2019 MERCEDES-BENZ SLC300 R172

Typical onset: 40,000-80,000 mi

Symptoms: Excessive oil consumption (1 qt per 1,000 mi or worse), Blue smoke on startup or acceleration, Misfires and rough idle as scoring worsens, Check engine light with combustion lean/misfire codes

Fix: Engine rebuild or short block replacement. This is the M274's Achilles heel—cylinder wall coating fails and pistons score the bores. Expect 25-35 labor hours for short block swap including ancillaries, fluids, and break-in. Some owners pursue MB goodwill assistance even out of warranty.

Estimated cost: $8,000-15,000

Typical onset: 60,000-100,000 mi

Symptoms: ATF pooling under vehicle near front or radiator area, Transmission slipping or delayed engagement when fluid gets low, Burnt ATF smell if leak goes unnoticed

Fix: Replace cooler lines and top off 7G-Tronic+ fluid. Lines crack at fittings or corrode at brackets. Access is reasonable on SLC. Plan 2-3 hours labor plus fluid and lines. Inspect radiator end-tank seals while you're there.

Estimated cost: $400-800

Typical onset: 50,000-90,000 mi

Symptoms: Clunk or thud on shifts, especially reverse-to-drive, Vibration at idle in gear, Excessive driveline movement visible when rocking throttle

Fix: Replace transmission mount. The hydraulic mount on the M274/722.9 combo deteriorates and the rubber tears. Straightforward job: 1.5-2 hours with proper lift access. OE or quality aftermarket both hold up well.

Estimated cost: $300-550

Typical onset: 70,000-110,000 mi

Symptoms: Extended cranking or no-start condition, Rough running and loss of power under load, P0087 fuel rail pressure too low code, Metallic ticking from engine if pump internals shed debris

Fix: Replace high-pressure fuel pump on the back of the cylinder head. This is a direct-injection system weakness; pump can fail internally or seize. Also replace low-pressure filter in tank while diagnosing. Budget 3-4 hours labor for pump, more if injectors are contaminated.

Estimated cost: $1,200-2,200

Typical onset: 50,000-90,000 mi

Symptoms: Metallic rattle at idle or light throttle, especially cold start, Rattle disappears under boost, No performance loss initially

Fix: Wastegate arm bushing wears in the IHI turbo, causing audible play. Can be ignored if no boost leaks or performance issues, but eventual actuator or turbo replacement may be needed. Turbo R&R is 6-8 hours due to tight SLC engine bay. Some techs modify the arm or replace actuator only.

Estimated cost: $1,500-3,000

Symptoms: Roof operates slowly or stops mid-cycle, Hydraulic fluid stains in trunk or behind seats, Warning message on dash: 'Roof operation not possible', Pump runs excessively without completing cycle

Fix: Inspect all hydraulic lines, pump seals, and cylinder seals. SLC inherited the SLK's folding hardtop and its quirks. Common leak points: pump-to-reservoir hoses and cylinder rod seals. Diagnosis 1-2 hours, repair 3-6 hours depending on component access. Keep system lubricated and exercised to prevent seals drying out.

Estimated cost: $800-2,500