What Is the 0331 Head?

The 0331 head refers to a specific cylinder head casting number used on the 2000–2001 Jeep Cherokee (XJ) and Grand Cherokee 4.0L inline-six engines. On paper, it was just another version of the tried-and-true 4.0L design. In practice, though, this casting had a major design flaw: it was prone to cracking, particularly between cylinders #3 and #4, once the engine experienced overheating.

When the head cracks, coolant and oil can mix, leading to a chain reaction of engine problems that, if ignored, can result in catastrophic damage.

Symptoms of a Cracked 0331 Head

• Coolant Loss (with no visible leaks): Coolant levels drop quickly but you don’t see puddles under the Jeep.
• Coolant in the Oil: A frothy or milky film on the oil fill cap, or milky residue on the dipstick, indicates coolant mixing with the oil.
• White Exhaust Smoke: Thick white smoke from the tailpipe often means coolant is entering the combustion chambers.
• Engine Overheating: Because coolant escapes, the engine may consistently run hot or overheat.
• Rough Running or Misfires: Loss of compression and coolant contamination can cause misfires and reduced performance.

Ignoring these symptoms can quickly escalate the issue, leading to bearing failure, cylinder wall damage, and even a seized engine.

Why Does the 0331 Head Crack?

1. Design Flaw: The original 0331 casting had a weak spot between cylinders #3 and #4, making it much more prone to cracking than previous or later designs.
2. Overheating: Overheating is often the trigger. The XJ’s cooling system must be kept in top shape; otherwise, the added stress can cause the flawed head to fail prematurely.

How to Identify the 0331 Head

• Look for the Casting Number: Remove the valve cover and check for 0331 stamped into the head.
• Check for a TUPY Mark: Chrysler later revised the design with a stronger version manufactured in Brazil. These are stamped with TUPY between cylinders #3 and #4 on the casting.

If the head has “0331” but no “TUPY” mark, it’s the flawed version.

Fixing the 0331 Head Problem

If you confirm that your Jeep has a cracked 0331 head, there are proven solutions:

• Replace with a TUPY Head: Swap in the revised TUPY 0331 head, which corrected the weak casting flaw.
• Clearwater Cylinder Head: Aftermarket companies like Clearwater produce durable replacement heads that eliminate the original problem.
• Avoid Reinstalling a Standard 0331 Head: Unless it’s a TUPY-marked version, another used 0331 will likely fail again.

Preventing Problems

• Maintain the Cooling System: Flush coolant regularly, replace thermostats, water pumps, and hoses, and keep the radiator in good condition.
• Monitor Temperature Gauges: Don’t ignore creeping engine temps—catch overheating early.
• Use Quality Parts: Cheap radiators or thermostats can cause hot spots that accelerate wear.

By keeping your cooling system in top shape, you reduce the risk of overheating and avoid costly repairs.