1996 GMC SIERRA 2500

Typical onset: 150,000-200,000 mi

Symptoms: Overheating with no apparent cause, Coolant in oil or oil in coolant, Loss of power, hard starting, or sudden no-start condition, White smoke from exhaust, Engine stalling when hot, restarts when cool (PMD issue)

Fix: The 6.5L is notorious for cracked cylinder blocks between cylinders (non-repairable) and failed Pump Mounted Driver (PMD) modules. PMD relocation is preventive and takes 2-3 hours. Block cracks typically require complete engine replacement or long block swap (15-20 hours labor). Many owners pre-emptively relocate the PMD and install a high-flow water pump, but block cracks are a ticking time bomb.

Estimated cost: $6,500-12,000

Typical onset: 120,000-180,000 mi

Symptoms: Transmission fluid leaking near radiator, Transmission overheating, slipping, or delayed engagement, Milky or pink fluid in radiator (coolant and ATF mixing), Sudden transmission failure after towing or heavy load

Fix: The factory cooler inside the radiator fails, allowing coolant to mix with ATF and destroying the 4L80E transmission. Steel lines rot out at frame crossmember mounts. Proper fix requires new radiator, external auxiliary cooler, and flushing lines (5-7 hours). If contamination occurred, transmission rebuild adds 12-16 hours. Always install an external cooler and bypass the internal one as preventive maintenance.

Estimated cost: $800-1,200 (lines/cooler only), $3,200-5,500 (with transmission rebuild)

Typical onset: 150,000-220,000 mi

Symptoms: Harsh shifting or delayed 1-2 upshift, Transmission clunk when shifting into drive or reverse, Excessive driveline vibration, Fluid leaks from tail housing or pan

Fix: The 4L80E is robust but wears normally with heavy use. Crossmember-mounted transmission mount fails, causing harsh shifts and driveline angles to change. Mount replacement is 1.5-2 hours. Internal wear (pump, clutches, bands) requires rebuild at 12-16 hours. These trucks were worked hard; many need rebuilds by 200k if not serviced religiously every 30k miles.

Estimated cost: $150-300 (mount only), $2,800-4,200 (rebuild)

Typical onset: 140,000-200,000 mi

Symptoms: White smoke from exhaust on startup, Coolant loss with no visible leak, Rough idle, misfire codes on cylinders, External coolant seepage at intake valley or head edges, Oil contamination in coolant reservoir

Fix: The 7.4L Vortec develops intake manifold gasket leaks (coolant into valley) and occasional head gasket failures, though less common than small-block Chevys. Both heads off requires 14-18 hours; intake gaskets alone are 6-8 hours. While you're in there, replace water pump, thermostat, and hoses. This engine is otherwise durable but expect valve guide wear and oil consumption above 200k.

Estimated cost: $1,800-2,800 (intake gaskets), $3,200-4,800 (head gaskets both sides)

Typical onset: 100,000-160,000 mi

Symptoms: Diesel: hard starting, low power, black smoke, fuel in oil (IP failure), Diesel: loss of prime, air in system, stalling, Gas: fuel pump whine, stalling, no-start, gauge reading empty when tank is full

Fix: The 6.5L diesel has a chassis-mounted lift pump that fails and an expensive injection pump prone to wear. Lift pump is 1.5 hours; IP replacement is 6-8 hours and requires careful timing. Gas 7.4L uses in-tank pump; sending unit failures are common (tank drop, 2.5-3 hours). Diesel fuel filter housings crack and leak air. Budget for IP replacement on any high-mile diesel.

Estimated cost: $400-700 (lift pump/sending unit), $1,800-2,800 (diesel injection pump)

Typical onset: 120,000-180,000 mi

Symptoms: Clunking from front end on acceleration or deceleration, Vibration at highway speed, Grinding or howling noise from front axle, Leaking diff cover or axle seals

Fix: U-joints wear and fail, especially on 4WD models used for towing. Driveshaft U-joint replacement is 2-3 hours for both shafts. Front differential carrier bearings wear in the 10.5-inch 14-bolt (2500HD), causing noise; rebuild requires 6-8 hours. Axle seals leak; common but not urgent unless hubs are damaged. These trucks were built to work — expect wear items.

Estimated cost: $400-700 (U-joints), $1,200-2,200 (front diff rebuild)