The 1999 Navigator shares the heavy-duty Ford Expedition platform with the 5.4L Triton V8 and 4R100 transmission. While capable when maintained, this first-generation model suffers from catastrophic spark plug ejection issues, transmission cooler line failures, and intake manifold coolant leaks that can escalate quickly if ignored.
Spark Plug Ejection from Cylinder Head
Common · high severityTypical onset: 60,000-150,000 mi
Symptoms: Loud popping or hissing sound from engine bay under load, Sudden misfire with Check Engine light, Complete loss of compression in one cylinder, Metal debris visible on spark plug if inspected
Fix: Early 2-valve 5.4L engines used shallow spark plug threads that strip under heat cycling. Requires time-sert or helicoil thread repair, often cylinder head removal for proper repair. Budget 6-10 hours labor depending on which cylinder and whether head comes off. Some shops attempt in-situ repairs but longevity is questionable.
Estimated cost: $1,200-2,800
4R100 Transmission Oil Cooler Line Failure
Common · high severityTypical onset: 80,000-140,000 mi
Symptoms: Transmission fluid puddle under front of vehicle, Rapid loss of ATF causing slipping or no movement, Pink or red fluid mixing with coolant in overflow tank, Transmission overheating warnings if caught early
Fix: Steel cooler lines rust through at crimped fittings or where they pass over crossmember. Line rupture dumps 12+ quarts fast. Replace both pressure and return lines preventively, flush system, refill. If coolant contaminated ATF, internal transmission damage likely requires rebuild. 3-4 hours for lines only, add 15-20 hours if transmission rebuild needed due to coolant ingestion.
Estimated cost: $400-800 (lines only), $2,500-4,000 (with rebuild)
Intake Manifold Coolant Cross-Over Leak
Common · medium severityTypical onset: 70,000-130,000 mi
Symptoms: Coolant smell from engine bay, especially when hot, Slow coolant loss with no visible external leaks, White smoke from exhaust on cold start, Rough idle or misfire if coolant enters cylinders
Fix: Plastic coolant crossover passages in the intake manifold crack or gaskets fail. Requires intake manifold removal, replace with updated aluminum crossover kit and new gaskets. While you're in there, replace thermostat and hoses. 5-7 hours labor. Delay risks hydrolocking a cylinder if enough coolant pools.
Estimated cost: $800-1,400
Front Hub Bearing and ABS Sensor Failure
Common · medium severityTypical onset: 90,000-150,000 mi
Symptoms: Grinding or humming noise that increases with speed, ABS and/or traction control warning lights, Erratic speedometer behavior, Wheel play detectable when checking for looseness
Fix: Integrated hub/bearing assemblies fail from heavy vehicle weight (5,800+ lbs). ABS tone ring corrodes or sensor fails simultaneously. Replace hub assembly as complete unit with sensor. 2-3 hours per side. Do both fronts if one fails over 120K miles. Four-wheel drive adds complexity accessing upper ball joint area.
Estimated cost: $400-700 per side
DPFE Sensor (EGR System) Failure
Occasional · low severityTypical onset: 60,000-120,000 mi
Symptoms: Check Engine light with P0401 or P1401 codes, Rough idle when warm, Slight hesitation on acceleration, Failed emissions test
Fix: Delta Pressure Feedback EGR sensor fails from heat exposure and clogged passages. Located on firewall side of engine. Clean EGR passages, replace DPFE sensor and inspect EGR valve. 1.5-2 hours. Common enough Ford issued TSBs. Won't strand you but kills fuel economy.
Estimated cost: $200-400
Rear Air Suspension Compressor and Bag Failure
Occasional · medium severityTypical onset: 100,000-160,000 mi
Symptoms: Rear end sags overnight or after sitting, Compressor runs excessively or constantly, Suspension warning light illuminated, Uneven ride height side-to-side
Fix: Air springs develop leaks at crimps or bags dry-rot. Compressor wears out cycling constantly trying to maintain pressure. Most owners convert to conventional coil springs ($400-600 kit, 3-4 hours). Keeping air system means replacing bags ($300-500 each) and possibly compressor ($500-800, 2 hours). Air system repair adds up fast.
Estimated cost: $500-700 (coil conversion), $1,200-2,200 (air system repair)
Heater Blend Door Actuator Failure
Occasional · low severitySymptoms: Clicking noise from behind dashboard, Heat only on one side or no heat at all, AC blows warm on one side, Temperature control stuck in one position
Fix: Plastic blend door actuators strip gears or motors fail. Requires partial dash disassembly to access HVAC box. 4-6 hours depending on which actuator (there are multiple). Some attempt access from below but proper fix means pulling dash components. Navigator's luxury interior makes this tedious.
Estimated cost: $500-900
Buy only with documented maintenance history and budget $2,000-3,000 for deferred issues — the spark plug ejection risk alone makes pre-purchase inspection critical, but solid trucks if you catch problems early.
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.