Mercedes-Benz SL 350
Four-valve V6 in two basic variants: 3.5L naturally aspirated (DES 35, 185–225 kW) with direct injection and stratified charge — refined, linear, no classic naturally aspirated feel. And the 3.0L BiTurbo family (DEH 30 LA, 245–295 kW) with Nanoslide coating, twin turbochargers and water-air charge air cooler. The BiTurbo sings sonorously from 2,500 rpm and screams at the top — exceptional for a V6. Weak points: timing chain wears early when oil change intervals are pushed. Oil cooler in the V-block doesn't seal forever. Camshaft solenoids leak. On the 3.5L stratified-charge variant add NOx sensor failures from short-trip use. Early DES 35 (2011–2013) had piston slap issues. The BiTurbo unit with Nanoslide is considered more robust. Maintained with short oil change intervals (10,000–12,000 km) the engine easily lasts beyond 200,000 km.
Naturally Aspirated V6 Roadster — the Relaxed SL Entry Point
306 hp naturally aspirated V6 in the SL R231: no BiTurbo punch, but linear power delivery without surprises. The SL 350 is the entry point to the range — less power than the 400 or 500, but cheaper to buy and run. With the roof down the V6 sounds pleasantly restrained: no V8 growl, but a refined six-cylinder note. Ideal for Sunday drivers and those who prefer the journey to the destination.
Engine Weaknesses 10
Known weak point: the oil cooler gasket in the V-block of the M276 ages and starts leaking. Repair is extremely labour-intensive because the engine must be partially disassembled — cost €1,700–3,800.
Symptoms: Oil spots under the vehicle, rainbow sheen on wet road, dropping oil level, oil smell
Known damage pattern on all newer Mercedes engines: solenoid valves leak, oil migrates into the wiring harness. First the lambda sensors die, then the ECU. Repair documented up to €1,970.
Symptoms: Check engine light with camshaft fault code, lambda sensor failure, oil film on connectors, in extreme cases ECU failure
On early M276 DES 35 (build years 2011–2013) pistons can loosen due to cylinder bore wear. Knock noise on cold start, worst case engine damage.
Symptoms: Clacking and knocking on cold start from the engine area, getting louder over time, rough idle when cold
The two-stage chain drive system of the M276 receives insufficient oil on early engines, leading to premature chain wear.
Symptoms: Rattling on cold start, rough idle, in the worst case chain jump
The M276 has a plastic thermostat housing that cracks with age and loses coolant. The thermostat itself can also fail open.
Symptoms: Coolant loss, puddle under the vehicle, fault code P0128, engine runs too cold on motorway, temperature gauge fluctuates
The M276 PCV system with oil separator is known for failures from defective membranes or blocked valves. Increased oil consumption and power loss from false air ingestion.
Symptoms: Noticeably rising oil consumption, blue smoke from exhaust, crankcase breather fault code P052E, power loss
On the M276 DES 35 (stratified-charge variant) the NOx sensors in the exhaust stream fail. Moisture accumulates at the sensor — short-trip driving prevents it from burning off. Repair costs €900–1,200 per sensor.
Symptoms: Check engine light, NOx sensor fault code, possibly increased fuel consumption and harder starting
The exhaust manifold can crack from thermal shock — especially on sportily driven examples and early build years. Welding repair is possible but not lasting.
Symptoms: Unusually loud exhaust note, power loss, exhaust smell in the engine bay or cabin, incorrect lambda sensor readings
On M276 BiTurbo variants, the wastegate linkage on the turbochargers can develop play and rattle. A progressively louder metallic noise from the turbo area. Turbocharger replacement required.
Symptoms: Metallic clattering and rattling from the engine area, especially under load, gets worse over months, boost pressure fault code
Direct injection on the M276 V6 causes carbon deposits on the intake valves. Cleaning is necessary at higher mileages.
Symptoms: Stumble when accelerating from part throttle, rough idle, slight increase in fuel consumption
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).