WJ series 4.0L Engine

JWJ series 4.0L Engine Overview

Engine Specs

• Engine Name: AMC 242

• Type: Inline-6 (I6), naturally aspirated

• Displacement: 4.0 liters (3964 cc)

• Valvetrain: OHV (12-valve)

• Fuel System: Sequential multi-point fuel injection

• Compression Ratio: 8.8.1

• Horsepower:

  • ~195 hp @ 4600 rpm

• Torque:

  • ~230 lb-ft @ 3000 rpm

Transmission Options

• Automatic:

  • 42RE 4-speed auto (for 4.0L models)

• Manual:

  • Not offered in the WJ (auto only for 4.0L in this series)

4WD Systems

• Available with.

  • Selec-Trac or Quadra-Trac I/II

  • NV242 or NV247 transfer cases depending on trim

Reliability & Reputation

• The 4.0L I6 engine is legendary for:

  • Simplicity

  • Longevity (often over 300,000 miles with proper care)

  • Low-end torque, great for off-road use

• Downsides:

  • Poor fuel economy (~14–18 mpg)

  • Somewhat underpowered for the WJ’s weight compared to the V8 options

 Common Issues

• Rear main seal leaks

• Valve cover gasket leaks

• Crank position sensor failures

• Cooling system wear (radiator, water pump, thermostat)

 Fun Fact:

The WJ series 4.0L Engine was one of the last vehicles to use this classic AMC engine before it was discontinued in 2006. It had minimal changes over its ~20-year run.

Let me know if you’d like:

• A comparison between the 4.0L and the 4.7L V8 in the WJ

• Maintenance tips

• Torque specs for engine work

• Or a PDF of the WJ service manual

 Comparison: WJ series 4.0L Engine vs. 4.7L V8 in the WJ

Summary:

• Choose the WJ series 4.0L Engine if you want simplicity, durability, and ease of repair.

• Choose the 4.7L V8 if you need more power or towing capability.

 Maintenance Tips for the WJ series 4.0L Engine

 Regular Maintenance Intervals:

• Oil Change: Every 3,000–5,000 miles (use 10W-30 or 5W-30)

• Coolant Flush: Every 2–3 years

• Spark Plugs: Every 30,000 miles (use Champion Copper Core plugs)

• Transmission Fluid (42RE): Every 30,000 miles

• Differential Fluid: Every 30,000–50,000 miles

 Common Preventive Repairs:

• Radiator replacement after ~100,000 mi (plastic end tanks fail)

• Water pump tends to go around 120k

• Crankshaft Position Sensor failure often causes stalling/no-start

• Exhaust manifold may crack over time (common but not catastrophic)

 Torque Specs (WJ series 4.0L Engine)

Here are a few useful torque specs for common jobs:

Let me know if you need the full torque spec list or a repair procedure for a specific component.

 Recommended Mods for the WJ series 4.0L Engine

• Cold Air Intake (slight throttle response improvement)

• Exhaust header (replaces crack-prone OEM)

• Upgraded radiator (aluminum, 3-core)

• PCM tuning (limited options, but available for minor gains)

• Lift kit (pairs well with low-end torque for off-roading)

 Manuals and Diagrams

If you’re working on your WJ, I can provide:

• PDF service manuals

• Wiring diagrams

• Fuse box layouts

• Vacuum line diagrams

• Factory torque specs

• OBD-II trouble code meanings

1. Performance & Mod Potential

2. Drivetrain & Axle Info

3. OBD-II Trouble Codes (Common for 4.0L)

4. Service Manual Links (if requested)

5. Off-Roading Tips for WJ with 4.0L

 1. Performance & Modding the WJ series 4.0L Engine

The 4.0L I6 isn’t a horsepower monster, but it has huge potential for reliability and torque-based mods:

 Common Performance Mods:

A 4.6L stroker (using a 4.2L crank, 4.0L block, and 4.0L head) is a popular and proven way to get V8 power with I6 reliability.

 2. Drivetrain, Axles & Transfer Cases in the WJ series 4.0L Engine

Transmission:

• 42RE 4-speed automatic

  • Electronically controlled

  • Common, reliable with regular service

  • Lockup torque converter

  • Weakest point if neglected — keep it cool and clean

Transfer Cases:

• NP242 Selec-Trac (most desirable for off-roading):

  • Full-time and part-time 4WD modes

  • 2WD / 4 Full-time / 4 Part-time / Neutral / 4LO

• NP247 Quadra-Trac II (less common with 4.0L):

  • No 2WD mode

  • Computer-controlled clutch packs

Axles:

• Dana 35: Weaker, used on lower trims

• Dana 44A: Aluminum center section, stronger but limited aftermarket

For serious off-roading or larger tires (33″+), many swap the Dana 35 for a Ford 8.8 or Dana 44 solid axle.

 3. Common OBD-II Trouble Codes (4.0L I6)

These are codes that often pop up in a WJ with the 4.0L:

Use a high-quality scan tool (like Autel, Innova, or JScan) to get manufacturer-specific codes.

 4. Off-Roading Tips (WJ 4.0L Specific)

The WJ is highly capable stock, and with the 4.0L, you’ve got a reliable torque base.

 Recommended Setup:

• Tires: 31″ or 32″ A/T or M/T

• Lift: 2″ budget boost or 3″ spring lift

• Recovery: Tow hooks, recovery strap, hitch point

• Skid plates: Transfer case and fuel tank protection

• Limited-slip or lockers: Consider a rear locker for real trails

• Selec-Trac: Ideal for part-time use when needed

 5. Service Manual / Diagrams (upon request)

I can provide:

•  Factory service manual PDF (full teardown instructions)

•  Wiring diagrams

•  Fuse & relay chart

•  Torque spec chart

•  Maintenance schedule checklist