The 1978 K5 Blazer is a tank-simple truck built on the proven GM square-body platform, but suffers from TH350/400 transmission failures, weak 14-bolt front axles under abuse, and significant block/bottom-end wear when neglected — especially the small-journal 400 V8.
TH350/TH400 Automatic Transmission Failure
Common · high severityTypical onset: 90,000-140,000 mi
Symptoms: Delayed or harsh 1-2 shift, slipping in 2nd gear, No reverse or weak reverse engagement, Burnt fluid smell, metal debris in pan, Overheating after towing or highway use
Fix: Full rebuild with clutch pack, band, and torque converter replacement. External cooler often added to prevent repeat failure. 12-16 labor hours for removal, rebuild, and reinstall.
Estimated cost: $1,800-3,200
Small-Block Bottom-End Wear (Especially 400 V8)
Common · high severityTypical onset: 120,000-180,000 mi
Symptoms: Heavy knocking on cold start that quiets when warm, Oil pressure drops below 10 psi at hot idle, Metal shavings in oil, visible during filter cuts, Blue smoke under acceleration
Fix: The 400 V8 has undersized main journals prone to early wear. Requires short-block replacement or full rebuild with crank turning, bearing replacement, and piston/ring work. 20-28 labor hours for R&R and machine work.
Estimated cost: $3,500-5,800
Frame Rust and Body Mount Rot
Common · high severitySymptoms: Body sags or tilts at corners when parked, Popping/creaking over bumps from cab area, Visible rust-through on frame behind cab mounts, Doors won't close properly, alignment shifts
Fix: Body mounts rot from road salt and moisture. Frame can rust through at mount points and rear spring hangers. Body-off inspection often reveals hidden damage. 8-12 hours for mount replacement if frame is solid; frame plating or replacement adds 20+ hours.
Estimated cost: $800-1,400 (mounts only); $4,000+ if frame repair needed
NP205 Transfer Case Front Output Seal Leak
Occasional · medium severityTypical onset: 80,000-120,000 mi
Symptoms: Gear oil dripping from front of transfer case, Oil on front driveshaft and skid plate, Low fluid level causes whining in 4WD, Leak worsens after hard off-road use
Fix: Front output seal wears from driveshaft yoke movement. Requires draining case, removing front driveshaft, and seal replacement. Often done with fluid change. 2-3 labor hours.
Estimated cost: $250-450
Carburetor Rochester Quadrajet Float and Choke Issues
Common · low severitySymptoms: Hard starting when cold, floods when hot, Black smoke and 8-10 mpg fuel economy, Stalling at idle when warm, Gas smell from engine bay, leaking carb base
Fix: Q-Jet is solid when healthy but float needles wear and choke pull-offs fail. Full rebuild kit with float, needle, accelerator pump, and choke service. 3-4 hours for removal, rebuild, reinstall, and tuning.
Estimated cost: $400-700
Steering Box Wander and Excessive Play
Occasional · medium severityTypical onset: 100,000-150,000 mi
Symptoms: More than 2 inches of wheel play before response, Constant steering correction on highway, Clunking when turning lock-to-lock, Fluid leak at sector shaft seal
Fix: Saginaw steering box wears internally; sector shaft adjustment only a temporary fix. Replacement with rebuilt unit is long-term solution. Alignment required after. 3-4 labor hours.
Estimated cost: $500-850
Fuel Tank Sending Unit Failure
Occasional · low severitySymptoms: Fuel gauge pegged empty or full regardless of level, Erratic gauge movement over bumps, Gauge works intermittently
Fix: Tank-mounted sending unit corrodes or float arm breaks. Requires tank drop, sender replacement, and testing. 2.5-3.5 hours depending on rust and access.
Estimated cost: $300-500
Buy it if the frame is solid and the trans shifts clean; avoid small-journal 400 V8s unless freshly rebuilt, and budget $2k-4k for deferred maintenance on any sub-$8k truck.
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.