The 1998 Navigator shares the F-150/Expedition platform with Ford's 5.4L Triton V8—a first-year model plagued by catastrophic spark plug blowout issues and transmission cooler failures that can grenade both units if ignored.
Spark Plug Ejection / Thread Failure (5.4L 2-valve)
Common · high severityTypical onset: 60,000-120,000 mi
Symptoms: Loud pop or bang from engine bay, sudden misfire, Check engine light with multiple misfire codes, Hissing sound from cylinder head, loss of compression, Plug literally shoots out of head under load
Fix: Ford used shallow spark plug threads that strip under heat cycling. HeliCoil or TimeSert thread repair is 2-3 hours per hole if caught early. If the plug blows completely, you're looking at removing the head for proper machining and insert installation—8-12 hours labor. Some shops use save-a-thread kits with the head on, but quality varies. Preventive: replace plugs every 30k with anti-seize and proper torque.
Estimated cost: $300-800 per plug (in-situ repair), $2,000-3,500 if head removal required
Internal Transmission Oil Cooler Failure (4R100)
Common · high severityTypical onset: 80,000-150,000 mi
Symptoms: Strawberry milkshake appearance in coolant overflow, Transmission slipping or delayed engagement after cooler ruptures, Engine overheating or transmission temp spikes, Pink foam under radiator cap
Fix: The cooler built into the radiator fails internally, mixing ATF and coolant—'the strawberry milkshake of death.' Once this happens, both fluids are contaminated. You need radiator replacement, complete transmission flush (sometimes multiple cycles), and often full rebuild because coolant destroys clutches and seals. Externally mounted aftermarket cooler is cheap insurance—1 hour install. If caught immediately you might survive with flush only, but most people don't catch it in time. Budget 12-18 hours for transmission R&R and rebuild.
Estimated cost: $2,500-4,500 for radiator + transmission rebuild, $150-300 for preventive external cooler
Intake Manifold Coolant Crossover Leak
Common · medium severityTypical onset: 70,000-130,000 mi
Symptoms: Coolant smell in cabin, no visible external leaks, Slow coolant loss, need to top off every few weeks, White smoke on cold start that clears up, Rough idle when cold, smooths out when warm
Fix: Plastic coolant crossover passages in the intake manifold crack and leak internally. Coolant drips into cylinders overnight causing rough starts. Requires complete upper intake removal, crossover replacement, and all gaskets—6-8 hours labor. Dorman makes an updated aluminum replacement part. While you're in there, replace the thermostat housing (also plastic, also cracks).
Estimated cost: $800-1,400
DPFE Sensor / EGR System Failure
Common · low severityTypical onset: 50,000-100,000 mi
Symptoms: Check engine light with P0401 (insufficient EGR flow) or P1401, Rough idle, stumbling on acceleration, Poor fuel economy, lack of power, May stall at idle when hot
Fix: The DPFE (Delta Pressure Feedback EGR) sensor fails constantly on these—it's a Ford design flaw across all Triton engines. The sensor itself is a 20-minute swap. However, carbon buildup in EGR passages often mimics or compounds the problem. A thorough fix includes DPFE replacement and cleaning/replacing the EGR valve and tube—2 hours total. Some techs bypass entirely in non-emissions states, but that's owner's call.
Estimated cost: $150-400
Ball Joint Failure (Front Suspension)
Common · high severityTypical onset: 80,000-120,000 mi
Symptoms: Clunking over bumps, especially when turning, Wandering steering, vehicle pulls to one side, Excessive tire wear on inside or outside edge, In severe cases, wheel visibly cocked at angle when parked
Fix: This 6,000+ lb truck eats ball joints—both uppers and lowers. Ford used pressed-in joints that require special tools or complete control arm replacement. Most shops replace the whole arm with aftermarket units that have greaseable joints—3-4 hours per side including alignment. DO NOT ignore this; a separated ball joint at highway speed means loss of steering control. Inspect every oil change after 60k miles.
Estimated cost: $600-1,200 per side (control arm replacement), $1,200-2,000 both sides with alignment
Rear Air Suspension Failure
Occasional · medium severityTypical onset: 90,000-140,000 mi
Symptoms: Rear sagging overnight or after sitting, Compressor runs constantly, won't shut off, Ride height sensor fault codes, Harsh ride, bottoming out over bumps
Fix: Air springs crack, leak, and fail. The compressor overworks trying to maintain pressure and burns out. You can replace individual components (bags run $150-250 each, compressor $300-500, 3-4 hours labor), but at this age most owners convert to coil springs using Strutmasters or similar kits—2-3 hours install, eliminates the system permanently. Coil conversion is cheaper long-term and more reliable.
Estimated cost: $400-800 for coil conversion kit installed, $1,000-1,800 to repair air system
Fuel Pump Failure
Occasional · high severityTypical onset: 100,000-180,000 mi
Symptoms: No-start condition, cranks but won't fire, Stalling or dying at idle after warm-up, Hesitation, surging under acceleration, Whining noise from fuel tank area
Fix: In-tank fuel pump dies from age and debris. The 98-02 Navigators require dropping the 30-gallon tank—there's no access door. Plan on 3-4 hours labor including lines, straps, and shield removal. Use OE Ford or Bosch pumps; cheap aftermarket units fail within a year. Replace fuel filter at the same time (it's on the frame rail, separate job normally but easy while you're under there).
Estimated cost: $600-1,000
Only buy if it's been meticulously maintained with records showing recent spark plug service, external trans cooler, and suspension work—otherwise you're inheriting $5,000+ in deferred maintenance on a 26-year-old luxury SUV.
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.