1994 GMC SIERRA 3500

Typical onset: 120,000-180,000 mi

Symptoms: No-start or stalling when warm (PMD failure on injection pump), Excessive blow-by and oil consumption, Knocking from bottom end, especially under load, Metal in oil, low oil pressure

Fix: PMD relocation is preventive ($300-500), but once bottom end goes it's rebuilt or replacement time. Pistons crack, main and rod bearings wear prematurely due to block flex and oiling issues. Full rebuild: 24-32 hours labor, includes machine work, gaskets, bearings, pistons.

Estimated cost: $4,500-7,500

Typical onset: 80,000-150,000 mi

Symptoms: Transmission fluid spraying under truck or puddling, Milky transmission fluid (coolant cross-contamination), Overheating transmission, slipping gears, Pink coolant in overflow tank

Fix: Steel cooler lines rust through at frame brackets. Internal radiator cooler fails, mixing coolant and ATF — requires radiator replacement, full trans flush, sometimes converter replacement. External lines: 1.5-2 hours. Internal leak with flush: 6-8 hours.

Estimated cost: $400-800 (lines only), $1,200-2,200 (radiator + trans service)

Typical onset: 60,000-120,000 mi

Symptoms: Clunk or bang when shifting into drive or reverse, Vibration at idle in gear, Visible sag of transmission tailshaft, Driveline vibration under acceleration

Fix: Rubber mount deteriorates, allowing excessive transmission movement. Common on heavy-duty models due to weight and stress. Replacement requires supporting transmission, 1.5-2 hours labor.

Estimated cost: $200-400

Typical onset: 150,000-200,000 mi

Symptoms: Coolant loss with no visible leak, White smoke from exhaust, Overheating, especially under load, Bubbles in coolant overflow, rough idle

Fix: Big-block gaskets fail between cylinders or into coolant jackets. Both heads typically done together. 14-18 hours labor, includes resurfacing heads if warped, new bolts, coolant system flush.

Estimated cost: $2,200-3,800

Symptoms: Hard starting, long crank times, Loss of power, surging under load, Black smoke, poor fuel economy, Fuel in oil (lift pump diaphragm failure)

Fix: Lift pump on driver frame rail fails, starving injection pump. Fuel filter housing o-rings leak air. Both are maintenance items but often neglected. Lift pump: 1-1.5 hours. Filter housing reseal: 0.5 hours. Injection pump if damaged: 6-8 hours.

Estimated cost: $250-500 (pump/filters), $1,800-2,800 (injection pump)

Typical onset: 140,000-200,000 mi

Symptoms: Low oil pressure at idle when warm, Deep knocking noise from bottom of engine, Metal shavings in oil, metallic glitter on dipstick, Sudden catastrophic failure if ignored

Fix: Weak crankshaft and insufficient main cap strength cause premature bearing wear. Requires full teardown, crank inspection/grinding, bearing replacement. Often part of full rebuild. Main bearings alone: 18-22 hours if crank is reusable.

Estimated cost: $3,500-5,500

Symptoms: Fuel gauge erratic or stuck, Fuel pump noise, loss of pressure, Rust flakes in fuel filter on gassers, Tank leaking at seams or filler neck

Fix: Steel tanks rust from inside out, especially in humid climates. Requires tank drop (2-3 hours). 6.5L has frame-mounted lift pump, easier access. 7.4L in-tank pump requires tank removal for replacement.

Estimated cost: $400-900 (pump), $600-1,200 (tank replacement)