1994 CHEVROLET IMPALA SS

Typical onset: 60,000-100,000 mi

Symptoms: no-start or random stalling, rough idle and misfires especially in wet weather, check engine light with multiple misfire codes, loss of power under load

Fix: The Optispark sits low on the front of the engine behind the water pump, making it vulnerable to water intrusion and heat. Replacement requires removing the water pump first. Labor is 4-6 hours. Use a vented aftermarket unit to reduce repeat failures. While you're in there, replace the water pump.

Estimated cost: $800-1,400

Typical onset: 80,000-150,000 mi

Symptoms: transmission fluid in coolant creating strawberry milkshake in overflow tank, transmission slipping or delayed engagement after radiator failure, drips or puddles of ATF under front of vehicle, overheating transmission

Fix: The 4L60E's cooler lines rust through where they connect to the radiator, and the internal trans cooler in the radiator can rupture, mixing fluids and destroying the transmission. If caught early (external leak only), replace lines and flush both systems, 2-3 hours labor. If radiator failed internally, you need radiator, lines, full transmission flush or rebuild. Many owners install external auxiliary cooler as prevention.

Estimated cost: $400-800 for lines/radiator; $2,500-4,000 if transmission contaminated

Typical onset: 120,000-180,000 mi

Symptoms: 2-3 shift flare or slipping, no 3rd or 4th gear, delayed engagement into drive or reverse, hard shifts or banging into gear

Fix: The 4L60E behind the LT1 is marginal for the torque and vehicle weight, especially if towing or driven hard. The 3-4 clutch pack and input shaft wear out most commonly. A rebuild with upgraded components takes 8-12 hours labor. Change fluid every 30,000 miles and add auxiliary cooler to extend life.

Estimated cost: $2,200-3,500

Typical onset: 100,000-150,000 mi

Symptoms: oil puddle under rear of engine, oil coating on transmission bellhousing, drips from oil pan seam, low oil level between changes

Fix: The LT1's rear main seal is a two-piece design that leaks with age. Requires transmission removal, 6-8 hours labor. Oil pan gasket leaks are common too; the pan needs to come off (exhaust and crossmember work), 4-5 hours. Budget for both if the rear main is being done since you're already there.

Estimated cost: $800-1,200 rear main; $500-800 oil pan; $1,200-1,800 both together

Typical onset: 100,000-140,000 mi

Symptoms: no-start with crank but no fuel pressure, stalling at idle or under acceleration, engine cuts out intermittently, whining noise from fuel tank

Fix: The in-tank pump fails from age and ethanol fuel. Tank must be dropped, 2-3 hours labor. Replace the fuel filter (mounted under driver side near tank) at the same time—it's often neglected and a clogged filter can kill the new pump. Recommend sending unit replacement, not just the pump.

Estimated cost: $600-900

Typical onset: 80,000-120,000 mi

Symptoms: coolant leak from front of engine, temperature gauge climbing above 210°F, squealing from water pump bearing, coolant in oil or vice versa if intake gasket fails

Fix: Water pump is 4-5 hours because you have to remove the Optispark. Do them together. The LT1 also has plastic intake manifold gaskets that deteriorate and leak coolant into the valley or oil. If overheating, pressure-test the system—intake gasket replacement adds another 6-8 hours and requires upper intake removal.

Estimated cost: $700-1,000 water pump; $1,200-1,800 intake gaskets

Typical onset: 80,000-120,000 mi

Symptoms: clunk when shifting into drive or reverse, vibration at idle in gear, excessive driveline movement visible under acceleration, exhaust rattling against crossmember

Fix: The rubber transmission mount collapses with age, allowing the tailshaft to sag. Easy fix from underneath, 1-2 hours. While you're there, inspect the torque arm bushings and differential mount—they wear too but are less common failures.

Estimated cost: $200-350