1990 GMC SIERRA 3500

Typical onset: 150,000-250,000 mi

Symptoms: heavy knocking from crankcase, sudden loss of oil pressure, metal shavings in oil, engine seizure

Fix: Full tear-down reveals spun main or rod bearings, often scoring the crankshaft beyond simple bearing replacement. Requires crankshaft R&R with machining or short-block replacement. 20-30 labor hours for a full rebuild with machine work; many owners opt for reman long-block swap at 16-20 hours.

Estimated cost: $4,500-8,500

Typical onset: 120,000-180,000 mi

Symptoms: white smoke from exhaust, coolant loss with no visible leaks, oil milky appearance, rough idle and misfires

Fix: The 454 big-block runs hot and corrodes head gaskets, especially if radiator maintenance was neglected. Heads must be removed, inspected for warpage, and often require milling. Both head gaskets, intake gaskets, and all cooling system components typically replaced. 14-18 labor hours.

Estimated cost: $2,200-3,800

Typical onset: 100,000-180,000 mi

Symptoms: hard starting when warm, intermittent stalling, loss of power under load, black smoke, no-start condition

Fix: The mechanical Stanadyne DB2 or early electronic DS4 pumps fail from fuel contamination or internal wear. Pump replacement requires timing adjustment and often reveals contaminated fuel lines needing flush. 6-9 labor hours plus pump core charge.

Estimated cost: $1,800-3,200

Typical onset: 80,000-150,000 mi

Symptoms: transmission fluid leaking near radiator, pink coolant in overflow tank, transmission slipping or delayed engagement, milky transmission fluid

Fix: Steel cooler lines rust through at bends or the internal radiator cooler leaks, cross-contaminating ATF and coolant. Both fluids must be flushed, lines and radiator often replaced together. If cross-contamination occurred, transmission requires full rebuild. Line replacement alone: 2-3 hours; with radiator and flush: 4-6 hours; transmission rebuild adds 12-18 hours.

Estimated cost: $400-800 (lines only), $3,500-5,500 (if trans damaged)

Typical onset: 100,000-200,000 mi

Symptoms: clunk when shifting into gear, vibration at highway speed, driveline shudder under load, visible sagging of transmission tail

Fix: Rubber transmission mount separates or crossmember cracks from towing stress and road salt. Requires lifting transmission to replace mount and inspect crossmember for cracks requiring welding or replacement. 2-4 hours.

Estimated cost: $250-600

Symptoms: rough running, white or black smoke, injector knock, fuel in crankcase, hard starting

Fix: Water in fuel or use of biodiesel not rated for older systems causes injector failures and fuel pump issues. Full system flush, all filters replaced, injectors cleaned or replaced, and lift pump checked. If fuel diluted oil, oil change mandatory. 4-8 hours depending on injector work.

Estimated cost: $800-2,200

Typical onset: 180,000-250,000 mi

Symptoms: blue smoke on startup, excessive crankcase pressure, oil consumption 1 qt per 500-1000 miles, fouled spark plugs

Fix: Worn or stuck piston rings let combustion gases into crankcase and burn oil. Inspection often shows cylinder glazing or ridge wear. Full rebuild with bore honing, new rings, and often pistons if worn. 18-24 labor hours.

Estimated cost: $3,500-6,000