P0021 on Ford: How to Diagnose & Fix

Two distinct tick patterns affect 5.0L Coyote V8 in 2015-2023 Mustang depending on engine generation. Gen 2 Coyote (2015-2017): light tick at idle and on cold start, commonly attributed to valve lash / lifter operation; generally cosmetic and not associated with engine damage. Gen 3 Coyote (2018-2023): louder, more pronounced tick — same family but Ford changed to plasma transferred wire arc (PTWA) cylinder liners. The tick is increasingly linked to piston slap from clearance issues against the plasma liner, with some confirmed cases of cylinder wall scoring requiring engine replacement under warranty. Direct injection (added on Gen 3 alongside port injection) contributes a separate fainter HPFP tick that is normal. Distinguishing: Gen 2 tick stays constant warm; Gen 3 piston slap diminishes as engine warms but returns under load. Owner forums (Mustang6G, MPR Racing Engines) document the Gen 3 pattern extensively.

The 5.0L Coyote V8 can develop a ticking or rattling noise at startup, often from the cam phasers. This is similar to issues in the Mustang GT. The noise typically diminishes after the engine warms up but can indicate phaser wear.

The 2004-2010 5.4L 3-valve Triton uses variable cam timing (VCT) phasers fed by clean, high-pressure engine oil. With infrequent oil changes or normal high-mileage wear, the phasers' internal vanes and locking pins wear out, the plastic timing-chain guides crack, and the hydraulic chain tensioners bleed down. The result is the notorious 'death rattle' — a loud rattle/knock on cold start and a metallic ticking at hot idle.

Distinct from the cam-phaser cold-start rattle, the first-generation (2011-2016) 3.5L EcoBoost suffers genuine primary timing-chain wear ('stretch') because of its long chain routing and the high loads of a twin-turbo direct-injection V6. As the chain pins and rollers wear, the cam-to-crank relationship drifts and the PCM logs P0016 (and related correlation codes). The first symptom is usually a brief metallic rattle on cold start that worsens over time. Wear typically appears around 100,000-150,000 miles, often accelerated by extended oil-change intervals. Left unrepaired, the chain can skip teeth or break, causing catastrophic valve-and-piston contact.

The first-generation 3.5L EcoBoost (the 2011-2016 engine generation) used in the 2015-2018 Ford Transit 350/350 HD with the twin-turbocharged V6 is known for premature timing chain stretch. The design uses one long primary chain driving two secondary cam chains, and the long primary chain stretches as the links wear individually, causing the chain to effectively grow longer. A stretched chain causes VCT timing correlation codes, cold-start rattling, rough idle, and eventual jumped timing. Ford moved F-150 and other trucks to a second-generation 3.5L EcoBoost with a dual primary chain system in 2017, but the Transit kept the first-generation engine with revised timing components. Transit vans used for commercial/fleet service with high-idle hours accumulate chain wear faster than typical passenger vehicles.

The 2nd-generation 3.5L EcoBoost V6 in 2018-2020 Ford Expedition (Gen 4) addressed the timing chain stretch issue of early-generation 3.5L EcoBoost engines (2011-2016) but introduced a new problem: VCT (Variable Camshaft Timing) unit wear that causes a distinctive ticking/tapping rattle on cold starts after a 6+ hour soak. The rattle typically lasts 2-5 seconds and comes from the top front of the engine. Ford issued TSB 23-2143 (superseding 22-2200) addressing the condition for 2018-2020 Expedition/Navigator built on or before November 30, 2019, recommending replacement of all four VCT units. The cam phasers become sticky when oil drains back overnight, then rattle until fresh oil pressure reaches them.

The 3.5L/3.7L Duratec V6 in first-generation Ford Edge commonly develops timing chain stretch and variable camshaft timing (VCT) phaser rattles between 80,000-120,000 miles. The engine uses two timing chains (one per cylinder bank) that can stretch over time, causing the VCT phasers to rattle loudly on cold starts as oil pressure builds. The rattle typically lasts 2-5 seconds on startup before oil pressure stabilizes. Left unaddressed, chain slack can cause timing to jump, resulting in rough running, poor fuel economy, and potential catastrophic engine damage if a chain breaks or a guide fails.

Multiple Bronco owners report a brief but pronounced rattling or knocking noise on cold start caused by worn or defective cam phasers, often accompanied by timing chain slack. Ford issued service guidance for EcoBoost engines using updated phaser parts. While some vehicles only exhibit noise, others progress to rough running, check-engine lights, or repeat repairs if only partial components are replaced.

On the 2011-2019 transverse Explorer, Ford located the water pump INSIDE the engine, behind the timing cover, driven off the primary timing chain. When the pump's internal shaft seal fails, coolant first weeps from a passage on the passenger side of the block, but if ignored (or if the bearing fails) coolant is dumped directly into the crankcase, contaminating the oil ('milky' oil) and risking bearing/engine destruction. Because the pump is buried behind the timing cover, replacement is a 10-12+ hour teardown. This generated a class-action lawsuit (no recall was issued). Typical onset is roughly 100k-200k miles but failures as early as ~60k miles are documented in NHTSA complaints.