Smart forfour 0.9 Turbo W453

Described as a 'becoming common issue' by Smart specialist resources. The wastegate actuator regulates turbo boost pressure. When it sticks open, the engine loses all boost and feels very sluggish. When it sticks closed, the engine overboosts and enters safe mode (limp mode). Symptoms include pulsing under acceleration, intermittent power loss, and sometimes a cluster of misleading error codes (accelerator pedal, crankshaft sensor) because the wastegate communicates status to the ECU. The actuator part alone costs around €300. If only the actuator needs replacing, total repair runs €400-600 at a specialist. If the turbo internals are damaged from sustained overboost, a full turbo replacement costs €1,200-1,800 including labor. Water ingress into the actuator from the rear-engine position accelerates failure. Smart has no fix beyond replacement.

Both turbo and non-turbo 453 engines are affected. The variable valve timing solenoid controls intake camshaft position. It sticks due to corrosion from water ingress, a known design weakness. Symptoms include rough idle, poor throttle response, and occasional knocking sounds. Smart has acknowledged the problem and released two fixes: a tilted mounting plate (to help water drain off) and a protective cover. The solenoid itself is inexpensive (€50-80) but removal can be tricky due to corrosion. Independent repair runs €150-250, dealer repair €300-400. Typically occurs after 40,000-80,000 km.

The 6-speed Getrag 6DCT250 Twinamic uses two motor-driven clutch actuators (M14 and M15). These can corrode or develop internal faults, especially the lower motor (M15) which is more exposed. Symptoms include loss of odd or even gears, harsh shifts, juddering at low speeds, and adaptation error codes. The lower actuator (M15) is a 5-minute replacement with 4 Torx bolts. The upper actuator (M14) requires intake manifold removal, significantly increasing labor. Parts cost €100-200 per motor. The dry-clutch design also requires transmission fluid changes every 60,000 km, which many owners skip. Manual gearbox versions avoid this issue entirely. Problems typically appear between 50,000-90,000 km.

The H4B engine uses a timing chain marketed as a lifetime component, but the tensioner can weaken over time. The oil jet that lubricates the chain is susceptible to clogging from carbon buildup, accelerating wear. Symptoms include a metallic rattle on cold start that fades after a few seconds as oil pressure builds. Typically occurs beyond 120,000-150,000 km. If the chain skips, it causes valve-piston contact and catastrophic engine damage. Regular oil changes with correct 5W-30 specification help prevent this. Chain, guides, tensioner, and sprocket replacement costs €800-1,200 at an independent garage, €1,200-1,500 at a dealer. Most forfours will not reach this mileage given their city car usage pattern.

Smart uses small, lightweight engine mounts that degrade faster than those in conventional cars, particularly under urban stop-and-go conditions. Symptoms include excessive vibration at idle, engine movement visible from outside, and clunking when accelerating or shifting gears. Typically becomes noticeable after 80,000 km. Mount parts are inexpensive (€20-50 each), but labor can be involved due to the rear-engine packaging. Total repair including labor runs €200-500 depending on how many mounts need replacing. Reinforced aftermarket mounts are available but transmit more vibration to the cabin.

The three-cylinder engine uses individual coil packs that can fail with relatively short service life compared to larger engines. Symptoms include rough running, misfires, check engine light, and reduced power. A single coil pack costs €20-40, and replacing all three as a set is recommended once one fails. Total repair runs €150-350 depending on whether done at an independent shop or dealer. Not a serious issue but worth monitoring, especially in cars with higher mileage or those used primarily for short trips.