The 2004 Explorer is a body-on-frame SUV built on the UN152 platform. The 4.0L SOHC V6 is plagued by timing chain cassette failures and head gasket issues, while both engines share transmission cooler and 5R55 automatic weaknesses that can grenade the unit if ignored.
4.0L SOHC Timing Chain Cassette Failure
Common · high severityTypical onset: 80,000-150,000 mi
Symptoms: rattling on cold start that quiets after warmup, check engine light with camshaft position codes, rough idle and loss of power, catastrophic engine damage if chain jumps timing
Fix: Requires front cover removal to replace both cassettes, guides, tensioners, and chains. Book time is 8-10 hours; most shops replace water pump and front seals while in there. Engine damage from jumped timing means rebuild or replacement.
Estimated cost: $1,800-2,800 for preventive replacement; $4,000-6,500 for engine rebuild if damage occurs
Transmission Oil Cooler Line Failure Inside Radiator
Common · high severityTypical onset: 60,000-120,000 mi
Symptoms: transmission fluid in coolant (strawberry milkshake appearance), coolant in transmission fluid, slipping or no movement after radiator leak, overheating transmission
Fix: Internal cooler in radiator corrodes and allows fluid mixing. Requires radiator replacement, full trans flush, often filter and fluid. If coolant contaminated the trans, rebuild or replacement needed. 3-4 hours for radiator and flush; add 12-16 hours if trans rebuild required.
Estimated cost: $800-1,200 for radiator and flush only; $2,500-4,000 if transmission rebuild needed
5R55W/S Transmission Intermediate Servo Bore Wear
Common · medium severityTypical onset: 90,000-140,000 mi
Symptoms: harsh 2-3 shift or slipping on 2-3 upshift, flare or delayed engagement into 3rd gear, transmission slipping under load, burnt fluid smell
Fix: Aluminum bore wears where servo piston rides, causing pressure loss. Requires transmission removal and case machining or sleeve kit installation. Quality rebuild includes all clutches and bands. 8-12 hours labor.
Estimated cost: $2,200-3,500
4.0L SOHC Head Gasket Failure
Occasional · high severityTypical onset: 100,000-180,000 mi
Symptoms: white smoke from exhaust, coolant loss with no visible leaks, overheating, rough idle and misfires, oil in coolant or vice versa
Fix: SOHC heads are prone to warping and gasket failure between cylinders or into coolant passages. Requires both heads removed, machined, new gaskets, timing components. While apart, replace chains and guides. 14-18 hours labor.
Estimated cost: $3,200-5,000
Front Lower Control Arm Bushing Deterioration
Common · low severityTypical onset: 70,000-110,000 mi
Symptoms: clunking over bumps from front end, steering wander and poor return to center, uneven tire wear on inside edges, vibration during braking
Fix: Rubber bushings crack and separate from control arms. Replace both lower control arms with bushings pressed in (most shops replace the whole arm vs. pressing bushings). 2-3 hours labor plus alignment.
Estimated cost: $500-800
Rear Axle Pinion Seal Leak
Occasional · low severityTypical onset: 80,000-130,000 mi
Symptoms: gear oil drips or pooling under rear differential, whining noise from rear axle when low on fluid, visible oil on rear diff cover and around pinion yoke
Fix: Pinion seal hardens and leaks. Requires driveshaft removal, pinion nut torque measurement for preload, seal replacement, and fluid refill. Critical to maintain proper preload. 2-3 hours labor.
Estimated cost: $350-550
4.6L V8 Spark Plug Ejection (Coil-On-Plug)
Occasional · medium severityTypical onset: 60,000-100,000 mi
Symptoms: sudden misfire and check engine light, hissing sound from engine bay, loss of power, plug found sitting on top of valve cover or dangling by wire
Fix: Aluminum head threads strip and spark plug blows out under combustion pressure. Requires helicoil or time-sert thread repair kit. Single cylinder is 2-3 hours; multiple failures common if first one ignored. Some shops remove head for proper repair.
Estimated cost: $400-700 per cylinder with time-sert; $1,500-2,500 if head removal required
Avoid the 4.0L V6 unless chains and cooler lines are documented as replaced; the 4.6L V8 is more reliable but prices reflect it—budget $2k/year for deferred maintenance on any high-mileage example.
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.