The 1977 Cadillac Seville was Cadillac's first attempt at downsizing with Oldsmobile's 350ci V8, but the diesel variant (LF9) that became standard in 1978-79 created lasting confusion—many gas engines were swapped out. The THM400 transmission is bulletproof, but the Oldsmobile gas engine itself has serious bottom-end durability issues when not meticulously maintained.
Oldsmobile 350 V8 Bottom-End Failure (Bearing Knock)
Common · high severityTypical onset: 80,000-140,000 mi
Symptoms: Deep knocking from crankcase that follows engine RPM, Low oil pressure especially when hot, Metallic rattling on cold start that worsens, Metal shavings in oil filter during changes
Fix: Full engine rebuild required—crankshaft typically needs machining, new main and rod bearings, often pistons/rings if cylinder walls are scored. 18-25 labor hours for complete bottom-end rebuild with machine shop work. Many owners opt for remanufactured long blocks instead (12-15 hours swap time).
Estimated cost: $3,500-6,500
THM400 Transmission Oil Cooler Line Failure
Common · high severityTypical onset: anywhere after 60,000 mi
Symptoms: Transmission fluid pooling under front of car, Pink fluid on radiator or near front crossmember, Sudden loss of drive gears after highway run, Transmission slipping or delayed engagement
Fix: Steel cooler lines rust through where they pass frame rails or at crimp fittings. Requires replacement of lines from transmission to radiator-mounted cooler. Add 4 hours if internal radiator cooler has cross-contaminated coolant into trans (requires trans flush and possible rebuild). 2-3 hours for lines only.
Estimated cost: $350-800
Intake Manifold Gasket Leaks (Oldsmobile 350)
Common · medium severityTypical onset: 70,000-120,000 mi
Symptoms: Coolant seeping at front or rear of intake manifold, White smoke from exhaust on startup, Coolant loss with no external leaks visible, Rough idle or misfire on one bank
Fix: The Oldsmobile small-block uses thin composite gaskets that deteriorate. Intake manifold removal required—8-10 hours due to Seville's tight engine bay with AC compressor and power steering interference. Replace all intake gaskets, valley pan gasket, and inspect EGR passages for blockage.
Estimated cost: $800-1,400
Fuel System Varnish and Carburetor Issues
Common · medium severitySymptoms: Hard starting when hot, Stalling at idle after warmup, Fuel smell in engine bay, Hesitation or stumble on acceleration, Flooding or fuel leaking from carb base
Fix: Rochester Quadrajet carburetors get varnish buildup from ethanol fuel, especially in cars that sit. Fuel filters clog quickly on awakened barn finds. Complete carb rebuild with ethanol-resistant parts takes 4-6 hours including removal, bench rebuild, and tuning. Replace fuel filter, check rubber fuel lines for cracking (3-4 hours for full system hose replacement).
Estimated cost: $600-1,200
Rear Transmission Mount Collapse
Occasional · medium severityTypical onset: 80,000+ mi
Symptoms: Clunk when shifting from park to drive, Excessive driveline vibration at highway speeds, Transmission tunnel heat or noise, Visible transmission sag when inspecting from below
Fix: The rubber transmission mount deteriorates from heat and age. Requires raising transmission slightly with jack, removing crossmember bolts, and replacing mount. Straightforward 1.5-2 hour job, but access is tight around exhaust.
Estimated cost: $200-400
Brake Master Cylinder and Booster Failure
Occasional · high severityTypical onset: anywhere—age-related
Symptoms: Soft or spongy brake pedal, Pedal sinks slowly to floor when held, Brake fluid leaking at booster-to-master junction, Hard pedal with poor stopping power (booster failure)
Fix: Original Bendix master cylinders and boosters fail from age more than mileage. Master cylinder replacement is 2 hours with bleeding. Booster adds another 2-3 hours due to firewall stud access. Many techs replace both together as preventive measure on 45+ year old cars.
Estimated cost: $500-1,100
Buy only if the engine has documented rebuild history or verified low mileage with proof of fanatical oil changes—too many basket cases with spun bearings out there, and a fresh engine costs more than the car's worth.
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.