The 1978 Grand Prix sits on GM's A-body platform with a variety of Pontiac, Chevy, and Olds engines. These are solid cruisers when maintained, but age-related failures in the TH350/TH200 transmissions, carburetor systems, and engine bottom-ends dominate the repair landscape after 40+ years.
TH200/TH350 Automatic Transmission Failure
Common · high severityTypical onset: 80,000-140,000 mi
Symptoms: Slipping on 1-2 or 2-3 upshift, especially when warm, No reverse or delayed engagement, Dark, burnt-smelling transmission fluid, Metal shavings in pan during service
Fix: Full rebuild with updated clutch packs, bands, and seals. TH200 (lighter-duty, found behind smaller V6/V8s) fails sooner than TH350. Expect 12-16 hours labor for R&R and rebuild. Often discovered when diagnosing slipping that started as minor.
Estimated cost: $1,800-3,200
Worn Main and Rod Bearings (All Engines)
Occasional · high severityTypical onset: 100,000-150,000 mi
Symptoms: Deep knocking or rumbling from bottom end, worse under load, Low oil pressure at idle when hot, Metallic debris in oil filter during changes, Sudden catastrophic failure if ignored
Fix: Requires engine teardown: oil pan, crankshaft removal, bearing replacement, crank polishing if journals are scored. Budget 20-28 hours for complete bottom-end rebuild if pistons/rings also need attention. Neglected oil changes and low-quality filters accelerate this on original engines.
Estimated cost: $2,500-4,500
Carburetor Issues (Rochester 2-bbl, 4-bbl Quadrajet)
Common · medium severitySymptoms: Hard starting when cold or hot-soak vapor lock, Rough idle, stalling at stop signs, Black smoke or fuel smell from exhaust, Flooding, gas puddling in intake
Fix: Ethanol fuel destroys old rubber parts, clogs jets. Full carb rebuild with modern ethanol-compatible kit plus float adjustment: 3-5 hours. Many shops prefer swapping to remanufactured unit. Choke pull-off and accelerator pump are frequent culprits.
Estimated cost: $400-800
Transmission Oil Cooler Line Leaks and Cooler Failure
Common · medium severityTypical onset: 60,000-120,000 mi
Symptoms: Red ATF dripping or pooling under front of car, Low transmission fluid with no visible external leak (cooler contaminating radiator), Transmission overheating, especially towing or summer driving, Milky or pink fluid in radiator (coolant/ATF mixing)
Fix: Steel lines rust through at frame rails or rubber sections crack. Replace lines and consider external cooler upgrade if original radiator-integrated cooler has failed. Lines alone: 2-3 hours. If coolant contamination occurred, trans rebuild often required. Flush radiator thoroughly.
Estimated cost: $250-600
Engine Mount Deterioration
Common · low severitySymptoms: Excessive engine movement visible when revving in Park, Clunking when shifting from Park to Drive or Reverse, Vibration felt through steering wheel or floor, Fan contacting shroud intermittently
Fix: Original rubber mounts collapse after decades. Replace both engine and transmission mounts as a set. Engine mounts: 2-3 hours. Trans mount another 1 hour. Poly replacements last longer but transmit more vibration.
Estimated cost: $350-600
Fuel System Rust and Clogging
Occasional · medium severitySymptoms: Sputtering, hesitation, or stalling after sitting, Fuel starvation under acceleration or long drives, Fuel smell near tank, Clogged fuel filter repeatedly
Fix: Steel tanks rust from inside if car sat with old fuel or water contamination. Requires tank removal, cleaning or replacement, new sending unit, fuel lines inspection, filter. Often combined with fuel pump replacement. 6-10 hours labor depending on line condition.
Estimated cost: $600-1,400
Buy one if the transmission shifts cleanly, the engine doesn't knock, and the frame is solid—but budget $2k-4k for deferred maintenance on any survivor; parts are cheap, but labor adds up quickly on 45-year-old drivetrains.
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.