The 1987 C20 is a heavy-duty square-body workhorse known for longevity if maintained, but transmission mounts fail early, fuel systems need attention on older units, and high-mileage engines — especially the 454 and 6.2L diesel — often need lower-end work or rebuilds before 200K.
Transmission Mount Failure
Common · medium severityTypical onset: 60,000-100,000 mi
Symptoms: Clunk when shifting between drive and reverse, Vibration at idle in gear, Visible sagging or torn rubber on crossmember mount
Fix: Replace transmission mount and inspect crossmember for cracks. Typically 1.5-2 hours labor with basic hand tools and a transmission jack or floor jack.
Estimated cost: $150-300
TH400/SM465 Transmission Cooler Line Leaks
Common · medium severityTypical onset: 80,000-150,000 mi
Symptoms: Transmission fluid puddles under front of truck, Low fluid level on dipstick, Burnt transmission smell if run low
Fix: Steel cooler lines rust through at bends or fittings. Replace lines and flush cooler if contaminated. 2-3 hours labor including removal of skid plates if equipped.
Estimated cost: $250-500
Small-Block Chevy (350/400) Lower End Wear
Occasional · high severityTypical onset: 150,000-200,000 mi
Symptoms: Heavy rod knock at idle, Low oil pressure below 10 psi hot, Metal shavings in oil filter, Blue smoke on deceleration
Fix: Main and rod bearings wear, sometimes requiring crankshaft machining. Full rebuild typical: remove engine, disassemble, machine crank and block, new bearings, rings, gaskets. 20-30 hours labor for complete R&R and rebuild.
Estimated cost: $2,500-4,500
454 Big-Block Piston Ring Failure and Oil Consumption
Common · medium severityTypical onset: 100,000-150,000 mi
Symptoms: Heavy blue smoke on startup and acceleration, Oil consumption 1+ quart per 500 miles, Fouled spark plugs, Loss of compression in one or more cylinders
Fix: Rings lose tension and cylinders glaze, especially if idled extensively. Short-block replacement or in-chassis ring job. If cylinders are tapered beyond spec, needs bore and oversize pistons. 18-25 hours for engine-out rebuild.
Estimated cost: $3,000-5,500
6.2L Detroit Diesel Injection Pump Failure
Occasional · high severityTypical onset: 120,000-180,000 mi
Symptoms: Hard starting when warm, Rough idle and black smoke, Loss of power under load, Fuel in oil (indicates failed pump seals)
Fix: Stanadyne DB2 pump internal wear or electronics failure. Replace or rebuild pump, bleed fuel system. 6-8 hours labor. Pump cores expensive; used or reman units vary widely.
Estimated cost: $1,200-2,200
Fuel Tank Sending Unit and Pickup Tube Corrosion
Common · low severitySymptoms: Fuel gauge reads empty or erratic, Engine stalls or surges with partial tank, Won't draw fuel below half tank
Fix: Steel tanks rust internally, pickup screens clog, sending unit floats corrode. Drop tank, replace sending unit and filter sock. 2-3 hours labor depending on frame-mounted vs. behind-axle location.
Estimated cost: $300-600
Frame Rust and Body Mount Deterioration
Common · medium severitySymptoms: Cab shifting or creaking over bumps, Visible rust perforation on frame rails behind cab, Doors misaligned or hard to close, Cracked body mounts visible from underneath
Fix: Frame rails rust at cab mounts, fuel tank straps, and rear spring hangers in salt states. Body mounts compress and crack. Replace mounts: 4-6 hours. Frame repair requires welding, plating, or section replacement: 8-20 hours depending on extent.
Estimated cost: $400-2,500
Buy one if the frame is solid and you can verify maintenance history; budget $2K-4K for deferred engine or transmission work on high-mileage examples, but a well-kept C20 will outlast modern trucks.
AI-assisted summary drawn from NHTSA recall data, our labor-times database, and platform knowledge. Not a substitute for a pre-purchase inspection on a specific vehicle.