1979 CADILLAC DEVILLE

Typical onset: 60,000-100,000 mi

Symptoms: Clunk when shifting from Park to Drive or Reverse, Excessive drivetrain vibration at idle, Transmission seems to shift hard or slam into gear, Visible sagging of transmission tail housing

Fix: The rubber transmission mount deteriorates and allows the TH400 to drop, which then stresses the transmission cooler lines, driveshaft U-joints, and can crack the transmission case or tailhousing. Requires lifting transmission to replace mount (2-3 hours), but often discovered after cooler line rupture or worse damage. Inspect every 50k miles preventively.

Estimated cost: $250-600

Symptoms: Hesitation or stumble on acceleration from stop, Stalling when warm at idle or low speed, Black smoke from exhaust under acceleration, Hard starting when hot

Fix: The Rochester Quadrajet 4-barrel becomes problematic due to deteriorated accelerator pump, clogged idle circuits, and failing choke pull-off. Internal gaskets harden and cause vacuum leaks. Full rebuild kit plus proper adjustment requires 4-6 hours by someone who knows these carbs. Many shops default to reman or new carb instead ($400-700 part). Ethanol fuel accelerates deterioration.

Estimated cost: $350-900

Typical onset: 100,000-150,000 mi

Symptoms: Lifter tick that doesn't quiet down after warmup, Loss of power and poor fuel economy, Low oil pressure at idle when hot, Excessive blow-by or oil consumption over 1 qt per 1000 miles

Fix: The 425 and especially the Olds 403 used soft flat-tappet cams that wear prematurely, wiping lobes and destroying lifters. Requires complete valve job with cam replacement and lifter set (20-25 hours labor). If neglected, leads to bearing wear and full rebuild. Often combined with worn piston rings from 70s tolerance stacking. Short block or complete engine rebuild runs 35-45 hours.

Estimated cost: $2,500-6,500

Typical onset: 70,000-120,000 mi

Symptoms: Sudden red fluid puddle under vehicle, Transmission slipping or not engaging after startup, Smoke from fluid hitting exhaust components, Low transmission fluid warning or burnt smell

Fix: Steel cooler lines rust through at bends or chafe points near crossmember, catastrophically draining TH400 in minutes. If caught immediately no damage, but most drivers lose 6+ quarts before noticing, causing clutch pack burn. Line replacement is 2-3 hours, but often requires transmission rebuild if driven while low ($1,800-2,800 additional). Replace lines preventively if any surface rust visible.

Estimated cost: $300-3,500

Symptoms: Engine dies at highway speed under load, Won't restart until it cools for 20-30 minutes, Surging or stumbling at sustained speed, Won't rev past 3000 RPM

Fix: In-tank sock filter and inline filter clog from varnish in 45-year-old fuel systems. Mechanical fuel pump can also develop weak diaphragm. Replace both filters and pump if pressure under 5.5 PSI (3 hours including tank drop for sock). Often requires complete fuel system flush and tank cleaning if sitting for years.

Estimated cost: $400-800

Symptoms: Rough idle that changes when touching vacuum lines, Climate control not switching modes properly, Cruise control not engaging or surging, Check engine light (if still functional)

Fix: Dozens of vacuum lines for EGR, AIR pump, cruise control, and HVAC controls become brittle and crack. Creates multiple vacuum leaks affecting driveability and all vacuum accessories. Systematic replacement of all lines requires 4-6 hours and vacuum diagram (often missing). Not catastrophic but makes car annoying to drive.

Estimated cost: $350-700