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In a Subaru engine, the camshaft is the shaft that turns crankshaft rotation into precisely timed valve movement. Its lobes push the valvetrain to open the intake valves for fresh air and fuel, then open the exhaust valves so burned gases can leave. Camshaft position determines valve lift, opening and closing timing, and the overlap between intake and exhaust events.
Many Subaru engines add Active Valve Control System (AVCS), which changes the camshaft’s phase angle while the engine runs. The camshaft lobes still have a fixed shape, but AVCS advances or retards the entire camshaft relative to the crankshaft to suit engine speed, load, temperature and emissions demands.
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What the camshaft does during each engine cycle
A four-stroke engine completes intake, compression, power and exhaust strokes. The crankshaft converts the pistons’ up-and-down motion into rotation. The camshaft uses that rotation to operate the valves at the correct point in each cylinder’s cycle.
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As a piston travels down on the intake stroke, an intake cam lobe rotates into position and opens the intake valve. The cylinder can then draw in air, or an air-fuel charge depending on the Subaru engine’s fuel system. The lobe’s height and shape determine how far and how quickly the valve opens.
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Compression and power
The intake valve closes before compression is completed. Both valves remain closed while the mixture is compressed and ignited. Keeping the valves shut prevents pressure from escaping during combustion, allowing the expanding gases to push the piston down.
Exhaust event
Near the end of the power stroke, an exhaust cam lobe opens the exhaust valve. The piston then moves upward and pushes combustion gases out of the cylinder. The exhaust valve closes before the next intake event is complete.
Overlap
For a short portion of the cycle, the intake and exhaust valves can be open at the same time. This overlap helps exchange gases at particular engine speeds, but too much overlap at low speed can reduce idle quality and torque. Camshaft timing determines when overlap begins, how long it lasts and how much valve lift occurs during it.
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How Subaru’s boxer layout affects camshafts
Subaru’s horizontally opposed engines place cylinders in two banks on opposite sides of the crankcase. Each bank has its own cylinder head and valvetrain. That physical arrangement is why a Subaru engine can have more than one camshaft even though it uses one crankshaft.
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The exact count depends on the engine family and model year. A Subaru DOHC service manual describes separate intake and exhaust camshafts on the right and left banks: two camshafts per bank and four total. That four-cam arrangement is not universal across every Subaru engine.
SOHC and DOHC layouts
| Layout | Camshaft arrangement | What it means for service and control |
|---|---|---|
| SOHC (single overhead camshaft) | One camshaft operates the valves in each cylinder head | Fewer camshafts, with the rocker or follower design determining how one shaft operates intake and exhaust valves |
| DOHC (dual overhead camshafts) | Separate intake and exhaust camshafts in each cylinder head | Allows direct control of intake and exhaust timing; a two-bank boxer engine can therefore use four camshafts |
Do not identify a Subaru engine by cam count alone. Confirm the engine code, model year and cylinder-head design before ordering parts or planning a timing service.
Camshaft parts and construction
A camshaft has journals, lobes and a drive connection. Journals support the shaft in the cylinder head through bearing surfaces and caps. Lobes convert rotary motion into the lift required by the valves, usually through buckets, rocker arms or followers, depending on the engine design.
The Subaru DOHC service information describes hollow camshaft construction and sintered-steel lobes. Those details reduce mass or provide wear-resistant lobe surfaces, but the materials and dimensions differ among engine families. Camshafts are precision parts: a worn lobe, damaged journal or incorrect clearance can alter valve lift and timing even if the timing marks are aligned.
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How the camshaft connects to the crankshaft
The timing belt or timing chain links camshaft rotation to the crankshaft. The crankshaft provides the reference rotation; the camshaft must turn at the correct speed and phase so each valve event matches piston position. In a four-stroke engine, the camshaft completes one revolution for every two crankshaft revolutions.
Subaru uses different timing-drive designs across engines and model years. Some use a belt, others a chain, and the tensioners, guides, sprockets and service procedures are correspondingly different. A slipped belt or chain, incorrect installation, stretched chain or failed tensioner can put valve timing out of alignment even when the camshaft itself is undamaged.
How AVCS changes camshaft timing
AVCS is Subaru’s variable valve-timing system. It does not reshape the cam lobes or independently change their lift. Instead, an oil-controlled cam sprocket changes the camshaft’s phase angle relative to the sprocket and crankshaft.
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The engine control module (ECM) selects a target cam angle from operating information such as engine speed, vehicle speed, throttle angle, airflow, coolant temperature and camshaft-position feedback. Exact inputs and strategies vary by engine and calibration.
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How the hydraulic mechanism works
- The ECM compares the desired cam angle with the actual position reported by the camshaft-position sensor.
- It commands an oil-flow control solenoid, sometimes called an oil-control valve, to direct engine oil through passages in the cam sprocket.
- Oil pressure moves the sprocket’s internal phaser, advancing or retarding the camshaft relative to the sprocket.
- The camshaft then opens and closes the valves earlier or later in relation to crankshaft position.
- The sensor reports the resulting position, allowing the ECM to correct the command as speed and load change.
By varying phase, AVCS can provide stronger low- and medium-speed torque, better high-speed output, improved emissions performance and improved fuel efficiency. The ECM can also alter intake and exhaust overlap as engine speed changes. The mechanical lobe profile remains fixed; AVCS changes when that profile acts on the valves.
