Smart roadster spark plug replacement is one of the most straightforward maintenance tasks you can tackle on the 452, yet it is also one of the most commonly neglected. The 698cc three-cylinder turbocharged engine is a compact, highly stressed unit that depends on a strong, consistent spark to run cleanly at boost. Worn or incorrect plugs cause misfires, poor fuel economy, hesitant throttle response and, in turbocharged applications, the risk of running rich enough to damage the catalytic converter. This guide covers the correct plug specification, gap settings, which brands to trust, step-by-step replacement instructions and how often you should be doing this job.
Why Spark Plugs Matter More on a Turbocharged Engine
On a naturally aspirated engine a marginal spark plug merely dulls performance slightly. On the Smart Roadster’s turbocharged 698cc three-cylinder, the consequences are more severe. Boost pressure compresses the air-fuel mixture in the combustion chamber, making it harder to ignite reliably. A worn electrode, an incorrect gap or an incompatible heat range all reduce the likelihood of a complete, properly timed burn. The result is a partial misfire: unburnt fuel passes through the exhaust, overheats the catalytic converter and throws the lambda sensor into a feedback loop that the Bosch MEG 1.1 ECU struggles to correct cleanly.
Because the engine only has three cylinders, each misfire represents one third of your total power strokes. You will feel it — a rough idle, a flat spot under acceleration, or an audible stumble at high revs. On higher-output variants running 1.33 or 1.43 bar of boost, the ignition system is working at its absolute limit, making plug condition even more critical. If you are also dealing with sensor faults, it is worth reading about how a faulty lambda sensor interacts with combustion quality before assuming the plugs are the only culprit.
Correct Spark Plug Specification for the Smart Roadster 452
The Smart Roadster 698cc engine is derived from the Mitsubishi 3B20 family and shares its basic ignition geometry. The OEM specification calls for a plug with an M14 thread, 19mm hex, 26.5mm reach and a projected electrode design to promote flame kernel development under boost.
Recommended Plugs
- NGK BKR6EK — the most commonly recommended OEM-equivalent choice. Heat range 6, standard copper core, projected tip. Reliable, affordable and widely available.
- NGK BKR6EKPB — the platinum-tipped version of the above. Longer service interval (around 30,000 miles) but not meaningfully better in day-to-day performance.
- Bosch FR7LDC — an acceptable alternative if NGK is unavailable, same heat range equivalent, copper electrode.
- Denso K20PR-U — another valid OEM-level substitute used by some independent specialists.
Heat Range Considerations
Standard 60kW cars use heat range 6 without issue. If you are running a remap at higher boost levels, some tuners recommend stepping up to a heat range 7 (colder plug, such as the NGK BKR7EK) to reduce the risk of pre-ignition under sustained high load. Do not fit iridium fine-wire plugs intended for high-revving naturally aspirated engines — the narrow electrode can erode quickly under the thermal and pressure stresses of turbo boost.
Electrode Gap
Factory gap specification is 0.8 mm (0.031 in). Always check with a feeler gauge before fitting, even on new plugs — manufacturing tolerances vary. On remapped cars running elevated boost, some owners reduce the gap slightly to 0.7 mm to ensure reliable ignition under the higher cylinder pressures.
How to Change the Spark Plugs: Step-by-Step
The Smart Roadster’s engine sits mid-rear, behind the seats and beneath a removable engine cover. Access is considerably better than on many modern cars once you have the cover off, and the three plugs are arranged in a neat row along the top of the cylinder head.
Tools Required
- 16mm spark plug socket with rubber insert (to grip the ceramic)
- 3/8-inch drive ratchet and extension bar (minimum 150mm)
- Torque wrench
- Feeler gauge
- Small amount of copper anti-seize compound
- Dielectric grease for the HT boots
Step-by-Step Procedure
- Allow the engine to cool completely. Working on a hot engine risks burning yourself and can cause aluminium threads to seize or pull when you remove the plugs. Wait at least two hours after driving.
- Remove the engine cover. The cover is secured by two quarter-turn fasteners at the rear and clips along the side trim. Lift it clear and set it aside.
- Locate the three ignition coil packs. On the 452 each cylinder has its own coil-on-plug (COP) unit sitting directly above the plug. Disconnect the electrical connector by pressing the tab and pulling firmly — do not lever against the coil body itself.
- Remove the coil packs. Each is retained by a single 8mm bolt. Lift the coil straight up; the HT boot will release from the plug with a slight twist if it feels stuck.
- Blow out any debris. Use compressed air or a can of air duster in the plug wells before removing the plugs, to prevent grit falling into the open cylinders.
- Unscrew the old plugs. Insert the plug socket with extension, break the plug loose anticlockwise and remove by hand once it is free. Inspect the old plug: a tan or light grey deposit on the electrode is healthy; black sooty deposits suggest rich running; white or chalky deposits suggest lean running or coolant contamination.
