1978 CADILLAC FLEETWOOD

Typical onset: 100,000-150,000 mi

Symptoms: Heavy knocking from bottom of engine especially on cold start, Low oil pressure at idle, Metallic rattling that worsens under load, Oil contaminated with brass/copper particles

Fix: Complete engine teardown required. Main bearings wear prematurely due to marginal oiling in the Cadillac 425 and softness of late-70s bearing material. Crankshaft often needs grinding or replacement. Most shops pull the engine for access. Expect 24-32 hours labor for R&R plus internal work. Many owners opt for complete rebuild or used engine swap at this point.

Estimated cost: $3,500-6,500

Typical onset: 80,000-120,000 mi

Symptoms: Blue smoke on startup and acceleration, Burning 1+ quart every 500-800 miles, Fouled spark plugs, Poor compression readings across multiple cylinders, Carbon buildup visible in tailpipe

Fix: Rings gum up from low-detergent oils and extended oil change intervals common in the era. Also victim of weak ring tension spec'd to reduce friction for CAFE standards. Requires engine teardown, cylinder honing, new rings on all pistons. If cylinders are scored, needs bore and oversize pistons. Engine-out job: 20-28 hours labor.

Estimated cost: $2,800-4,200

Typical onset: 60,000-100,000 mi

Symptoms: Clunk when shifting from Park to Drive or Reverse, Excessive driveline vibration at highway speed, Transmission tail visibly sagging, Hard shifts especially into gear from neutral

Fix: The rubber transmission mount deteriorates from heat, age, and the sheer weight of the THM400. Mount literally tears apart. Requires lifting transmission slightly with jack, removing crossmember bolts, replacing mount. Straightforward job: 1.5-2.5 hours labor. Often done alongside transmission service.

Estimated cost: $180-350

Symptoms: ATF puddle under front of car, Transmission overheating, Low transmission fluid level, Slipping or delayed engagement, Pink fluid mixing with coolant in radiator (if internal cooler fails)

Fix: Steel cooler lines rust through at bends and mounting points, especially in salt states. External lines are 1-2 hour job with pre-bent replacements. If the radiator's internal cooler fails, ATF mixes with coolant—catastrophic for transmission. Requires radiator replacement plus complete transmission flush: 4-6 hours total.

Estimated cost: $200-450 (lines only), $800-1,400 (if radiator cooler fails)

Symptoms: Hard starting when cold or hot, Rough idle and stalling, Hesitation on acceleration, Black smoke from rich condition, Check Engine light (early EGR systems), Poor fuel economy below 8 MPG

Fix: These engines run either a Rochester Quadrajet or Varajet depending on application, buried under a maze of vacuum lines, EGR valves, and early computer controls. Vacuum leaks and gummed-up carbs are endemic. Proper fix involves carburetor rebuild, vacuum system diagnosis, and EGR cleaning. DIY-friendly if you have patience: 3-6 hours. Shops often spend longer chasing gremlins.

Estimated cost: $400-900

Typical onset: 70,000-110,000 mi

Symptoms: Coolant weeping from front or rear of intake, Coolant loss with no external puddle, Rough idle from vacuum leak, White residue around intake bolts, Overheating in traffic

Fix: The large cast-iron intake manifold on both the 403 and 425 uses paper/composite gaskets that fail from heat cycling and corrosion. Coolant passages run through intake ends. Requires removal of carburetor, distributor, all accessories, and intake manifold. Clean mating surfaces, install quality gaskets with proper torque sequence. 6-9 hours labor.

Estimated cost: $650-1,200

Symptoms: Intermittent stalling after sitting, Hard starting, Engine dies under load, Fuel starvation at highway speed, Rust particles in fuel filter

Fix: Old fuel varnish in tanks that sat, combined with rusting steel tanks and lines, clogs filters and carburetors. Fuel filter on these is inline or in carb inlet—change every 10,000 miles or annually. If tank is rusty, needs drop, clean, and seal or replace: 4-6 hours. Preventive filter changes are 0.5 hour.

Estimated cost: $25-80 (filter), $600-1,100 (tank service)