1984 CADILLAC DEVILLE

Typical onset: 60,000-100,000 mi

Symptoms: White smoke from exhaust indicating coolant burning, Overheating with no external leaks, Oil contaminated with coolant (milky appearance), Loss of coolant with no visible leaks, Rough idle and misfires

Fix: The HT4100 was an aluminum disaster with inadequate head bolt torque and thin cylinder walls. Head gaskets fail regularly, but cracked blocks between cylinders are the real killer. Most need complete engine replacement or rebuild with updated bolts and proper torque specs. Rebuild involves 25-35 hours labor including removal, machining, reassembly. Many shops won't touch these anymore—recommend sourcing a used replacement or swapping to the older 368ci if possible.

Estimated cost: $3,500-6,500

Typical onset: 70,000-120,000 mi

Symptoms: Pink milky transmission fluid indicating coolant mixing, Transmission slipping or delayed engagement, Coolant in transmission pan during service, Overheating transmission, Radiator leaking where cooler lines enter

Fix: The internal transmission cooler in the radiator develops leaks that allow coolant and ATF to mix—this destroys the transmission rapidly. Once contamination occurs, you need radiator replacement, complete transmission flush and often full rebuild since clutches don't tolerate coolant. External cooler install is mandatory to prevent recurrence. 12-18 hours for trans rebuild, 2-3 for radiator and lines, 1-2 for external cooler addition.

Estimated cost: $2,200-4,000

Typical onset: Any mileage—age-related

Symptoms: Hard starting or extended cranking, Stalling at idle or when coming to stops, Surging idle between 500-1200 RPM, Check engine light with codes 23, 24, 34, 44, Black smoke from rich running

Fix: The early Digital Fuel Injection system uses throttle position sensors, coolant temp sensors, and an idle speed control motor that all fail with age. Throttle body itself can develop vacuum leaks at the gasket. TPS replacement is 0.5 hours, ISC motor 1 hour, full throttle body rebuild/replace 2-3 hours. Parts availability is getting scarce—many require junkyard hunting.

Estimated cost: $300-900

Typical onset: 80,000-150,000 mi

Symptoms: Excessive blow-by from crankcase, Oil consumption over 1 quart per 500 miles, Knocking from bottom end, Hard starting requiring extended glow plug cycles, White or blue smoke on startup

Fix: The 350 diesel in these years is the notorious Oldsmobile conversion that grenades itself—head bolts pull threads from the block, main bearings fail, injection pumps die. When one thing goes, the whole engine is typically toast. Full rebuild requires 30-40 hours and specialized diesel knowledge most shops don't have anymore. Realistically, you're looking at engine swaps to a gas V8 or parting out the car.

Estimated cost: $4,500-7,500

Typical onset: 80,000-120,000 mi

Symptoms: Severe clunking when shifting from park to drive, Excessive engine movement visible under hood during acceleration, Vibration through steering wheel at idle, Difficulty shifting into gear

Fix: The FWD setup uses hydraulic mounts that collapse and leak fluid. Front mount is accessible but rear mount requires supporting the drivetrain. Front mount replacement 1.5-2 hours, rear mount 3-4 hours. Often multiple mounts need replacing simultaneously. Must be careful not to misalign drivetrain during installation.

Estimated cost: $400-800

Symptoms: Automatic climate control stuck in one mode, Blower motor runs constantly on high, No heat or AC despite system functioning, Temperature fluctuations unrelated to setting

Fix: The electronic climate control programmer (computer) behind the glove box fails from capacitor leakage and bad solder joints. Repair requires removal and either rebuild with new capacitors (if you can find someone) or sourcing a used unit. 2-3 hours removal and replacement. Many owners just bypass to manual controls.

Estimated cost: $350-650