The E31 850i with the M70 V12 is a magnificent grand tourer that demands respect and deep pockets. Nikasil cylinder bore issues dominate the repair history, and when they surface, you're looking at engine-out work that rivals the car's current value.
Nikasil Cylinder Bore Failure (M70 V12)
Common · high severityTypical onset: 60,000-120,000 mi
Symptoms: Cold-start misfire that clears after warm-up, Loss of compression in one or more cylinders, Blue smoke on startup, Rough idle that worsens over time
Fix: Pre-1996 M70 blocks used Nikasil bore coating that degrades from sulfur in US fuel. Proper fix is engine removal and either sleeved block or short-block replacement with Alusil block. Engine-out labor alone is 25-30 hours. Many shops won't touch it.
Estimated cost: $8,000-15,000
Valve Stem Seal Deterioration
Common · medium severityTypical onset: 80,000-140,000 mi
Symptoms: Oil consumption 1 quart per 1,000 miles or worse, Blue smoke on deceleration or startup, Fouled spark plugs on multiple cylinders
Fix: Requires cylinder head removal on both banks to replace seals. With heads off, smart money does full valve job and timing chain guides simultaneously. 35-40 hours labor with heads R&R.
Estimated cost: $5,500-8,500
Transmission Oil Cooler Line Failure
Common · high severitySymptoms: Transmission fluid in coolant (strawberry milkshake in expansion tank), Coolant in transmission fluid (slipping, erratic shifts), Trans overheat warnings on dash
Fix: The auxiliary trans cooler lines rot internally or the cooler itself fails inside the radiator. Cross-contamination ruins both systems. Requires radiator replacement, complete trans fluid system flush, sometimes trans rebuild. 8-12 hours if you catch it early.
Estimated cost: $2,000-6,000
Engine and Transmission Mounts (Hydraulic Mounts)
Common · medium severityTypical onset: 70,000-110,000 mi
Symptoms: Excessive vibration at idle in Drive, Clunk on throttle application or lift, Visible engine movement during acceleration, Transmission thunk on shifts
Fix: The hydraulic mounts fail internally and need all replaced as a set. Six mounts total (engine and trans). Access is tight on some positions. 6-8 hours labor.
Estimated cost: $1,200-1,800
Dual Mass Flywheel Failure (Manual Transmission)
Occasional · high severityTypical onset: 80,000-120,000 mi
Symptoms: Rattling at idle in neutral, Judder during clutch engagement, Difficulty shifting into first or reverse, Metallic grinding from bell housing
Fix: DMF wears out springs between masses. Requires transmission removal (12-15 hours), and you replace flywheel and clutch as a unit. OE parts are $2,500+ alone.
Estimated cost: $3,500-5,000
Fuel System Issues (Pumps, Filter, Lines)
Occasional · medium severityTypical onset: 90,000-150,000 mi
Symptoms: Hard starting when hot, Loss of power above 4,000 RPM, Surging or cutting out under load, Check engine light with lean codes
Fix: Twin fuel pumps in tank, hard-to-access inline filter, and aging rubber lines cause progressive starvation. Filter replacement is 2 hours due to location. Pump replacement requires tank drop (6-8 hours).
Estimated cost: $800-2,200
Throttle Position Sensor and Idle Control Issues
Occasional · low severityTypical onset: 100,000+ mi
Symptoms: Erratic idle speed (hunting between 500-1200 RPM), Stalling when coming to a stop, Hesitation off idle, Check engine light with TPS or idle control codes
Fix: Both throttle bodies (one per bank) have TPS and idle valves that carbon up or fail electronically. Cleaning helps temporarily, but replacements needed eventually. 3-4 hours labor for both sides.
Estimated cost: $600-1,200
Buy only if you have a trusted M70 specialist nearby, proof of Alusil block swap or post-9/95 build date, and a separate fund equal to the purchase price for repairs.
AI-assisted summary drawn from NHTSA recall data, our labor-times database, and platform knowledge. Not a substitute for a pre-purchase inspection on a specific vehicle.