1991 GMC SIERRA 2500

Typical onset: 120,000-180,000 mi

Symptoms: Heavy knocking on cold start that quiets when warm, Low oil pressure at idle, Metallic rattling under load, Oil consumption increasing rapidly

Fix: Main and rod bearings fail due to marginal oiling design in early 6.5L blocks. Requires full engine-out rebuild with bearing replacement, crank inspection/machining, and often piston replacement if cylinder wall scoring present. 18-24 labor hours for complete teardown and rebuild. Many shops recommend short block replacement instead due to labor overlap.

Estimated cost: $3,500-6,500

Typical onset: 100,000-150,000 mi

Symptoms: White smoke from exhaust on startup, Coolant consumption with no external leaks, Rough idle and misfire codes, Overheating under load, Milky oil on dipstick in advanced cases

Fix: Big-block 454 head gaskets fail from thermal cycling and casting porosity issues common to this era. Both heads typically need work simultaneously. Head removal, milling, new gaskets, ARP studs recommended. 14-18 labor hours. Engine-out not required but makes access significantly easier — add 6 hours if going that route.

Estimated cost: $2,200-3,800

Typical onset: 90,000-140,000 mi

Symptoms: Transmission fluid in radiator (strawberry milkshake in overflow), Hard shifts or slipping after coolant contamination, External leaks at cooler line fittings at radiator, Transmission overheating towing or in summer heat

Fix: Factory transmission cooler integrated into radiator fails internally, mixing coolant and ATF. Requires radiator replacement, full transmission fluid flush (often multiple flushes), filter service, and external auxiliary cooler installation to prevent repeat failure. If contamination went unnoticed, internal transmission damage requires rebuild. 4-6 hours for radiator and flushing; add 12-16 hours if rebuild needed.

Estimated cost: $800-1,200 (cooler/flush only), $2,500-3,800 (if rebuild required)

Typical onset: 80,000-150,000 mi

Symptoms: Hard starting, especially when hot, Stalling after warmup, Fuel gauge erratic or reading empty, Loss of power under acceleration, Long crank times

Fix: Mechanical lift pump on 6.5L diesel fails from worn diaphragm, causing low fuel pressure and air intrusion. In-tank sending unit also fails. Requires dropping fuel tank (heavy, labor-intensive on 2500), replacing lift pump and sending unit. 3-5 labor hours depending on rust and fuel load in tank.

Estimated cost: $600-1,100

Typical onset: 150,000-200,000 mi

Symptoms: Black smoke under acceleration, Severe loss of power, Engine won't start or starts then dies immediately, Rough idle with visible misfiring, Fuel in crankcase oil

Fix: Stanadyne DB2 injection pump on 6.5L suffers from worn internal components and seal failures. Pump replacement requires removal, timing procedure, and often injector service while apart. Core charge applies if remanufactured unit used. 6-9 labor hours including timing and bleeding system.

Estimated cost: $1,800-3,200

Symptoms: Rough idle and stalling, Hesitation on acceleration, High idle stuck at 1500+ RPM, Black smoke from tailpipe, Poor fuel economy suddenly worsening

Fix: TBI injectors clog or fail, throttle position sensor drifts, and idle air control valve sticks. These trucks still use batch-fire TBI — relatively simple but parts are aging out. Injector cleaning/replacement, TPS and IAC replacement typical. 2-4 labor hours for complete TBI service and sensor replacement.

Estimated cost: $400-850

Typical onset: 60,000-100,000 mi

Symptoms: Clunk when shifting from park to drive, Vibration at highway speed, Driveline shudder on acceleration, Visible sag of transmission tailhousing

Fix: Rubber transmission mount deteriorates from heat and age. Simple replacement requiring transmission support and crossmember removal. Often done alongside other transmission work. 1.5-2.5 labor hours.

Estimated cost: $200-400