Mercedes-Benz SL 500
4.7-litre BiTurbo V8 with direct injection (CGI), 90-degree bank angle and hot-V layout — turbos sit between the cylinder banks. Timing chain and tensioners are the first thing to check on used examples: cold-start rattle is a warning sign. Alusil bores react badly to wrong oil or extended intervals; piston slap develops gradually from oil film loss, often triggered by fuel-washing injectors. Ignition coils rarely last beyond 120,000 km. The oil separator should be checked after 100,000 km — when faulty it forces oil vapour into the intake and accelerates intake valve carbon build-up. Sound character: subtle BiTurbo rumble, no naturally aspirated roar. Shorten maintenance intervals to 10,000 km, oil MB 229.5 with low Noack value. Engine long-lived with correct maintenance — problem cars are almost always those with missed oil changes.
V8 BiTurbo Roadster — the M278 at Its Most Honest
In the R231 the M278 sheds its S-Class insulation. Roof open, a rich, deep BiTurbo rumble develops that barely exists in the closed saloon interior. 320 kW, no turbo lag thanks to twin-scroll charging, rear-wheel drive — and a short, direct body. The AIRMATIC tune is comfort-oriented, not a true sports car. But few roadsters at this price point deliver such a sovereign, stress-free high-speed feeling. The best M278 for drivers seeking sound and open sky.
Engine Weaknesses 9
All M278 built before Feb. 2013 are affected by prematurely wearing chain tensioners. Mercedes responded with a goodwill campaign: new tensioners plus a check valve in the cylinder head.
Symptoms: Metallic rattle on cold start (2–5 seconds), later also rattling when warm and on hot restart
No M278 with over 120,000 km known without cylinder bore damage. Silitec coating is sensitive to knock. Full rebuild approximately €14,000–18,000, replacement engine approximately €20,500.
Symptoms: Increased oil consumption (1–1.6 L/1,000 km), engine knock, rough idle, brass swarf in oil
The M278 is the best-known example of Mercedes' oil migration problem. Oil seeps from the camshaft adjuster solenoid capillary-style through the entire engine wiring harness to the ECU. Costs up to €11,000 if ECU and harness both need replacing.
Symptoms: Cold-start rattle, fault codes P0010/P0011 camshaft adjuster, oil traces on connectors, sporadic engine misfires, complete ECU failure
CGI direct injectors carbon up from 120,000 km and dribble after shutdown. Post-drip washes the oil film from the bore — a direct precursor to bore damage.
Symptoms: Rough idle, fuel smell after shutdown, black wet spark plugs on individual cylinders, high consumption, misfire fault codes
The wastegate linkage bush wears and initially causes rattling on cold start that develops into a permanent noise.
Symptoms: Metallic rattle from the turbo area on cold start, sluggish acceleration, P0299 boost pressure fault code
The hydraulic engine mounts on the M278 V8 lose their damping fluid after around 100,000 km. The heavy V8 (>250 kg) then causes noticeable vibrations throughout the vehicle.
Symptoms: Vibrations at idle in the cabin, clunking on throttle application and lift-off, visibly collapsed engine mount
Ignition coils fail from around 100,000 km — usually one at a time, rarely all together. Misfires on individual cylinders, MIL flashes. Left untreated, fouled plugs wash the oil film from the bore.
Symptoms: Stumble at idle and part throttle, flashing engine warning light, misfire fault codes (cylinders 5 and 7 most commonly affected), occasional fuel smell
The oil separator (crankcase ventilation) wears and feeds too much oil vapour into the intake. Accelerates intake valve carbon build-up and measurably increases oil consumption.
Symptoms: Increased oil consumption, blue smoke on cold start, oil film visible in intake pipe, smoke from exhaust after extended parking
As a pure direct-injection engine, the M278 builds up carbon deposits on the intake valves. Walnut blasting every 60,000 km is recommended.
Symptoms: Rough idle, slight power reduction, occasional misfires when cold
Vehicle Weaknesses 8
Leaking front ABC dampers were documented on early R231 examples. A new damper from Mercedes costs 1,900 EUR; reconditioned units are available for half the price.
The ABC hydraulic oil absorbs moisture. The pH drops, rubber components dissolve and contaminate valves and lines. Annual change is mandatory.
Active Body Control loses hydraulic pressure through non-use or ageing. Valves become coated with deposits, pressure accumulators lose membrane tension. Regular Pentosin change every 50,000 km is essential.
The folding roof can develop leaks through aged seals, particularly at the side guides and in the A-pillar area.
High quiescent current draw. Battery discharges quickly with prolonged standstill. AGM battery must be maintained regularly.
Pressure lines are partly routed incorrectly from the factory and become porous through pinching. Hydraulic oil escapes and the roof stops in any position.
The vario roof develops creaking, rattling or clicking noises. Recalibration by the dealer eliminates the noises. Cost approximately 540 EUR.
The bonding of the roof seal at the transition to the windscreen detaches with age. Water runs along the A-pillar into the interior.
Reports & Tests
35 owner complaints filed with NHTSA (2012–2020). Most reported: Engine (7), Powertrain (7), Wheels (7).