Intake-only and dual AVCS
Some Subaru engines vary intake-cam timing only. Others use AVCS on both intake and exhaust camshafts. Dual AVCS gives the control system more authority over overlap and exhaust-scavenging behavior, but it also adds control hardware and diagnostic points. Whether an engine has intake-only or dual AVCS must be verified for that specific engine family.
Camshaft versus crankshaft
| Part | Primary job | Relationship to the other shaft |
|---|---|---|
| Crankshaft | Converts piston movement into rotational torque sent through the drivetrain | Provides the base rotational reference for piston position |
| Camshaft | Operates intake and exhaust valves at timed intervals | Turns at half crankshaft speed and must remain correctly phased to it |
A crankshaft-position sensor tells the ECM where the crank is; a camshaft-position sensor tells it where a particular cam is. The ECM uses both signals to synchronize fuel injection, ignition and variable valve timing. Replacing a camshaft sensor does not replace a damaged camshaft, correct a slipped timing drive or repair a blocked AVCS oil passage.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchWhat a camshaft-related fault can mean
When commanded cam timing does not match actual timing, Subaru diagnostic procedures treat the problem as a control-and-alignment issue rather than automatically blaming one component. A fault may involve the sensor, wiring, oil-control solenoid, oil passages, camshaft condition, cam sprocket, timing belt or chain, tensioner, or installation alignment.
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Common symptom patterns
- Warning-light codes for camshaft position or variable valve timing
- Hard starting, rough idle, hesitation or reduced power
- Unusual camshaft or timing-drive noise
- Intermittent symptoms that change with engine temperature or oil pressure
- A no-start condition after timing-drive work
These symptoms overlap with ignition, fuel, compression and air-leak problems. A code identifies a circuit or performance condition; it does not by itself prove that the sensor is defective.
Practical diagnostic sequence
- Identify the exact engine. Use the vehicle identification number, engine code and model year. Camshaft sensors, connectors, sprockets and timing procedures are not interchangeable across all Subaru engines.
- Record diagnostic codes and freeze-frame data. Note which bank and which camshaft are named, along with engine speed, temperature and operating conditions when the fault occurred.
- Inspect the sensor circuit. Check connector fit, oil contamination, damaged wiring, power supply, ground and signal integrity. A replacement sensor must match the engine and connector.
- Check engine oil condition and level. AVCS relies on clean oil moving through narrow control passages. Use the oil grade and service interval specified for the engine; do not assume a new sensor can compensate for restricted oil flow.
- Test the oil-control solenoid and passages. Verify electrical operation and inspect for sticking, sludge or blockage. A solenoid that cannot direct oil will prevent the cam phaser from reaching its commanded angle.
- Compare commanded and actual cam angle. Scan data can show whether the cam responds when the ECM commands movement. A fixed reading, slow response or excessive error points the diagnosis toward control, oil flow or mechanical timing.
- Verify timing-drive alignment. If the error is persistent, inspect belt or chain marks, tensioner operation, guides and sprockets using the engine-specific procedure. Do not rotate or disassemble the drive based on a generic Subaru diagram.
- Inspect the camshaft and valvetrain. If alignment and control hardware pass inspection, check lobes, journals, caps, followers and clearances for wear or damage.
Because several failures can produce the same code, replacing a camshaft-position sensor is a reasonable repair only when testing supports a sensor or circuit fault. It is not a guaranteed fix for an AVCS fault.
What camshaft removal involves
Camshaft service is not normally an external bolt-on repair. Subaru procedures require removing timing components and cam sprockets before the camshaft can be withdrawn. The technician must preserve timing references, release valve-train loads in the specified sequence and keep parts organized by position.
During installation, the journals require lubrication and the caps must be installed in their correct locations and sequence. The timing drive then has to be set to the engine’s precise alignment marks, followed by the required tensioning and rotation checks. Incorrect timing can cause poor running or, on interference designs, piston-to-valve contact.
When professional service is the safer choice
- The timing belt or chain must be removed to reach the camshaft.
- The engine has an interference-valve design or unknown timing history.
- Cam caps, journals or lobes show scoring, discoloration or measurable wear.
- AVCS data shows a persistent angle error after electrical and oil checks.
- You do not have the engine-specific locking, tensioning and torque information.
How to identify the camshaft configuration in your Subaru
- Find the engine code and model year in the vehicle documentation or identification label.
- Check whether the engine uses SOHC or DOHC cylinder heads.
- Determine whether AVCS controls the intake camshaft, exhaust camshaft or both.
- Confirm whether the timing drive is a belt or chain.
- Use the exact service manual for that engine when locating sensors, solenoids, timing marks and camshaft caps.
This verification prevents common mistakes such as ordering a sensor for the wrong bank, assuming every boxer engine has four camshafts, or applying a belt procedure to a chain-driven engine.
Quick Recap
Key points to remember
- The camshaft’s lobes turn crankshaft rotation into timed valve motion.
- Camshaft phase controls valve lift timing, closing timing and intake-exhaust overlap.
- A DOHC Subaru boxer engine can have four camshafts, but cam count varies by engine family and year.
- AVCS hydraulically advances or retards camshaft phase in response to ECM commands.
- Camshaft timing faults require model-specific checks of sensors, oil-control hardware, cam condition and the timing drive.
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