The 1991 Mazda Navajo is a rebadged Ford Explorer sharing the same 4.0L OHV V6 and chassis. Expect typical Ford Ranger/Explorer vulnerabilities: transmission cooler failures, bottom-end engine wear, and some body/brake booster issues specific to early-90s Ford truck platforms.
Transmission Oil Cooler Failure Leading to Transmission Destruction
Common · high severityTypical onset: 80,000-140,000 mi
Symptoms: Transmission slipping or erratic shifting, Milky pink fluid in radiator or transmission pan, Sudden loss of forward gears after coolant mixing
Fix: The in-radiator transmission cooler develops internal leaks, allowing coolant and ATF to mix. Coolant destroys clutch packs and contaminates the entire system. Requires transmission rebuild or replacement, radiator replacement, and complete fluid flush of all cooling system and transmission lines. 12-16 hours labor for full job including transmission R&R and rebuild.
Estimated cost: $2,200-3,800
Bottom-End Engine Failure (Rod Bearings, Main Bearings)
Occasional · high severityTypical onset: 120,000-180,000 mi
Symptoms: Deep knocking noise from lower engine block, worse under load, Low oil pressure at idle when warm, Metallic debris in oil or filter, Sudden catastrophic knock followed by seized engine
Fix: The 4.0L OHV Cologne V6 develops rod and main bearing wear, especially if oil changes were deferred. Once knocking starts, short block replacement or full rebuild is required. In-chassis bearing replacement is possible but rarely economical given labor access issues. Most shops recommend used or reman long block. 18-24 hours for engine removal, rebuild/replacement, and reinstallation.
Estimated cost: $3,500-5,500
Head Gasket Failure (Both Banks)
Occasional · medium severityTypical onset: 100,000-150,000 mi
Symptoms: White smoke from exhaust on cold start, Coolant loss with no visible leaks, Oil contamination in coolant overflow or vice versa, Overheating under load or highway driving
Fix: Head gaskets fail due to age, overheating history, or inadequate coolant maintenance. Requires removal of both heads, resurfacing, new gaskets, and replacement of all head bolts. While heads are off, valve job and cam inspection recommended. 14-18 hours labor for both heads including machine work wait time.
Estimated cost: $1,800-2,800
Brake Booster Vacuum Leak and Master Cylinder Failure
Common · high severityTypical onset: 90,000-140,000 mi
Symptoms: Hard brake pedal requiring excessive force, Hissing sound from brake pedal area when pressed, Brake pedal slowly sinking to floor when held, Engine rough idle improving when brake pedal released
Fix: The brake booster diaphragm deteriorates or develops vacuum leaks, eliminating power assist. Master cylinder often fails simultaneously due to age and corrosion. Both components should be replaced together. NHTSA had recall for booster issues on this platform. 3-5 hours labor for booster and master cylinder replacement including bleeding.
Estimated cost: $600-1,100
Fuel Tank Sender and Pump Assembly Corrosion
Occasional · medium severitySymptoms: Fuel gauge erratic or stuck on empty/full, Extended cranking when hot or after sitting, Engine stalling at low fuel levels, Fuel odor near rear of vehicle
Fix: Fuel tank sender units corrode, and in-tank pumps fail from age and ethanol fuel exposure. Tank assembly was subject to NHTSA recall for corrosion and leakage concerns. Requires tank drop, sender/pump module replacement, and often fuel filter. Inspect tank itself for rust-through. 4-6 hours labor for complete fuel pump and sender replacement.
Estimated cost: $650-1,200
Automatic Transmission Mount Collapse
Common · low severityTypical onset: 70,000-120,000 mi
Symptoms: Clunk when shifting from Park to Drive or Reverse, Vibration at idle in gear, Excessive driveline movement visible from underneath, Transmission tail housing contacting crossmember
Fix: Rubber transmission mount deteriorates and collapses, allowing excessive drivetrain movement. Causes harsh shift feel and potential driveline vibration. Simple replacement but requires supporting transmission weight. 1.5-2.5 hours labor.
Estimated cost: $180-350
Pass unless you find one with documented external trans cooler install, recent engine work, and solid service records — the transmission cooler time bomb and engine bottom-end issues make these expensive to rescue once problems start.
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.