1984 CADILLAC SEVILLE

Typical onset: 60,000-100,000 mi

Symptoms: Overheating with no obvious coolant leaks, White smoke from exhaust, Severe oil consumption (quart per 500 miles), Coolant in oil or oil in coolant, Loss of power and misfires, Cracked cylinder heads or blown head gaskets

Fix: The HT4100 is infamous for weak head bolts, porous aluminum blocks, and inadequate cooling. Head gaskets blow, heads crack, and blocks develop internal cracks. Most need a complete engine rebuild or replacement with a used unit. Expect 25-35 hours labor for engine removal, rebuild, and reinstall. Many owners swap in a 350 Chevy small-block instead.

Estimated cost: $4,500-8,000

Typical onset: 80,000-120,000 mi

Symptoms: Hard starting when cold, Excessive white or blue smoke, Sudden loss of power, Coolant mixing with oil, Cracked cylinder heads, Catastrophic bottom-end failure

Fix: The 5.7L Olds diesel has weak head bolts and head gasket failures, plus the DX block prone to cracking. Injection pump failures are expensive. Water in fuel destroys the IP quickly. Most need complete rebuilds including crank, pistons, rings, heads. 30-40 hours labor. Many are scrapped rather than fixed.

Estimated cost: $5,000-9,000

Typical onset: 70,000-110,000 mi

Symptoms: Transmission fluid leaking from radiator area, Pink milky transmission fluid, Harsh or erratic shifting, Transmission slipping after coolant contamination, Fluid puddles under front of car

Fix: The TH325-4L and TH200-4R transmissions use a cooler inside the radiator that fails, mixing coolant and ATF. This destroys the transmission if not caught early. Requires radiator replacement or external cooler bypass, plus full transmission fluid flush. If contamination happened, expect a transmission rebuild. 8-12 hours for cooler fix, 18-25 hours if trans needs rebuild.

Estimated cost: $800-1,200 cooler only, $2,500-4,000 with trans rebuild

Typical onset: 90,000-130,000 mi

Symptoms: Slipping in 3rd or 4th gear, No overdrive engagement, Harsh 1-2 shift, Delayed engagement into gear, Burnt transmission fluid smell, No movement in any gear

Fix: Both overdrive automatics suffer weak 3-4 clutch packs and valve body issues. The 200-4R particularly has notorious throttle valve and converter clutch problems. Rebuilds require updated clutches, bands, and valve body. 16-22 hours labor for R&R and rebuild. Used replacements often fail quickly.

Estimated cost: $2,800-4,500

Symptoms: Intermittent stalling, No-start when hot, Check engine light with multiple codes, Erratic idle, Poor fuel economy, Limp mode operation

Fix: The early DFI system uses an ECM and multiple sensors that fail with age. Throttle position sensors, MAP sensors, and coolant temp sensors are common culprits. The ECM itself can fail. Diagnosis is time-consuming with period-correct scan tools. 3-6 hours diagnostic plus 2-4 hours replacement depending on component. NOS or used parts often required.

Estimated cost: $400-1,200

Typical onset: 80,000-120,000 mi

Symptoms: Oil spots under car after sitting, Oil coating on transmission bellhousing, Low oil level between changes, Oil dripping from back of engine or pan rail

Fix: Both engines leak from rear main seal and oil pan gaskets as they age. Rear main requires transmission removal on the 4.1L and diesel (12-16 hours), oil pan is 4-6 hours due to limited access with crossmember and exhaust in the way. Often done together to save duplicate labor.

Estimated cost: $800-1,400 oil pan, $1,200-2,000 rear main

Symptoms: Rough cold idle, Hesitation on acceleration, Flooding and fuel smell, Poor fuel economy, Black smoke from exhaust

Fix: The 368 uses a Quadrajet that needs periodic rebuild due to old fuel gumming passages and worn accelerator pump. Choke pulloff and fast-idle systems fail. Rebuild kits available, 3-4 hours labor for removal, rebuild, and tuning. The 368 is the most reliable engine option but getting hard to find carb parts.

Estimated cost: $400-700