The 1987 K5 Blazer is a solid, body-on-frame truck that will run forever if you feed it—but the TH700-R4 transmission is its Achilles' heel, and these small-block Chevy engines are reaching the age where bottom-end rebuilds are common due to wear and neglect.
TH700-R4 Transmission Failure (3-4 Clutch Pack Burnout)
Common · high severityTypical onset: 120,000-180,000 mi
Symptoms: No 3rd or 4th gear, or slipping/flaring on 2-3 or 3-4 upshift, Transmission overheating, burnt fluid smell, Check engine light with codes for incorrect gear ratio
Fix: Full transmission rebuild or replacement. The 700-R4 is notorious for 3-4 clutch pack failure, often traced to inadequate factory cooler or low line pressure. Expect 8-12 hours labor for R&R and rebuild, plus upgraded cooler installation (mandatory).
Estimated cost: $1,800-3,200
Engine Bottom-End Wear (Rod and Main Bearings)
Common · high severityTypical onset: 150,000-250,000 mi
Symptoms: Knocking or tapping noise from crankcase, especially cold start or under load, Low oil pressure at idle (under 10 psi hot), Metallic debris on oil drain plug magnet or in filter
Fix: Short block replacement or in-frame rebuild with new rod and main bearings, honed cylinders, new rings if bore is acceptable. These engines often lived hard lives towing or plowing. Plan 16-24 hours labor for full teardown, machine work, and reassembly.
Estimated cost: $3,500-6,500
Fuel Pump Failure (Mechanical or In-Tank Electric)
Common · high severityTypical onset: 80,000-140,000 mi
Symptoms: Hard starting, especially hot restart, Stalling or hesitation under acceleration, Fuel pressure below spec (carb models lose volume, TBI models below 9-13 psi)
Fix: TBI models have in-tank electric pump; requires dropping tank (2-3 hours). Carbureted models use mechanical pump on block (1 hour). Ethanol fuel accelerates diaphragm deterioration. Always replace fuel filter simultaneously.
Estimated cost: $250-600
Transmission Oil Cooler Line Corrosion and Leaks
Common · medium severitySymptoms: Transmission fluid spots under radiator area or along frame rail, Low transmission fluid level with no visible leak at pan, Rusty or weeping steel cooler lines at fittings or mid-span
Fix: Steel cooler lines rot from road salt and age. Replace with pre-bent or custom-bent lines, upgrade rubber hoses at same time. 2-3 hours labor. Critical to catch before transmission runs dry and destroys itself.
Estimated cost: $300-550
6.2L Detroit Diesel Injection Pump Failure
Occasional · high severityTypical onset: 100,000-180,000 mi
Symptoms: Hard starting, extended cranking, white smoke, Loss of power, rough idle, black smoke under load, Fuel in oil (dilution) from internal pump seal failure
Fix: Stanadyne DB2 injection pump wears out or loses timing. Rebuild or reman replacement required; 4-6 hours labor. These are getting scarce and expensive. Verify timing and lift pump function first before condemning injection pump.
Estimated cost: $1,200-2,500
NP208 Transfer Case Front Output Seal and Chain Wear
Occasional · medium severityTypical onset: 120,000-200,000 mi
Symptoms: Transfer case fluid leak at front output (toward transmission), Grinding or rattling noise in 4WD, especially during turns, Jumping out of 4WD under load
Fix: Front seal is common leak point; 2 hours to drop and reseal. Drive chain stretches and wears sprockets over time, especially if run low on fluid. Full rebuild with new chain, bearings, and seals is 6-8 hours.
Estimated cost: $350-1,400
Body Mount and Frame Rust (Northern Climates)
Common · medium severitySymptoms: Body sitting crooked or sagging on frame, Visible rust perforation on cab corners, door bottoms, or tailgate, Cracked or disintegrated rubber body mounts
Fix: Body mounts rot and compress, frame rails rust through behind cab or at rear spring hangers (salt belt). Body mount replacement is 4-6 hours; frame repair requires welding and fabrication. Inspect carefully before purchase.
Estimated cost: $400-2,000
Buy one if the frame is solid and the transmission has already been rebuilt with a good cooler—otherwise budget $5K in deferred maintenance on any high-mile example.
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.