The 1984 Pontiac 6000 is an early A-body front-driver that shows GM's teething pains with transverse powertrains. The 2.8L V6 paired with the THM125C 3-speed automatic is the common culprit—expect transmission and intake manifold gasket headaches if you're shopping high-mileage examples.
THM125C Automatic Transmission Failure
Common · high severityTypical onset: 80,000-120,000 mi
Symptoms: Slipping between gears, especially 1-2 shift, Delayed engagement when shifting into drive, Transmission fluid dark or burnt smell, Complete loss of forward gears
Fix: The 3-speed THM125C is notoriously weak, especially behind the V6. Clutch packs wear, governor seals leak, and the final drive fails. Rebuild runs 12-16 hours; used units are gambles. Cooler lines and mounts often need replacement simultaneously.
Estimated cost: $1,800-2,800
2.8L V6 Intake Manifold Gasket Leaks
Common · medium severityTypical onset: 70,000-110,000 mi
Symptoms: Coolant smell in cabin or under hood, White exhaust smoke on cold start, Coolant loss with no visible external leaks, Rough idle or stumbling when warm
Fix: The composite gaskets deteriorate and allow coolant into the intake plenum and oil passages. If caught early, it's a gasket job (6-8 hours). Neglect it and you'll hydro-lock cylinders or trash bearings, leading to engine rebuild territory.
Estimated cost: $600-1,100
2.5L Iron Duke Piston Ring / Cylinder Wear
Occasional · high severityTypical onset: 100,000-140,000 mi
Symptoms: Heavy blue smoke on startup and acceleration, High oil consumption—quart every 500-800 miles, Loss of compression and power, Fouled spark plugs
Fix: The Tech IV 2.5L wears rings and scores cylinders, especially if oil changes were skipped. A proper fix is new rings, hone, and pistons—essentially a rebuild. Short-block swaps are common but labor-intensive at 18-22 hours for engine removal and reinstall.
Estimated cost: $2,200-3,500
Transmission and Engine Mount Deterioration
Common · medium severityTypical onset: 60,000-100,000 mi
Symptoms: Clunking when shifting into gear, Excessive engine movement visible from driver's seat, Vibration at idle that smooths out at speed, Difficulty shifting manual transmission (if equipped)
Fix: Hydraulic and rubber mounts collapse from age and fluid contamination. Front mount is most critical—failure lets the engine torque hard into the firewall. Replace all three mounts as a set; 3-4 hours labor with the car on a lift.
Estimated cost: $350-650
Fuel System Varnish and Clogging
Occasional · medium severitySymptoms: Hard starting after sitting, Stumbling or hesitation under load, Stalling at idle, Check engine light with lean codes (if equipped with early feedback carb)
Fix: Carbureted models (most '84s) suffer varnish buildup in idle circuits and accelerator pumps if the car sat. TBI models clog injectors and filters. Filter is 0.5 hours; full carb rebuild or injector service adds 2-4 hours depending on system.
Estimated cost: $150-550
Front Cradle and Subframe Rust
Occasional · high severitySymptoms: Visible rust perforation on front crossmember, Clunking or flexing feeling over bumps, Control arm or steering rack mounting points cracked, Alignment issues that won't stay corrected
Fix: Salt-belt cars rot the front subframe where it cradles the engine and transmission. Welding patches is temporary; proper repair requires subframe replacement—a 10-14 hour ordeal with full drivetrain support. Many cars are totaled at this point.
Estimated cost: $1,500-2,800
Buy only if rust-free, low-mileage, and you have a transmission fund—these are interesting period pieces but not reliable daily drivers without preemptive work.
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.