1986 CADILLAC SEVILLE

Typical onset: 50,000-90,000 mi

Symptoms: Overheating with no obvious coolant leaks, White smoke from exhaust, Oil contaminated with coolant (milky appearance), Loss of compression in multiple cylinders, Aluminum debris in coolant, Coolant consumption without external leaks

Fix: The HT-4100's thin aluminum cylinder walls crack and warp, while head bolts pull threads from the block. Most shops recommend complete engine replacement or rebuild with upgraded head studs and block sleeves. Labor alone is 18-24 hours for R&R, more for proper rebuild. Many owners swap to the Oldsmobile 307 V8 or Chevy 350 as permanent fix.

Estimated cost: $3,500-7,000

Typical onset: 60,000-120,000 mi

Symptoms: Hard starting when engine is hot, Stalling at idle or when coming to stops, Rough idle with surging, Check engine light with fuel system codes, Fuel smell from engine bay

Fix: The TBI units leak internally and externally, injectors clog, and the throttle position sensor fails. Rebuilding the TBI unit takes 2-3 hours but often needs the entire upper intake assembly cleaned. NOS or quality rebuilt units are scarce. Expect diagnostic time hunting intermittent issues.

Estimated cost: $600-1,200

Typical onset: 70,000-110,000 mi

Symptoms: Slipping between gears, especially 2nd to 3rd, No 4th gear (overdrive), Delayed engagement when shifting to Drive, Shuddering during light throttle acceleration, Transmission fluid appears burned or has metal particles

Fix: The 440-T4 was undersized for the application and suffers from worn clutch packs, failed torque converter lockup, and cracked cases. Rebuild requires 10-14 hours and must include upgraded components or it'll fail again within 30,000 miles. Many shops won't warranty these transmissions.

Estimated cost: $2,200-3,800

Symptoms: Speedometer or tachometer reads erratically or not at all, Segments of digital display go dark, Warning messages displayed incorrectly, Entire cluster goes dark then comes back, Fuel gauge stuck or jumps wildly

Fix: Cold solder joints on the circuit boards fail over time, and the vacuum fluorescent displays themselves deteriorate. Requires cluster removal (2 hours) and either bench repair by specialist or replacement unit. Replacement clusters are getting scarce and expensive.

Estimated cost: $400-900

Typical onset: 60,000-100,000 mi

Symptoms: Oil spots on driveway consistently at rear of engine, Low oil level requiring frequent top-ups (quart every 500-800 miles), Oil coating on bellhousing and transmission case, Visible drips from oil pan rail

Fix: The HT-4100's rear main seal fails regularly, requiring transmission removal (8-10 hours labor). Oil pan gaskets leak due to aluminum warping. Often done together when transmission is out anyway. The job is worth doing if the engine is otherwise healthy, but many aren't.

Estimated cost: $1,200-2,000

Typical onset: 80,000-130,000 mi

Symptoms: Rear of car sags when parked overnight, Compressor runs constantly or won't run at all, Uneven ride height side-to-side, Warning light for suspension system, Hissing sound from rear when car is turned off

Fix: Electronic level control compressor burns out, air lines crack and leak, and height sensors fail. Compressor replacement is 2-3 hours, lines another 1-2 hours each. Many owners convert to conventional springs and shocks for reliability. OEM parts are NLA, aftermarket quality varies.

Estimated cost: $800-1,600