The 1977 Blazer is a solid square-body truck with robust drivetrain bones, but they're getting old — expect transmission cooling issues, tired TH350/400 automatics, and smallblock rebuilds as these 40+ year-old workhorses cross 150k+ miles.
TH350/TH400 Transmission Cooler Line & Radiator Tank Failure
Common · high severityTypical onset: any mileage — age and heat cycles matter more
Symptoms: Pink or milky transmission fluid — coolant contamination, Transmission overheating on highway drives, Sudden loss of forward gears after coolant mixing
Fix: Replace radiator with integral cooler or add external cooler, flush transmission completely, often requires trans rebuild if coolant got inside — 6-10 hours total if trans needs R&R and internal service
Estimated cost: $800-2,200
Transmission Rebuild — Worn Clutches and Seals
Common · medium severityTypical onset: 120,000-180,000 mi
Symptoms: Slipping on 1-2 or 2-3 shift under load, Delayed engagement into Drive or Reverse, Burnt ATF smell, dark or metallic fluid
Fix: Full rebuild with clutch packs, bands, seals, filter, and fluid — 8-12 hours labor for R&R and bench rebuild
Estimated cost: $1,800-3,200
350/400/454 Bottom End Wear — Bearings and Ring Seal
Occasional · high severityTypical onset: 150,000-220,000 mi
Symptoms: Low oil pressure at idle when hot — under 10 psi, Knocking or ticking from crankcase, worsens with RPM, Excessive blowby, oil consumption over 1 qt per 500 miles
Fix: Short block replacement or full rebuild with crank polish, new bearings, piston rings, gaskets — 16-24 hours depending on condition and accessibility
Estimated cost: $3,500-6,500
Fuel System Varnish and Carburetor Gumming
Common · low severitySymptoms: Hard starting after sitting overnight or longer, Rough idle, stumbling on acceleration, Fuel smell from stuck float or leaking accelerator pump
Fix: Carburetor rebuild kit, fuel filter, inspect and clean fuel lines and tank if sitting — 2-4 hours labor for carb R&R and rebuild
Estimated cost: $350-650
Sagging or Broken Transmission Mount
Common · low severityTypical onset: 80,000-140,000 mi
Symptoms: Clunk when shifting from Park to Drive or Reverse, Vibration through floor at idle in gear, Visible droop of transmission tailhousing
Fix: Replace transmission crossmember mount — 1-1.5 hours with basic hand tools
Estimated cost: $150-280
Frame Rust and Body Mount Rot
Occasional · high severitySymptoms: Body visibly sagging or misaligned doors, Rust holes through frame rails near rear spring hangers or crossmember, Collapsed or crumbling rubber body bushings
Fix: Body mount replacement takes 4-6 hours; frame rust repair requires welding, plates, or section replacement — depends on severity, can be 12+ hours
Estimated cost: $600-3,500
Buy it if the frame is solid and the transmission shifts clean — budget $2k-4k for deferred drivetrain work on anything over 120k miles, but these are still fixable 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.