The 1991 Jeep Comanche shares its MJ platform with the XJ Cherokee, offering decent reliability when maintained, but suffers from chronic transmission cooling issues, oil consumption problems on higher-mileage 4.0L engines, and the typical Renix fuel system quirks that plague early fuel-injected Jeeps.
Automatic Transmission Overheating and Failure (AW-4 / 999)
Common · high severityTypical onset: 80,000-140,000 mi
Symptoms: Delayed engagement into gear after warm-up, Burnt transmission fluid smell or dark fluid on dipstick, Slipping between gears under load or on hills, Transmission oil cooler leaking into radiator (strawberry milkshake fluid)
Fix: Factory transmission cooler in radiator fails internally, mixing coolant and ATF. Requires radiator replacement, external auxiliary cooler installation, full transmission flush or rebuild if contamination occurred. Rebuild typically 12-16 hours labor; preventive cooler install 2-3 hours.
Estimated cost: $400-700 for preventive cooler setup; $1,800-3,200 for full transmission rebuild after contamination
4.0L Engine Oil Consumption and Piston Ring Failure
Common · medium severityTypical onset: 120,000-200,000 mi
Symptoms: Blue smoke on startup or acceleration, Oil level dropping 1+ quart between changes, Fouled spark plugs (oil-soaked threads), Loss of power under load
Fix: High-mileage 4.0L engines develop worn piston rings and cylinder glazing. Requires complete engine rebuild with new pistons, rings, bearings, and machining. In-chassis overhaul runs 18-24 hours; short block swap 12-16 hours.
Estimated cost: $2,800-4,500 for full rebuild; $2,000-3,200 for short block replacement
Renix Fuel System Issues (Injectors, Pressure Regulator, Filter)
Common · medium severityTypical onset: 70,000-150,000 mi
Symptoms: Hard starting when engine is hot, Rough idle with intermittent stalling, Fuel smell in engine bay or visible leaks at regulator, Check engine light with lean/rich codes
Fix: Renix-era fuel pressure regulators leak internally, and clogged filters starve injectors. Fuel filter replacement every 30k miles critical. Regulator replacement 1 hour; injector cleaning/replacement 2-3 hours; filter 0.5 hour.
Estimated cost: $150-300 for filter and regulator; $400-700 for injector service
Head Gasket Failure (4.0L I6)
Occasional · high severityTypical onset: 100,000-180,000 mi
Symptoms: White smoke from exhaust (coolant burning), Overheating without visible external leaks, Oil cap shows milky residue (coolant mixing), Bubbles in coolant reservoir when running
Fix: The 4.0L head gasket fails between cylinders or into coolant passages, especially if overheated. Requires cylinder head removal, resurfacing, new gasket set, and often new head bolts. 8-12 hours labor including machining downtime.
Estimated cost: $1,200-2,000
Transmission and Engine Mount Deterioration
Common · low severityTypical onset: 60,000-120,000 mi
Symptoms: Excessive vibration at idle in gear, Clunking when shifting from park to drive, Engine rocks noticeably during acceleration, Visible cracks or oil saturation on rubber mounts
Fix: Rubber motor and transmission mounts degrade from age and oil exposure. Transmission mount especially prone to tearing. Each mount replacement 0.5-1.5 hours depending on accessibility.
Estimated cost: $250-500 for all mounts
Rear Leaf Spring and Shackle Wear
Common · medium severitySymptoms: Sagging rear end when loaded, Clunking over bumps from rear suspension, Excessive rear axle movement side-to-side, Visible rust or cracked bushings at shackles
Fix: Leaf springs flatten over time, and shackle bushings wear out from age and rust. Affects ride quality and handling, especially when hauling. Full leaf spring replacement 3-5 hours per side; shackle bushings 2-3 hours.
Estimated cost: $600-1,200 for springs and hardware; $200-400 for shackles only
Disc Brake Rotor Warping and Caliper Seizure
Occasional · medium severityTypical onset: 50,000-90,000 mi
Symptoms: Steering wheel pulsation during braking, Vehicle pulls to one side under braking, Brake pedal feels spongy or requires excessive travel, One wheel significantly hotter than others after driving
Fix: Front rotors warp from heat cycling and are prone to thickness variation. Caliper slide pins seize from lack of lubrication. Resurface or replace rotors, rebuild or replace calipers. Rotor replacement 1-2 hours; caliper rebuild 2-3 hours per axle.
Estimated cost: $300-600 for rotors and caliper service
Buy one with documented transmission cooler upgrade and strong compression test results — they're tough trucks when the major gremlins are addressed, but neglected examples become money pits fast.
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.