The 2004 4Runner is split between two drivetrains: the bulletproof 4.7L V8 (2UZ-FE) that runs forever, and the troublesome 4.0L V6 (1GR-FE) known for catastrophic head gasket failures. Your experience hinges entirely on which engine you have.
V6 Head Gasket Failure (4.0L 1GR-FE only)
Common · high severityTypical onset: 90,000-150,000 mi
Symptoms: Milky oil or coolant, White smoke from exhaust, Overheating with no external leaks, Rough idle and misfires, Coolant loss with no visible puddles
Fix: Both head gaskets typically fail due to inadequate torque spec from factory. Requires cylinder head removal, resurfacing, new gaskets, and updated torque procedure. Often find warped heads needing machine work. 16-20 hours labor. Many shops replace both heads preventively once one side fails.
Estimated cost: $3,500-5,500
Secondary Air Injection (AIR) System Failure
Common · low severityTypical onset: 80,000-130,000 mi
Symptoms: P0441, P0446, or P0410-series codes, Check engine light on cold starts, Rough idle for 30-60 seconds after cold start, Failed emissions testing
Fix: Air pump, one-way valves, and vacuum switching valve all deteriorate. Pump is noisy when failing. System is emissions-only—won't strand you but won't pass inspection. Pump replacement 2-3 hours, full system overhaul 4-6 hours if lines are corroded.
Estimated cost: $800-1,800
Transmission Oil Cooler Line Rust & Leaks
Common · medium severitySymptoms: ATF puddles under front of vehicle, Low transmission fluid warnings, Pink fluid dripping near radiator, Transmission slipping or delayed engagement
Fix: Steel cooler lines rust through where they run along frame, especially in salt states. Can also see internal radiator cooler failure mixing ATF and coolant ("strawberry milkshake of death"). External line replacement 2-3 hours. If cooler contaminated transmission, you're looking at flush or rebuild.
Estimated cost: $400-700 (lines only); $2,500-4,000 (if transmission damaged)
Lower Ball Joints Wear
Common · medium severityTypical onset: 90,000-140,000 mi
Symptoms: Clunking over bumps, Steering wander or looseness, Uneven tire wear on inside edges, Fails play test during alignment
Fix: OEM ball joints are not serviceable separately—requires entire lower control arm replacement per side. Aftermarket allows press-in joint replacement. Most shops do arms in pairs for alignment reasons. 3-4 hours labor for both sides with alignment.
Estimated cost: $800-1,400
Rear Differential Pinion Seal Leak
Occasional · low severityTypical onset: 100,000-180,000 mi
Symptoms: Gear oil dripping from front of rear diff, Oil coating underside of differential, Low rear diff fluid on check
Fix: Pinion seal hardens and leaks. Slow leak usually, not urgent but will damage bearings if ignored long-term. Requires driveshaft removal and pinion nut torque/preload reset. 2-3 hours labor. Good time to service diff fluid.
Estimated cost: $400-650
Frame Rust (Northern/Salt Belt Vehicles)
Occasional · high severitySymptoms: Visible surface rust turning to flaking/scaling, Holes developing in frame rails near rear axle mounts, Failing state safety inspections, Frame flex or cracking noises
Fix: Not a specific failed part but a platform-killer in rust states. Rear frame sections and crossmembers rot out. Toyota had frame replacement programs on Tacoma/Tundra but not 4Runner. Inspect carefully before purchase—structural frame damage is not economically repairable.
Estimated cost: Vehicle total loss in severe cases
Throttle Body Carbon Buildup & Idle Issues
Occasional · low severityTypical onset: 70,000-120,000 mi
Symptoms: Rough or fluctuating idle, Stalling when coming to stop, Hesitation on acceleration, P0505 idle control codes
Fix: Electronic throttle body accumulates carbon, sticking the valve. Cleaning takes 1-1.5 hours, requires throttle relearn procedure. Often accompanied by dirty MAF sensor. Preventable with regular intake system cleaning.
Estimated cost: $150-300
Buy the V8 model and it'll run 300k+ with basic maintenance; the V6 is a ticking time bomb that will need $4k in head work—budget accordingly or walk away.
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.