Knocking Noises

  1. Check Starter Bolts
  2. Check Flex Plate Bolts to make sure they are tight
  3. Check for cracks in the Flex Plate


  1. Bad Waterpump especially if it happens at high speed
  2. Worn fan clutch (if overheating at slow speeds) 
  3. Broken or missing fan shroud
  4. Clogged Radiator
  5. Use a 195 degree thermostat.   Don't use anything lower as you will get bad gas mileage.
  6. Air trapped in the upper radiator hose.   Disconnect and burp the system.

Jeep 4.0 Engines: How to Rebuild and Modify

Jeep 4.0 Engines: How to Rebuild and Modify (Sa Design)Has anyone picked up a copy of the Jeep 4.0 Engines book? Let us know your thoughts!In this Workbench title, veteran author and Chrysler/Jeep engine expert Larry Shepard covers the rebuild of an entire engine in exceptional detail. He also delves into popular high-performance modifications and build-ups. Step-by-step photos and captions cover each crucial step of the engine disassembly. He shows the inspection of all critical parts, including block, heads, rotating assembly, intake, and exhaust. Critical machining processes are covered, including decking the block, line boring, and overboring the block. The book provides exceptional detail during the step-by-step assembly so your engine is strong and reliable. Installing a larger-displacement rotating assembly (or stroker package) is one of the most cost-effective ways to increase performance, and the author covers a stroker package installation in detail.


Troubleshooting No-Start Issues

 Most Common Reasons

  1. CPS
  2. Starter or Starter Solenoid
  3. Fuel Pump
  4. Distributor (up to 99 only)
  5. PCM / ECU

Runs Erratic under Load / Accelaration

  1. If backfires through intake, check for Clogged Fuel Pump
  2. If it pings and runs hot
    1. Check Timing (Advanced or Retarded Timing)
    2. Make sure the Distributor case bolt is tight.   You shouldnt be able to turn the whole distributor by hand

Throttle Body Upgrades

So you want to squeeze as much horsepower and increase performance for your 4.0 in your XJ or MJ?   A bored throttle body is an easy upgrade to do just that.  

  • There are various sizes of bored throttle bodies ranging from 60mm all the way up to 68mm!  
  • HO 4.0Ls factory throttle bodys are 58mm.   Renix is 52mm.
  • The most commonly used size is the 62mm.   If your 4.0L is stroked, you may need something bigger than 62mm depending on your intake manifold bore.

Do It Yourself Links

Engine Cooling System

Questions about the engine cooling system?  

General Cooling System Info

 There are 2 types of Cooling systems on the XJ/MJ

  1. Open style - any normal cooling system used today.
  2. Closed style - no radiator cap and utilizes a pressure bottle. used in 87-90 XJs.


  • Change engine coolant every 12,500 miles or 12 months.
  • Thereafter change engine coolant at the beginning of every winter season.
  • Maintain ethylene glycol and water solution at a mixture of 50/50.