1978 CADILLAC SEVILLE

Typical onset: 40,000-80,000 mi

Symptoms: Excessive white or black smoke from exhaust, Hard starting when warm, runs rough, Coolant loss with no external leaks (head gasket failure), Metal shavings in oil, bearing knock, Loss of power and fuel in crankcase

Fix: The Olds 350 diesel used gasoline engine block castings with inadequate head bolts (10 vs 14), causing head gasket failures that let coolant into cylinders and crack blocks. Weak crankshafts and rod bearings fail early. Most require complete engine replacement or conversion to gas 350. Rebuild involves 35-45 hours labor including R&R, machine work, and all internal components. Many owners opt for gas engine swap which is actually cheaper long-term.

Estimated cost: $4,500-7,500

Typical onset: 80,000-120,000 mi

Symptoms: Slipping between gears, especially 2nd to 3rd, Delayed engagement when shifting to Drive or Reverse, Transmission overheating, burned fluid smell, Harsh shifts or no upshifts, Metal particles in transmission pan

Fix: Early Sevilles used THM 400, later ones THM 200-4R. The 200-4R has weak direct clutch packs and inadequate cooler capacity in heavy Seville application. Transmission rebuild requires 12-16 hours including R&R, with torque converter replacement essential. Upgraded clutches and aftermarket cooler recommended during rebuild. Fluid and filter service every 25K miles critical but often neglected.

Estimated cost: $2,200-3,800

Typical onset: 60,000-100,000 mi

Symptoms: Transmission fluid leaking from radiator area, Pink or red fluid in coolant overflow, Transmission slipping after engine reaches operating temperature, Milky appearance in transmission fluid (coolant contamination)

Fix: The factory transmission cooler integrated into the radiator often fails internally, allowing coolant and ATF to mix—this destroys the transmission rapidly. External cooler lines rust through at frame contact points. Fix requires radiator replacement or external cooler installation (recommended), plus full transmission fluid flush. If contamination occurred, transmission rebuild is necessary. Labor 3-5 hours for cooler/radiator work only.

Estimated cost: $600-1,200 (cooler only), $3,000-4,500 (if transmission contaminated)

Typical onset: 50,000-90,000 mi

Symptoms: Engine stalling, won't restart until cooled, Loss of power under load, Excessive black smoke, Rough idle and misfiring, Fuel in return lines full of debris

Fix: The diesel's Stanadyne injection pump runs extremely tight tolerances and fails when contaminated by water or debris. Fuel filter housing allows water accumulation. Failed injection pumps damage injectors. Complete fuel system service includes injection pump replacement/rebuild (8-10 hours), all eight injectors, fuel filters, and complete tank cleaning. Many require fuel tank removal to clean sediment.

Estimated cost: $2,800-4,500

Typical onset: 70,000-110,000 mi

Symptoms: Excessive vibration at idle, smooths out at higher RPM, Clunking when shifting between Park, Reverse, and Drive, Engine movement visible when accelerating, Fan blade contacting shroud, Transmission linkage binding or misaligned

Fix: The rubber mounts deteriorate from oil contamination and age, causing severe drivetrain movement. Diesel engines, being heavier, accelerate mount failure. Both engine mounts and transmission mount typically need replacement together. Labor 3-4 hours for all three mounts. Failed mounts can cause accessory belt misalignment and transmission linkage damage if ignored.

Estimated cost: $450-750

Typical onset: 120,000-180,000 mi

Symptoms: Blue smoke on startup or acceleration, Excessive oil consumption (1 quart per 500-800 miles), Loss of compression and power, Blowby visible at oil filler cap, Fouled spark plugs

Fix: The Olds 350 gas engine experiences normal wear but oil consumption becomes excessive. Cylinder ridge wear and glazed bores are typical. Requires complete engine rebuild with new pistons, rings, bearings, timing set, and machine work (bore/hone cylinders, deck heads). Total 28-35 hours labor including R&R and all internal work. Many opt for remanufactured long block (18-22 hours labor) as more cost-effective.

Estimated cost: $3,800-5,500 (rebuild), $4,200-5,800 (long block)