2008-2015 Mercedes-AMG C63 (W204) M156 6.2L spark plug replacement (16 plugs)
Tools
- 3/8 inch drive ratchet with 3 inch, 6 inch, and 10 inch extensions
- Torque wrench that covers 5 to 30 ft lbs
- Torque wrench that covers 60 to 200 in lbs
- 5/8 inch (16mm) thin-wall magnetic spark plug socket
- 10mm socket and E10 external Torx
- T27, T30 Torx bits
- Long swivel adapter or 3/8 wobble extension for rear cylinders
- Small pick set for coil connector clips
- Compressed air with blow gun (mandatory for plug well cleaning)
- Nitrile gloves and shop rags
- Dielectric grease
- Anti-seize (nickel-based)
Parts
- Spark plugs — NGK 96305 PLZKAR6A-11 (Mercedes-Benz OEM A004159590326), 16 required (two per cylinder)
- Ignition coils — Mercedes-Benz A2769060260, inspect and replace as needed (do not replace speculatively — the M156 coil failure rate is lower than the M113 that preceded it)
- Coil bolts if disturbed — reuse if in good condition
Steps
01Prep the workspace
The M156 uses two spark plugs per cylinder; one on the outboard side of each valve cover, one on the inboard side reached through the intake valley. Sixteen plugs total. Plan for a full workday if this is a first attempt; experienced hands cut the job in half but the first pass is always slow because of the number of interference points around the intake manifold. Disconnect the negative battery terminal in the trunk. Wait five minutes before touching any coil connector.
02Remove the engine cover and airbox
Lift the black plastic engine cover straight up off its four rubber grommets. Remove the airbox lid, the MAF sensor plug, and the intake air tube. Remove the airbox base (four 10mm bolts). Set aside on a rag.
03Remove the outboard coils and plugs (8 plugs, 4 per bank)
Each outboard coil sits on top of its cylinder, retained by two 10mm bolts. Lift each coil connector's bail lock with a pick, disconnect the harness, then remove the two coil bolts and lift the coil straight up. Blow the plug well clean with compressed air before removing the plug. Break each plug loose with the 5/8 inch thin-wall socket on a 6 inch extension. The rear plugs on each bank sit close to the firewall; a wobble extension prevents the socket from cocking off-axis and rounding the plug hex.
04Remove the intake manifold for inboard plug access
The eight inboard plugs are accessible only with the intake manifold removed. Disconnect the throttle body harness, the crankcase breather hoses (three total on the manifold), and the fuel rail cover pins. Loosen the eight E10 external Torx intake manifold bolts in a crossing pattern. Lift the manifold off the head studs and rotate it forward to rest across the front of the engine bay, supported by a rag on the radiator core support. Cover the eight open intake ports with clean rags immediately.
05Remove the inboard coils and plugs
Each inboard coil is retained by a single 10mm bolt. Disconnect the harness at each coil, remove the bolt, lift the coil out. Blow each inboard plug well clean before removing the plug. The inboard plugs live in deep wells and any debris that falls in ends up on the piston crown at the next start.
06Inspect the old plugs
Line up all sixteen plugs in firing order (M156 firing order 1-5-4-2-6-3-7-8). Look for asymmetric wear cylinder to cylinder. A single cylinder with a wet black plug points to a stuck injector or a failed coil on that specific hole. Uniform light tan across all sixteen means fueling and ignition are balanced and the plugs did their job to the end of their service life.
07Prep and install the new plugs
Confirm the new plug gap on each new plug with a wire gauge; M156 spec is 0.040 inch (1.0 mm) but NGK ships pre-gapped. Apply a small dab of nickel-based anti-seize to the plug threads only. Thread each plug in by hand until fully seated in the head. Torque the M156 spark plugs to 20 ft lbs (27 Nm). Verify against your service manual. The aluminum head threads are the weak point on the M156; over-torque strips the head, not the plug.
08Install coils with dielectric grease
Apply a thin film of dielectric grease inside each coil boot. Set each inboard coil back into its well, orienting the harness connector correctly. Fit the 10mm hold-down bolt. Torque the M156 inboard coil bolts to 71 in lbs (8 Nm). Verify against your service manual. Reconnect the coil harness with the bail lock fully engaged.
09Reinstall the intake manifold
Remove the port rags. Inspect each port for any dropped fastener or debris. Set the intake manifold onto the head studs with the OE port seals in place; replace any seal that is torn or hardened. Thread the eight E10 bolts by hand, then torque the M156 intake manifold bolts to 89 in lbs (10 Nm) in the manufacturer's sequence. Verify against your service manual. Reconnect breather hoses, fuel rail cover pins, and throttle body harness.
10Reinstall outboard coils
Set each outboard coil onto its plug well with dielectric grease in the boot. Fit both 10mm hold-down bolts per coil. Torque the M156 outboard coil bolts to 71 in lbs (8 Nm). Verify against your service manual. Reconnect the coil harness.
11Reassemble airbox and cover
Refit the airbox base, MAF, intake tube, and airbox lid. Snap the engine cover back onto its grommets. Reconnect the battery.
12Start and verify
Turn the ignition on without cranking for 30 seconds. Start the engine. Idle should stabilize within five seconds with a smooth eight-cylinder pulse; any hunting or lumpy idle indicates a coil connector not fully seated or a plug not correctly gapped. Scan for stored misfire codes with a Mercedes-capable tool (Xentry, iCarsoft MB II, or Autel MK906) and clear any residual codes.
Notes
Reference guide — verify every torque figure before you rely on it. A general procedure compiled by Wrenchpit, not a first-hand write-up from one owner's vehicle. The fastener torques below are commonly cited values, not measured ones, and they vary by model year, engine and trim. This job involves fasteners where an incorrect torque can let a component come apart in service. Look the figures up in your service manual and use a calibrated torque wrench. Part numbers should be checked against the labels on your old parts.
Applies to roughly 2008-2015 model years. Filed under 2010 because a guide carries a single model year.
The M156 6.2L V8 in the W204 C63 is the last of Mercedes-AMG's hand-built naturally aspirated V8s and is famous for both its sound and its service demands. Sixteen spark plugs at every plug interval is one of those demands. Factory recommendation is every 40,000 miles for the plug service, but community consensus (BenzWorld, MBWorld, and the dedicated M156 owner groups) is 20,000 to 30,000 miles for driven cars because the platinum electrode wears faster than expected under the M156's high compression and aggressive spark advance. The bigger M156 story is the well-documented head bolt failure that appears on early production engines, where the head bolts stretch under thermal cycling and eventually seep coolant into the oil galleries. Do not conflate the two; plugs are a maintenance item, head bolts are a separate multi-day service. If a plug service reveals coolant weeping from any spark plug well or coolant in the oil, stop the plug job and diagnose the head bolts. NGK 96305 is the correct plug in the correct heat range for a stock M156; do not substitute plugs from other AMG engines even if the seat and reach appear similar. Buy plugs in sealed packs from an authorized NGK distributor; the M156 plug is a widely counterfeited part and fake plugs run rich enough to trigger a bank-lean code within a few hundred miles.
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