1991 MERCEDES-BENZ 420SEL

Typical onset: 120,000-180,000 mi

Symptoms: Rattling noise on cold start that quiets after warmup, Rough idle, Chain slap audible from front of engine, Check engine light or poor performance if jumped timing

Fix: Chain, rails, tensioners, and upper guide must be replaced as a set. Requires front-end teardown including radiator and accessories. 12-16 hours labor. If ignored, chain can jump or break, causing catastrophic valve-to-piston contact.

Estimated cost: $2,200-3,800

Typical onset: 150,000+ mi

Symptoms: White smoke from exhaust, Coolant loss with no visible leaks, Overheating, Milky oil on dipstick or cap, Bubbles in expansion tank when running

Fix: M116 V8 head gaskets fail due to age and heat cycling. Both heads should be pulled, checked for flatness, and resurfaced if needed. Valve stem seals typically replaced during this job. 18-24 hours labor for both sides. High parts cost due to gasket set and related seals.

Estimated cost: $3,500-5,500

Typical onset: 100,000-150,000 mi

Symptoms: Harsh or delayed shifts, Slipping between gears, Stuck in one gear (limp mode), Flare on 2-3 shift, No upshift past second gear

Fix: The 722.3 transmission's valve body develops sticky valves and worn bores over time. Vacuum modulator can fail, causing erratic shift points. Valve body can be rebuilt or replaced; modulator is external and cheap. Drop pan, replace valve body, flush, and refill. 6-8 hours labor for full valve body service.

Estimated cost: $1,200-2,400

Typical onset: 120,000+ mi

Symptoms: Hard starting when warm, Rough idle and stumbling, Fuel smell under hood, Poor fuel economy, Engine dies when coming to a stop

Fix: The mechanical K-Jetronic fuel injection uses a fuel distributor that wears internally, causing rich/lean mixtures. Can be rebuilt by specialists or replaced with remanufactured unit. Diagnosis requires careful fuel pressure testing. 4-6 hours labor plus core charge if replacing.

Estimated cost: $800-1,800

Typical onset: Any mileage (age-related)

Symptoms: Rear sags when parked overnight, Uneven ride height side-to-side, Pump runs constantly, Hydraulic fluid leaks at accumulators or lines, Warning light on dash

Fix: Rear self-leveling spheres, lines, and pump seals age out regardless of mileage. Leaking spheres are most common. Can replace with spheres (~3 hours labor) or convert to coil springs for reliability. Keeping system requires periodic fluid changes rarely done.

Estimated cost: $600-1,400

Symptoms: Intermittent stalling or no-start, Rough running that comes and goes, Multiple sensor faults, Brittle, cracked insulation on underhood wiring, Electrical gremlins that move around

Fix: Mercedes used biodegradable wiring insulation in this era that turns to dust. Entire engine harness should be replaced or rebuilt. Aftermarket harnesses available or can be custom-rebuilt. 8-12 hours labor depending on whether AC and other systems are integrated.

Estimated cost: $1,200-2,200