cylinders served by the broken cam section receive no valve actuation. Intake valves stay closed (no air/fuel mixture enters) or exhaust valves stay open (compression lost). Affected cylinders stop firing instantly.
• Valve-to-piston contact: On interference engines — which include the majority of modern passenger car engines including most Honda, Toyota, VW, BMW, and Ford units — valves held open by a broken cam lobe can be struck by the rising piston. This bends or snaps valves, damages piston crowns, and can crack the cylinder head. On an interference engine, a broken camshaft almost always destroys the cylinder head.
• Secondary damage: Broken cam fragments can travel through the oiling system, scoring crankshaft bearings, connecting rod bearings, and cylinder walls. Oil pressure drops as debris blocks oil galleries, accelerating wear on every moving component.
• Complete engine seizure: In severe cases, particularly where the engine continues running briefly after the break, connecting rod bearing failure leads to the connecting rod punching through the engine block — effectively destroying the entire engine.

Why do camshafts break?

Repair cost for a broken camshaft on an interference engine typically ranges from £1,500–£5,000+ depending on the extent of secondary damage — cylinder head rebuild, new valves, piston replacement, and machine shop work add up quickly. On high-value engines (BMW M-series, Porsche, Mercedes AMG), costs can exceed the vehicle's market value.

Do camshafts make cars faster?

Yes — a performance camshaft is one of the most effective naturally aspirated engine modifications for increasing power and engine speed capability. The camshaft determines how much air and fuel the engine can breathe at different RPM ranges, and the stock camshaft in most production engines is a compromise designed for emissions compliance, idle quality, and low-RPM torque — not peak power.

How cam specifications affect performance

Three primary specifications define a camshaft's performance character:

• Lift: How far the valve opens, measured in millimetres. More lift allows more air/fuel mixture to enter the cylinder. A stock Honda B16 cam lifts the intake valve approximately 10.6 mm; a performance Skunk2 Stage 2 cam increases this to 11.5 mm — a modest change that contributes to a 15–20 hp gain when paired with supporting modifications.
• Duration: How long the valve stays open, measured in crankshaft degrees. Higher duration cams keep valves open longer, favouring high-RPM breathing at the cost of low-RPM torque and idle quality. A stock cam might have 200° of intake duration; an aggressive race cam might run 260–280°, moving the power band 1,500–2,000 rpm higher.
• LSA (Lobe Separation Angle): The angle between intake and exhaust lobe centrelines, measured in camshaft degrees. Tighter LSA (e.g., 106°) increases peak power and overlap — good for high-RPM naturally aspirated use. Wider LSA (e.g., 114°) produces a smoother idle and broader torque curve — better for street use and forced induction applications.

Realistic power gains from camshaft upgrades

A key point: a camshaft alone rarely delivers its full potential. The cam is one part of the engine's breathing system — head porting, intake manifold, exhaust system, and ECU calibration all interact. A Stage 2 cam installed in an otherwise stock engine and not retuned can actually reduce power at low RPM without gaining significantly at the top end. Always remap or retune after a camshaft change.

Can you degree a cam in the car?

Yes, you can degree a camshaft in the car — but it is significantly more difficult than doing it on an engine stand and requires patience, the right tools, and careful access to the front of the engine. Degreeing a cam verifies that the camshaft is installed at the correct phasing relative to the crankshaft, ensuring maximum overlap, peak lift, and valve events occur exactly where the cam manufacturer intended.

Why degreeing matters

Manufacturing tolerances in timing gears, sprockets, and timing chains mean that even a correctly installed cam can be off by 2–4 crankshaft degrees from its specified centreline. On a mild street cam this is barely noticeable. On a high-lift, high-duration performance cam, 4° of error can cost 10–15 hp at peak power and shift the power band noticeably. Degreeing confirms — and corrects — this.

Tools required

• Degree wheel (360° — typically 7–12 inches diameter, mounted on the crankshaft snout)
• TDC pointer (fixed reference point aligned to the degree wheel)
• Dial indicator and magnetic base (measures valve or lifter movement to 0.01 mm precision)
• Piston stop or TDC finder (establishes true top dead centre before mounting the degree wheel)
• Offset cam gears or adjustable cam sprocket (allows correction if the cam is found to be out of spec)

The degreeing process in the car

• Establish true TDC: Remove the spark plug from cylinder 1. Install a piston stop and rotate the crank by hand until the piston contacts the stop — note the degree wheel reading. Rotate in the opposite direction until it contacts again — note that reading. True TDC is exactly halfway between the two readings. Adjust the degree wheel pointer to read 0° at this point.
• Mount the dial indicator: Position the dial indicator directly over the lifter or cam follower for the intake valve of cylinder 1 (or whichever cylinder the cam manufacturer specifies for checking). On OHC engines, this typically means accessing the cam follower or shim directly — this can be very cramped in the car with the cam cover removed.
• Find the lobe centreline: Rotate the crank slowly and record the dial indicator reading every 10° before and after peak lift. Peak lift occurs at the lobe centreline. Record the crank degree at peak lift — this is your intake centreline (ICL).
• Compare to specification: The cam card (supplied with the cam) specifies the intended ICL — for example, 108° ATDC (after top dead centre). If your measured ICL is 112°, the cam is 4° retarded. If it reads 104°, it is 4° advanced.
• Correct with offset keys or adjustable sprocket: Advance the cam by rotating the adjustable sprocket or installing an offset woodruff key in the appropriate direction. Re-check after each adjustment. Repeat until the measured ICL matches the specification within ±0.5°.

In-car degreeing challenges

• Access: On transverse-mounted engines (most front-wheel-drive cars), the front of the engine faces the firewall or is partially blocked by the radiator. Removing the radiator significantly improves access and is often worth the extra hour.
• Degree wheel mounting: The crankshaft snout must be accessible to mount the degree wheel. On some engines, the harmonic balancer must be removed and reinstalled with the degree wheel behind it — check thread direction before applying force (some cranks use left-hand threads).
• Rotating the engine: With the cam cover off and the engine in the car, rotating the crank by hand requires a breaker bar on the crank bolt or a socket on the accessory belt pulley. Ensure all spark plugs are removed to reduce compression resistance.
• DOHC engines: On dual overhead cam engines, both intake and exhaust cams must be degreed independently — doubling the work. Verify both cams relative to the specified LSA on the cam card.

For most performance builds, degreeing the cam correctly — even in the car — is worth every bit of the effort. A cam installed even 4° out of phase is running at a significant disadvantage, and the adjustment takes less than an hour once the degree wheel is properly set up.