1.7 Valve Adjustment, Camshaft Timing & Variable Valve Timing (VVT/VVL)
Key Takeaways
- Tight valve lash holds a valve off its seat, removing its cooling path into the cylinder head and producing burned valves, low compression, and hot-start misfire; loose lash produces a ticking noise and reduced effective lift and duration.
- Hydraulic lifters have no running adjustment; their preload is set by torquing the rocker or lash adjuster to specification, and collapsed or noisy hydraulic lifters are diagnosed as component failures rather than adjusted.
- DTC P0016 is a crankshaft-to-camshaft position correlation fault and points to a stretched timing chain, worn chain guide, jumped belt, incorrect reassembly, or a phaser locked out of position, not to a defective sensor by default.
- P0011 indicates camshaft timing over-advanced or system performance on bank 1 intake, while P0014 indicates the same condition on the bank 1 exhaust camshaft; the most common root causes are contaminated oil, a clogged oil control valve screen, and low oil pressure.
- Variable valve lift systems such as Honda VTEC switch to a high-lift rocker profile through oil-pressure-actuated locking pins, so lift-system faults are usually oil pressure, oil level, or pressure switch failures rather than camshaft failures.
Why Valve Train Timing Is an Engine Performance Topic
ASE task A.11 requires verifying valve adjustment on engines with mechanical or hydraulic lifters, and task A.12 requires verifying camshaft timing and the operation of camshaft timing components including variable valve timing (VVT) and variable valve lift (VVL). These sit inside General Diagnosis rather than Engine Repair because valve train timing errors present as driveability and emissions complaints: hard starting, low power, rough idle, misfire codes, and catalyst-damaging raw fuel.
A cam that is a few degrees out of phase does not usually make noise. It produces low manifold vacuum, a lazy MAF or MAP reading relative to RPM, high or low fuel trims, and a P0016 family code. Recognizing that pattern is the tested skill.
Mechanical Valve Lash
Valve lash (valve clearance) is the designed gap in the valve train that guarantees the valve fully seats when it is closed. Thermal expansion consumes part of that gap at operating temperature.
Consequences of incorrect lash:
| Condition | Mechanism | Symptoms |
|---|---|---|
| Too tight | Valve is held slightly off its seat as components expand; the valve loses its primary heat path into the seat and head | Burned exhaust valve, low compression on one cylinder, hot-start misfire, hard hot starting, eventual valve failure |
| Too loose | Lost motion in the train delays valve opening and shortens effective duration and lift | Audible tick that follows camshaft speed, reduced high-RPM power, accelerated cam lobe and rocker wear |
Measurement technique. Lash is measured with a feeler gauge between the cam lobe base circle (or rocker tip) and the valve stem or bucket, with that cylinder's valve fully closed — typically at TDC of the compression stroke for that cylinder. A correctly sized gauge produces a light, uniform drag. Specifications differ between intake and exhaust valves and between hot and cold measurement; using the wrong column of the specification table is a classic error.
Adjustment methods:
- Screw-and-locknut rockers: turn the adjuster to the specified drag, then hold the adjuster while torquing the locknut and re-verify.
- Shim-over-bucket or shim-under-bucket: measure existing clearance, calculate the required shim thickness from the installed shim, and swap in the calculated shim. No infinite adjustment exists — the arithmetic must be done.
- Hydraulic lifters/lash adjusters: there is no running adjustment. Preload is established by torquing the rocker bolt or pedestal to specification with the lifter on the base circle. A ticking hydraulic lifter is diagnosed (oil supply, oil level, oil viscosity, lifter bleed-down, collapsed lifter) and replaced, not adjusted.
Verifying Camshaft Timing
Base camshaft timing is confirmed by aligning crankshaft and camshaft timing marks with the engine at the manufacturer's reference position, and by comparing scan-tool camshaft position data to crankshaft position data.
Indicators of retarded cam timing: low manifold vacuum that is steady but below specification, reduced low-end torque, extended cranking, and a lean or lazy fuel trim pattern.
