1991 BMW 850I E31

Typical onset: 60,000-120,000 mi

Symptoms: Excessive oil consumption (1qt per 500-1000 miles), Cold-start misfire that improves when warm, Loss of compression on multiple cylinders, Blue smoke from exhaust on deceleration

Fix: Nikasil cylinder liners were incompatible with high-sulfur fuel in the US. Requires complete engine rebuild with Alusil block swap or cylinder resleeve. Budget 60-80 hours labor for full rebuild including heads, gaskets, and associated seals. Many owners opt for used Alusil replacement engines (post-9/93 production) which cuts labor to 35-45 hours for R&R.

Estimated cost: $8,000-15,000

Typical onset: 80,000-150,000 mi

Symptoms: Transmission fluid puddle under vehicle (usually passenger side), Sudden loss of drive after fluid loss, Transmission overheating warnings if caught early, Pink or red fluid mixed with coolant in overflow tank (internal cooler failure)

Fix: The crimped hard lines running along the transmission tunnel corrode and split. External lines are 4-6 hours to replace; internal cooler-in-radiator failure requires radiator replacement plus full transmission flush, adding 3-4 hours. Always replace both external lines preventively when doing one. Trans mount replacement typically done at same time (add 1.5 hours).

Estimated cost: $800-2,200

Typical onset: 100,000+ mi

Symptoms: Intermittent misfire codes on random cylinders, Check engine light with multiple stored faults, Rough idle that changes with engine bay temperature, Fuel smell in cabin or engine bay, Hard starting when heat-soaked

Fix: Engine bay heat destroys the insulation on injector wiring and sensor connectors. The M70 has two engine computers (one per bank) which complicates diagnosis. Full engine harness replacement is 18-24 hours labor due to V12 packaging. Many techs repair individual sections as they fail, but this becomes a whack-a-mole game. Aftermarket replacement harnesses now available.

Estimated cost: $2,500-4,500

Typical onset: 70,000-130,000 mi

Symptoms: Erratic idle (500-1200 RPM hunting), Hesitation or stumble on acceleration, Trans won't downshift properly, Limp mode activation (max 3000 RPM), Cruise control inoperative

Fix: Each bank has its own throttle body with mechanical linkage connecting them. Plastic bushings wear, linkage goes out of sync, and throttle position sensors fail. Requires removal of upper intake plenum for access (6-8 hours total). Both throttle bodies should be rebuilt/replaced together including TPS, idle control valves, and linkage bushings. Sync adjustment critical afterward.

Estimated cost: $1,800-3,200

Typical onset: 80,000+ mi

Symptoms: Oil puddle under center of engine after sitting, Oil dripping from bell housing area, Valley area wet with oil when intake manifold removed, Burning oil smell but no visible smoke

Fix: The valley pan gasket between cylinder banks and rear main seal are notorious leakers. Valley pan requires upper intake removal (10-12 hours labor). Rear main seal requires transmission removal (14-18 hours). Smart money does both if transmission is already out for cooler lines or clutch work on manual cars. These leaks rarely strand you but create a mess.

Estimated cost: $1,200-3,500

Typical onset: 90,000+ mi

Symptoms: Hesitation under load at highway speeds, Long crank time before starting, Loss of power above 4000 RPM, Fuel odor near tank area, Rough running that improves with full tank

Fix: Dual fuel pumps live in-tank. Filter is often neglected (should be every 30k). Pressure regulator on the fuel rail fails causing rich running or fuel smell. Filter change is 1-2 hours. Pump replacement requires tank drop (6-8 hours for both pumps). Many techs find 15-20 year old pumps still working but weak. Consider preventive replacement if tank is dropped for other work.

Estimated cost: $600-2,800

Typical onset: 100,000+ mi

Symptoms: Rear sags when parked overnight, Compressor runs constantly when driving, Suspension warning light illuminated, Uneven rear ride height side-to-side, Hissing from rear when vehicle shut off

Fix: Air bladders in rear struts leak, compressor wears out, or accumulator sphere loses pressure. Complete conversion to conventional springs/shocks is 4-6 hours and eliminates future headaches. Repair of OE system (spheres, struts, compressor) costs more but maintains original ride. Most specialists recommend the coil conversion unless you're concours-focused.

Estimated cost: $1,200-3,000