1991 MERCEDES-BENZ 500SL R129

Typical onset: any mileage, age-related (25+ years)

Symptoms: intermittent misfires and rough running, check engine light with multiple cylinder misfire codes, hard starting or no-start conditions, crumbling insulation revealing bare wires, fuel odor from injector connector shorts

Fix: Complete engine harness replacement required. This is not a repair—the original biodegradable insulation disintegrates. Harness removal and reinstallation on M119 is labor-intensive, typically 12-16 hours for a thorough job including proper routing and connector cleaning. Aftermarket harnesses available but OEM-style replacement preferred.

Estimated cost: $2,500-4,000

Typical onset: 80,000-150,000 mi or after any overheat event

Symptoms: white smoke from exhaust, coolant loss with no external leaks, rough idle and misfire, milky oil or coolant in expansion tank, overheating history often precedes failure

Fix: Both head gaskets on M119 require heads-off service. Heads must be checked for warping and pressure tested. Typical job is 18-24 hours including resurfacing if needed. Always replace head bolts (torque-to-yield), valve cover gaskets, and timing chain components while in there. Common to find corroded coolant passages.

Estimated cost: $4,500-7,500

Symptoms: top operates slowly or stops mid-cycle, hydraulic fluid leaks in trunk area, top will not latch or unlatch, pump runs but top doesn't move, asymmetric operation (one side moves, other doesn't)

Fix: R129 top system uses hydraulic cylinders, lines, and a pump/reservoir. Common failures: cylinders seals leak (2-4 hours per cylinder), pump fails (3-4 hours), or micro-switches in the latch mechanism go bad (1-2 hours). Diagnosis time adds up—these systems require methodical testing. Fluid contamination can cascade failures.

Estimated cost: $800-3,500

Typical onset: any mileage, age and heat-cycle related

Symptoms: transmission fluid dripping or spraying near radiator, transmission slipping or delayed engagement after leak begins, burnt transmission fluid smell, low fluid level on dipstick

Fix: Hard lines that run to the auxiliary trans cooler crack at fittings or corrode through. Replacement requires removing airbox and accessing tight area near radiator support. 2-3 hours labor. Must refill and check trans fluid level precisely (requires running engine and cycling through gears). Ignored leaks destroy the transmission in under 50 miles.

Estimated cost: $400-700

Typical onset: 60,000-100,000 mi

Symptoms: clunk when shifting from P to D or R, excessive vibration at idle in gear, visible sagging of transmission tailshaft, harsh engagement into gear

Fix: Rear transmission mount is a large hydraulic-filled unit that deteriorates and collapses. Replacement requires supporting transmission and removing crossmember. 2-3 hours. Often done with engine mounts at same time since access is similar and they fail on same schedule.

Estimated cost: $500-900

Symptoms: rough idle and stumbling, fuel odor and visible leaks at distributor, hard starting when hot, poor throttle response, fuel pressure fluctuations

Fix: The Bosch CIS-E mechanical fuel injection uses a distributor block that can develop internal leaks and external weeping. Rebuild kits available but many techs replace the unit (4-6 hours). Fuel accumulator also fails (holds pressure for hot starts). System requires specific pressure testing tools. Also check fuel filter—often neglected and causes low pressure symptoms.

Estimated cost: $800-2,000

Typical onset: 80,000-120,000 mi

Symptoms: rear end sags when parked overnight, pump runs excessively or constantly, uneven rear ride height side-to-side, warning light on dash, hydraulic fluid leaks near rear shocks

Fix: Hydraulic self-leveling uses accumulators (pressure spheres) and level sensors that fail with age. Accumulators lose charge (1 hour each to replace), pump can fail (2-3 hours), or valve block leaks (4-5 hours). Can convert to conventional springs and shocks if done properly, but diminishes ride quality. OEM repair is preferred.

Estimated cost: $1,200-2,800