- Check and set the gap on new plugs. Use a feeler gauge to confirm 0.8 mm. Adjust by carefully bending the ground electrode — never apply force to the centre electrode.
- Apply a thin film of copper anti-seize to the thread. Do not get any on the electrode or the ceramic insulator.
- Thread plugs in by hand first. This is critical — cross-threading an aluminium head is an expensive mistake. If there is resistance when threading by hand, stop and realign.
- Torque to specification: 20–25 Nm. Do not over-tighten; the aluminium threads will not thank you.
- Apply dielectric grease inside each HT boot before pushing the coil packs back on. This prevents the boot bonding to the ceramic and makes future removal easier.
- Refit coil packs, reconnect electrical connectors and refit the engine cover.
- Start the engine and check for misfires. The idle should settle smoothly within a few seconds. Clear any stored misfire codes if you have a diagnostic reader.
Service Interval: How Often Should You Change Them?
Smart’s factory service schedule specified spark plug replacement at 40,000 km (approximately 25,000 miles). In practice, most experienced owners and independent specialists recommend changing standard copper plugs every 20,000–25,000 miles, or every two to three years, whichever comes first. The three-cylinder turbocharged engine runs the plugs harder than a comparable naturally aspirated unit, and the cost of three NGK BKR6EK plugs is negligible compared with the diagnostic time wasted chasing a misfire that turns out to be nothing more than worn electrodes.
If your car has covered more than 40,000 miles with no record of plug replacement — very common on the used market — do it immediately as part of a comprehensive recommissioning service. Combine it with an engine oil and filter change for maximum benefit, since both jobs give you the engine cover off and the engine in a cooled, accessible state at the same time.
Platinum or iridium plugs (where suitable) can extend the interval to 30,000–40,000 miles, but always inspect them physically at the standard interval regardless — the electrode condition tells you a great deal about how the engine is running.
Reading the Old Plugs: What the Deposits Tell You
Before you discard the old plugs, spend sixty seconds reading their condition. The deposits on the electrode and insulator tip are a direct record of combustion conditions inside that cylinder.
- Light tan or grey deposits, slight electrode wear — normal, healthy combustion. The engine is running at the correct mixture.
- Heavy black, sooty or oily deposits — rich running, oil burning or both. Check for a faulty MAP sensor, a leaking injector seal or excessive oil consumption. The MAP sensor is a known weak point on ageing 452s and can cause persistent rich running that rapidly fouls plugs.
- White or chalky deposits, blistered insulator — lean running or overheating. Could indicate a boost leak, a vacuum leak at the intake manifold or a failing wastegate holding boost too low.
- Erosion on one plug only — suspect a coil pack failure or a hairline crack in the HT boot on that cylinder. Replace the corresponding coil pack before fitting new plugs.
- Heavy carbon on one plug, clean on others — a stuck-open injector or a compression problem on that cylinder. Further investigation is warranted before simply refitting new plugs.
Ignition Coil Packs: Replace or Reuse?
The coil-on-plug units on the Smart Roadster are relatively robust but do fail with age, particularly the HT boots which become brittle and crack after ten to fifteen years. If your car is approaching or past fifteen years old, it is worth inspecting the boots carefully under a bright light. A cracked or perished boot allows the spark to track down the outside of the ceramic insulator rather than jumping the electrode gap, causing an intermittent misfire that is maddeningly difficult to diagnose.
Replacement coil packs are available as pattern parts for reasonable money. If you are already doing plugs on a high-mileage car and the boots look questionable, replacing all three coil packs at the same time makes sense — it is the same labour for a fraction of the cost of a diagnostic session. New coil packs also pair well with fresh plugs; fitting new plugs behind a marginal coil is a false economy.
Note that coil pack faults can also generate SAM-related fault codes and confused ECU readings. If you are experiencing intermittent electrical gremlins alongside ignition symptoms, it is worth checking whether your car’s ECU firmware is up to date, as the latest firmware revision handles certain fault conditions more gracefully than earlier versions.
Smart Roadster Spark Plug Replacement: Summary and Recommendations
Smart roadster spark plug replacement is a one-hour DIY job that requires basic tools and delivers a meaningful improvement in throttle response, idle quality and fuel efficiency on any car that has not had fresh plugs recently. Use NGK BKR6EK as your default choice, set the gap to 0.8 mm, torque to 25 Nm and repeat the job every 20,000–25,000 miles. Always read the old plugs before discarding them — they may be pointing you towards a more significant issue. Combine the job with an oil change and a visual inspection of the coil packs and HT boots, and you will have covered the three most cost-effective ignition system maintenance items in a single afternoon.
