1974 GMC K2500

Typical onset: 80,000-150,000 mi

Symptoms: Rough idle and loss of power, especially when warm, Metallic rattling from front of engine at startup, Backfiring through carburetor or intake, Check timing and it's retarded 8-12 degrees despite static setting being correct

Fix: Replace timing chain, gears, and tensioner. On small-blocks (307/350) this is 4-6 hours; big-blocks (396/402/454) run 5-7 hours due to accessory clearance. Must pull radiator and harmonic balancer. Original nylon-toothed cam gears are notorious for shedding teeth—upgrade to steel. I6 292 is similar labor but simpler access.

Estimated cost: $600-1,200

Typical onset: 60,000-120,000 mi

Symptoms: Gear oil dripping from front driveshaft area, Oil spray on transmission crossmember and exhaust, Gradual fluid loss in transfer case—check level drops between changes

Fix: Replace front output shaft seal and inspect bushing wear. The NP205 is tough but the seal gets hardened from heat cycling. Access requires dropping front driveshaft. Takes 1.5-2 hours if you're just doing the seal; add another hour if replacing the bushing. Fluid refill requires 4 quarts GL-5 90W.

Estimated cost: $180-350

Symptoms: Hesitation on acceleration, flat spots at part-throttle, Black smoke under load—running rich, Hard starting when hot, flooding condition, Fuel odor in cab or engine bay after sitting

Fix: Q-jet carburetors on these trucks develop worn throttle shafts causing vacuum leaks, accelerator pump failures, and stuck power valves. Full rebuild with kit takes 3-4 hours including removal/reinstall and adjustment. Alternatively, replace with remanufactured unit (1-2 hours swap time). Ethanol fuel accelerates deterioration of rubber parts.

Estimated cost: $350-800

Typical onset: 70,000-130,000 mi

Symptoms: Grinding or howling from front end, worse in turns, Wheel play detectable when rocking tire at 12-6 o'clock, Uneven tire wear on inside or outside edge, ABS-like pulsing feel through steering (wheel lockup from seized bearing)

Fix: Replace wheel bearings, seals, and inspect spindle for damage. Front 4WD hubs have both inner and outer bearings requiring proper preload adjustment. Dana 44: 2-3 hours per side including repacking. GM 10-bolt similar. Warn locking hubs often need rebuild or replacement at same time (add $150-250).

Estimated cost: $300-600 per side

Symptoms: Coolant leaks from radiator seams or tube ends, Engine runs too cold (160-170°F) or won't reach operating temp, Overheating under load or in traffic despite new thermostat, Rust flakes in coolant, visible core deterioration

Fix: Original brass/copper radiators are 50 years old and corroded internally. Replacement is 2-3 hours labor. Overcooling is common from stuck-open thermostats or missing shrouds—these trucks NEED the fan shroud for proper airflow. Many have been removed. Upgrading to aluminum 3-row is popular for big-block applications.

Estimated cost: $400-900

Symptoms: Soft or spongy brake pedal that slowly sinks to floor, Fluid level drops in master cylinder reservoir, Brake fluid on rear backing plates or inside front drums/rotors, Pull to one side during braking

Fix: Single-reservoir master cylinders (pre-dual-circuit) are dangerous when they fail. Rubber cups deteriorate with age regardless of mileage. Master replacement: 1.5-2 hours. Wheel cylinders: 1 hour per side rear, 1.5 hours per front if drums. Complete system overhaul (master + 4 wheel cylinders + flush) runs 5-6 hours. DO NOT DELAY THIS REPAIR.

Estimated cost: $350-750

Typical onset: 90,000-150,000 mi

Symptoms: Transmission won't downshift for passing power, Early or late shift points, harsh 1-2 shift, Transmission fluid in vacuum line to modulator, Slipping under acceleration after initially moving fine

Fix: Vacuum modulator diaphragm failure allows trans fluid into intake manifold (causes blue smoke) and ruins shift quality. Modulator replacement is 0.5 hours and cheap. If caught early, trans survives. Ignored, leads to internal clutch damage requiring rebuild (8-12 hours labor, $1,200-2,000 total). Also check detent cable adjustment.

Estimated cost: $80-150 (modulator only)

Symptoms: Visible rust perforation on frame rails behind cab, Body mounts collapsed—gap between bed and cab closes, Cab or bed sits crooked or lower on one side, Creaking and popping noises over bumps from loose body mounts

Fix: Snow-belt trucks have serious frame rust issues, especially at rear spring mounts and outriggers. Body mounts are rubber and rot out. Replacing all 6-8 mounts requires lifting cab/bed: 6-8 hours labor. Frame welding/plating for structural repair is 10-20 hours depending on severity and may not be economical. Inspect BEFORE purchase—this can total the truck.

Estimated cost: $800-1,500 (mounts only); frame repair $2,000-5,000+