1991 CADILLAC SEVILLE

Typical onset: 60,000-100,000 mi

Symptoms: white smoke from exhaust, coolant loss with no visible leaks, overheating, milky oil on dipstick, rough idle when cold

Fix: Early Northstar blocks have inadequate head bolt threading—bolts pull out under thermal stress. Proper repair requires Timesert thread inserts in all 20 bolt holes per head, new gaskets, and machining. 18-24 labor hours for both heads with inserts.

Estimated cost: $3,500-5,500

Typical onset: 80,000-140,000 mi

Symptoms: pink fluid under car, transmission slipping after warm-up, delayed engagement, transmission overheating warning, puddles at front of vehicle

Fix: Steel lines rust through where they pass the subframe or at crimp fittings. Replace all cooler lines as a set (not just the leaking one), flush cooler, inspect radiator end-tank for cracks. Often discover damaged 4T60-E internals from running low on fluid. 3-4 hours for lines alone, 12-16 if transmission needs rebuild.

Estimated cost: $400-600 lines only, $2,200-3,200 with transmission rebuild

Typical onset: 70,000-120,000 mi

Symptoms: clunk when shifting to drive or reverse, vibration at idle in gear, visible engine movement when revving, difficulty shifting, creaking sounds over bumps

Fix: Rubber mount deteriorates from heat and oil exposure. Front transaxle mount takes the most abuse. Replace all three mounts as a set—rear mount and torque strut also commonly failed by this age. 2-3 hours labor.

Estimated cost: $350-550

Typical onset: 90,000-150,000 mi

Symptoms: blue smoke on acceleration, quart every 500-800 miles, fouled spark plugs, rough running, carbon buildup on intake valves

Fix: Early Northstar pistons use weak ring design that carbons up and loses tension. Only real fix is engine rebuild with updated pistons and rings. 24-30 hours for complete teardown, hone cylinders, new bearings, gaskets, seals. Often discover scored cylinder walls requiring bore and oversize pistons.

Estimated cost: $4,500-7,000

Typical onset: 80,000-130,000 mi

Symptoms: hard starting when hot, stumbling under acceleration, stalling at idle, fuel smell, check engine light with lean codes

Fix: In-tank fuel pump assembly gets varnish buildup, regulator diaphragm leaks. Filter (inside tank on '91) clogs from old fuel. Requires tank drop, pump module replacement, clean tank. 4-5 hours labor.

Estimated cost: $600-900

Symptoms: intermittent no-start, stalling while driving, dies when hot, restarts when cool, cranks but won't fire, no spark or injector pulse

Fix: Sensor behind harmonic balancer fails from heat. Common on both 4.5L and Northstar. Requires removing balancer, 2-3 hours labor. Always replace reluctor ring if damaged and check balancer for wobble.

Estimated cost: $350-500