Indicators of a jumped or stretched chain: a P0016 (crankshaft position–camshaft position correlation, bank 1 sensor A) or a related P0017/P0018/P0019 for other banks and sensors. Timing chains do not literally stretch in the metal; the pins and bushings wear, which lengthens the chain pitch and retards cam phase. Worn guides and a failing hydraulic tensioner produce the same result, often with a cold-start rattle for the first few seconds before oil pressure loads the tensioner.
Diagnostic caution: P0016 is a correlation code. It reports that the two sensors disagree about mechanical phase. Replacing the camshaft position sensor because the code names it is one of the most common wasted repairs; confirm mechanical phase before replacing either sensor.
Variable Valve Timing (VVT) Operation and Diagnosis
A VVT system rotates the camshaft relative to its drive sprocket using a cam phaser — a hydraulic vane assembly fed by engine oil and metered by an oil control valve (OCV), also called a camshaft position actuator solenoid. The PCM pulse-width modulates the OCV to hold a commanded cam angle.
Advancing intake cam timing improves low-RPM torque; retarding it improves high-RPM breathing. Retarding the exhaust cam increases internal exhaust gas dilution, which is why many VVT engines have no external EGR valve at all — a fact ASE tests directly.
Diagnostic sequence:
- Check oil level, condition, and viscosity first. VVT is an oil-actuated system. Sludge, extended drain intervals, and incorrect viscosity are the leading causes of VVT complaints.
- Compare commanded versus actual camshaft angle on the scan tool. Command an advance; the actual angle should follow within a few degrees. No response with a good electrical command indicates a restricted OCV screen, low oil pressure, or a stuck phaser.
- Test the OCV electrically. Measure winding resistance against specification (commonly in the single-digit to low-teens ohm range depending on the manufacturer) and verify the PCM's PWM drive with a scope. A solenoid can pass a resistance check and still stick hydraulically.
- Verify oil pressure at operating temperature. VVT authority collapses when hot idle oil pressure falls below specification.
Common VVT codes:
| DTC | Meaning | Typical root causes |
|---|---|---|
| P0010 / P0020 | "A" camshaft position actuator circuit, bank 1 / bank 2 | Open or shorted OCV winding, harness damage, driver failure |
| P0011 | "A" camshaft position — timing over-advanced or system performance, bank 1 | Clogged OCV screen, contaminated oil, low oil pressure, stuck phaser |
| P0012 | "A" camshaft position — timing over-retarded, bank 1 | Same family; phaser cannot reach commanded advance |
| P0014 | "B" (exhaust) camshaft position — timing over-advanced or performance, bank 1 | Exhaust-side OCV or phaser fault |
| P0016 | Crankshaft/camshaft position correlation, bank 1 sensor A | Worn chain, worn guides, failed tensioner, jumped timing, phaser lock pin failure |
A phaser that cannot return to and hold its default parked position at shutdown produces a distinctive cold-start rattle for one to three seconds, because the locking pin is not engaged before oil pressure builds.
Variable Valve Lift (VVL)
VVL changes valve lift and duration, not just phase, by switching between camshaft lobe profiles. Honda VTEC uses oil pressure to drive locking pins that couple a mid rocker arm riding a high-lift lobe to the primary rockers above a threshold RPM. Toyota VVTL-i uses a similar high-lift slipper mechanism, and other manufacturers axially slide a lobe carrier on a splined camshaft to swap profiles.
Because engagement is oil-pressure dependent and gated by an oil pressure switch, VVL complaints ("the engine will not pull past 5,000 RPM") are diagnosed as oil level, oil pressure, spool valve, screen restriction, or pressure switch faults. Honda systems set a P1259 family VTEC system code, and the diagnostic path checks the spool valve screen and the pressure switch signal before any camshaft component is suspected.
An engine with mechanical bucket-and-shim valve adjusters has one exhaust valve set well below the specified clearance. Which failure is this condition most likely to produce?
A VVT-equipped engine sets P0011 (bank 1 camshaft timing over-advanced or system performance). Scan data shows the PCM commanding cam advance while actual camshaft angle stays at the parked position. Electrical testing confirms the oil control valve winding resistance and PWM drive signal are within specification. Which cause is most likely?
A high-mileage engine sets P0016 with a brief rattle on cold start that disappears after about two seconds. Which action should the technician take before replacing any position sensor?