1991 CHEVROLET C10

Typical onset: 80,000-150,000 mi

Symptoms: Transmission fluid in coolant or vice-versa (strawberry milkshake in reservoir), Transmission slipping or delayed engagement after overheat, Rust-through at steel cooler lines where they pass frame rail, Sudden loss of transmission fluid from corroded lines

Fix: Replace radiator with built-in cooler AND add external cooler, replace all cooler lines. Critical to flush entire transmission and torque converter—contaminated fluid kills clutches within 500 miles. 6-8 hours labor if caught early, 12-16 hours if transmission needs rebuild.

Estimated cost: $800-1,200 preventive / $2,500-3,800 with transmission rebuild

Typical onset: 100,000-180,000 mi

Symptoms: Hard starting when hot, starts fine cold, Rough idle with black smoke from one side of exhaust, Fuel smell in engine bay, visible leak at regulator diaphragm, Stumble on acceleration, loss of power above 3,000 RPM

Fix: Rebuild or replace both TBI injectors, replace fuel pressure regulator, new fuel filter. Injectors can be cleaned ultrasonically but replacement is more reliable at this age. 2-3 hours labor for injectors and regulator.

Estimated cost: $350-650

Typical onset: 150,000-220,000 mi

Symptoms: Metallic knocking on cold start that quiets when warm (rod bearings), Piston slap rattle first 30-60 seconds after start, worse in winter, Oil pressure drops below 10 PSI at hot idle, Metal flakes in oil, filter element shows bearing material

Fix: Full short-block rebuild or replacement required—rod bearings, main bearings, piston rings minimum. Many shops go straight to reman short block at this mileage. 18-24 hours labor for in-frame rebuild with heads off, more if machine work needed.

Estimated cost: $2,800-4,500 in-frame / $3,500-5,200 with reman short block

Typical onset: 90,000-140,000 mi

Symptoms: Fuel smell, especially after sitting overnight, Hard starting, long cranking before fire, Flooding condition, fuel leaking into intake manifold, Check engine light with rich mixture codes

Fix: Replace central fuel spider injector assembly under intake plenum. Upper intake must come off—gaskets, spider unit, fuel lines. This is the Achilles heel of the 4.3L Vortec. 4-6 hours labor.

Estimated cost: $650-950

Typical onset: 80,000-150,000 mi

Symptoms: Coolant loss with no visible external leak, White smoke at startup, sweet smell from exhaust, Rough idle, occasional misfire codes, Oil looks milky or coolant level drops slowly

Fix: Replace intake manifold gaskets and plenum gasket set. TBI engines use composite gaskets that deteriorate, allowing coolant into crankcase or vacuum leaks. 5-7 hours labor for V8s, includes new thermostat and coolant flush.

Estimated cost: $550-850

Typical onset: 60,000-120,000 mi

Symptoms: Random no-start when hot, starts fine after cooling 20 minutes, Misfire under load, especially in humid weather, Visible carbon tracks inside distributor cap, Intermittent stalling at idle or low speed

Fix: Replace cap, rotor, ignition module, and spark plug wires as a set. HEI modules fail with heat cycling. Cap should be replaced every 40-50k regardless—carbon tracking is inevitable. 1.5-2 hours labor.

Estimated cost: $250-425

Typical onset: 100,000+ mi or 20+ years

Symptoms: Clunk when shifting from Park to Drive or Reverse, Vibration at highway speed, driveline angle changes, Visible sagging of transmission tail housing, Exhaust contact at crossmember under load

Fix: Replace transmission mount and inspect crossmember for rust perforation—common in salt states. If crossmember is rotted, replacement requires frame work and is more involved. 1.5-2 hours for mount only, 4-6 hours if crossmember is shot.

Estimated cost: $180-350 mount only / $600-900 with crossmember