The 1990 Jeep Comanche is a body-on-frame compact pickup sharing Wrangler/Cherokee DNA. The 4.0L I6 is bulletproof if maintained, but the 2.5L I4 struggles with durability under load. Common headaches center on transmission cooling, rear main seals, and frame rust in salt states.
Automatic Transmission Overheating and Failure (AW4 / 999 / 904)
Common · high severityTypical onset: 80,000-140,000 mi
Symptoms: Transmission fluid dark or burnt smelling, Slipping between gears, especially 2nd-3rd on the AW4, Delayed engagement when shifting from Park to Drive, Complete loss of forward gears after overheating episode
Fix: Factory transmission cooler is undersized and often clogs. Coolant contamination from leaking radiator internal cooler destroys clutch packs. Prevention: install external cooler and bypass internal radiator cooler. Rebuild requires 12-18 labor hours; expect torque converter, clutch packs, seals, and bands.
Estimated cost: $1,800-3,200
Rear Main Seal Leak (4.0L I6 and 2.5L I4)
Common · medium severityTypical onset: 90,000-150,000 mi
Symptoms: Oil dripping from bell housing area onto crossmember, Oil spots on driveway after overnight parking, Visible oil coating on starter and lower transmission housing, Persistent oil smell from exhaust heat
Fix: Two-piece rear main seal design is notorious for leaking as Viton hardens. Requires transmission drop (add transfer case on 4WD). Also inspect oil pan gasket and valve cover while in there. 6-9 hours labor depending on 2WD vs 4WD and rust.
Estimated cost: $650-1,200
Cylinder Head Cracking (2.5L I4 only)
Occasional · high severityTypical onset: 70,000-110,000 mi
Symptoms: White smoke from exhaust on cold start, Coolant loss with no visible external leaks, Rough idle and misfires, often cylinder 3 or 4, Coolant in oil (milky dipstick) in advanced cases
Fix: The 2.5L head is prone to cracking between valves, especially if overheated even once. Requires head removal, pressure test, and replacement if cracked (common). Budget 10-14 hours: head gasket set, machine work if salvageable, timing components, and coolant flush. Many opt for junkyard head or upgrade to 4.0L swap instead.
Estimated cost: $1,400-2,800
Frame Rust and Leaf Spring Mounts
Common · high severitySymptoms: Visible rust perforation on rear frame rails behind cab, Leaf spring shackle mounts pulling away or cracking, Body lean or suspension sag on one side, Clunking over bumps from loose spring hangers
Fix: Unibody construction behind the cab is the Achilles heel in rust states. Rear frame rails and shackle boxes rot out, requiring plate welding or full frame section replacement. Inspect before purchase—surface rust is manageable, but structural rot means walk away. Repair labor varies wildly: 8-20 hours depending on severity, plus sandblasting and coating.
Estimated cost: $800-3,500
Throttle Body Injection Issues (Renix System, pre-1991)
Occasional · medium severityTypical onset: 100,000+ mi
Symptoms: Intermittent stalling at idle, especially when warm, Surging or hunting idle speed, Hard starting after sitting overnight, Check Engine Light with MAP sensor or TPS codes
Fix: The Renix engine management system (1987-1990) uses failure-prone MAP sensors and throttle position sensors. Coolant temp sensor also causes gremlins. Diagnose with Renix-specific scanner (OBD-I). Typical fix: replace MAP ($80 part), TPS ($60), and clean throttle body. 1.5-2.5 hours diagnostic and repair.
Estimated cost: $250-450
Fuel Tank Sending Unit Failure
Occasional · low severityTypical onset: 80,000-130,000 mi
Symptoms: Fuel gauge reads empty when tank is half full, Erratic gauge movement or stuck needle, Gauge reads full constantly regardless of fuel level
Fix: The in-tank sending unit float arm corrodes or the resistor fails. Requires dropping the fuel tank (remove skid plate on 4WD models). While in there, replace fuel pump if original. 2-3 hours labor. Common enough that many owners just learn to use the trip odometer.
Estimated cost: $280-500
Transfer Case Linkage and Vacuum Issues (NP231, 4WD)
Occasional · medium severitySymptoms: Cannot shift into or out of 4WD, Grinding when attempting to engage 4WD, 4WD indicator light flashing or not illuminating, Hissing sound from vacuum system under dash
Fix: Vacuum-actuated front axle disconnect system uses brittle plastic lines that crack. Shift linkage bushings wear out, causing misalignment. Check vacuum lines first (30 minutes), then inspect shift linkage and mode fork alignment. Some convert to manual cable-pull hubs. 1.5-3 hours depending on root cause.
Estimated cost: $150-600
Buy a 4.0L model with a clean frame and service records; avoid 2.5L I4 and any rust-belt trucks without thorough inspection—transmission and frame issues are expensive but predictable if you know what to look for.
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.