Quick Info
Model
450, 452
Estimated Cost
€60
Difficulty
Intermediate
Time Required
1 Hour
Safety Notes
Engine Temperature: Only perform this work on a cold or lukewarm engine. Working on a hot engine increases the risk of stripping the delicate aluminum threads in the cylinder head.
Specialized Tooling: Use a dedicated HT lead removal tool. Attempting to pull the spark plug wires by hand will likely result in internal wire damage or a snapped lead.
Cleanliness: Before removing the old plugs, use compressed air to blow out any debris from the spark plug wells to prevent dirt from falling into the combustion chambers.
Threading: Always start threading the new plugs by hand using just the socket and an extension. Never use a ratchet to start the threads, as you cannot feel if the plug is cross-threading.
Step-by-Step Guide
Introduction
The 698cc and 599cc engines in the Smart 450 and 452 series are unique in their use of two spark plugs per cylinder. This design ensures more complete combustion but makes maintenance twice as intensive. Often, mechanics or previous owners only change the "easy" top three plugs, leaving the hidden lower three to degrade over years.
Ignoring the spark plugs can lead to poor performance, "hunting" at idle, and increased stress on the ignition coils. Replacing all six plugs restores the crisp throttle response the Roadster is known for. This guide covers the access to both the upper set (visible from the engine bay) and the lower set (hidden behind the rear bumper), ensuring your engine receives a complete service.
Accessing the Engine and Upper Plugs
To begin, open the rear hatch and lift the floor carpet. Locate the metal engine cover and unscrew the single central holding bolt (or several Torx bolts depending on the model). Lift the metal plate to reveal the engine. The first three spark plugs are located on the top of the engine block, covered by thick rubber HT leads. Carefully clear away any loose dust or oil from the area surrounding the leads to ensure a clean working environment before you begin any mechanical extraction.
Removing the HT Leads and Upper Plugs
Slide the HT lead removal tool over the metal sleeve of the plug connector. Lever the tool against the engine block to pop the lead off the plug; repeat this for all three top leads. Once the leads are removed, use a 16mm spark plug socket with a long extension to unscrew the old plugs. It is highly recommended to use a socket with a rubber insert or a magnet to safely lift the plugs out of the deep wells without dropping them.
Accessing the Lower Spark Plugs
The remaining three spark plugs are located on the rear side of the engine, facing the bumper. To access these comfortably, you should remove the rear bumper (following the bumper removal guide). Once the bumper and the metal crash bar are out of the way, you will see the lower HT leads. Use the same removal tool to pop these off. Space is tighter here, so using a swivel-head ratchet or a shorter extension may be necessary to navigate around the exhaust heat shield.
Installing and Torquing the New Plugs
Check the gap of your new NGK LKR8A plugs (standard is 0.7mm). Place a new plug into your socket and lower it into the well. Spin the extension bar with your fingers until the plug is finger-tight. Once seated, use a torque wrench to tighten the plugs to exactly 22.5 Nm. Do not over-torque, as the aluminum threads are very fragile. Proper torquing is essential for heat dissipation from the plug to the cylinder head.
Reconnecting Leads and Reassembly
Push the HT leads back onto the new spark plugs until you hear and feel a distinct "click," ensuring a solid electrical connection. It is a good practice to apply a tiny amount of dielectric grease to the inside of the rubber boots to prevent them from sticking in the future. Reinstall the metal engine cover, secure the carpet, and if you removed the rear bumper, bolt it back into place. Start the engine to confirm a smooth idle before heading out for a test drive.
