1993 Jaguar XJ6 Cylinder Head: Diagnosing Failure and Inspecting a Used Replacement

Understanding Cylinder Head Failure in the 1993 Jaguar XJ6

The 4.0L AJ6 inline-six engine in the 1993 Jaguar XJ6 is generally considered robust, but like many engines of its era, it is susceptible to cylinder head and head gasket issues. Failure is often not due to a fundamental flaw in the head itself, but rather the failure of the head gasket that seals it to the engine block. This can be caused by age, overheating, or improper torque on the head bolts. When the gasket fails, it can allow high-pressure combustion gases to enter the cooling system, or let coolant leak into the cylinders or mix with the engine oil.

Symptoms of a Failing Cylinder Head or Gasket

• Engine Overheating: This is a classic sign. Combustion gases entering the cooling system can create air pockets and pressurize the system beyond its limits, reducing cooling efficiency.
• White Smoke from Exhaust: Sweet-smelling white smoke or steam exiting the tailpipe, especially after the engine is warm, indicates that coolant is leaking into the combustion chambers and being burned off.
• Milky, Foamy Oil: If you check the oil dipstick or oil filler cap and find a creamy, light-brown substance that looks like a milkshake, it means coolant is mixing with your engine oil. This is a serious condition that compromises the oil's ability to lubricate, leading to rapid engine wear.
• Unexplained Coolant Loss: If you are constantly adding coolant but can't find any visible leaks (puddles under the car), the coolant may be leaking internally into the engine.
• Rough Running and Misfires: Coolant entering a cylinder can foul the spark plug, preventing proper ignition and causing the engine to run poorly, especially on startup. Loss of compression from a failed gasket between two cylinders will also cause a noticeable lack of power.
• Bubbles in the Radiator or Coolant Reservoir: With the engine running, visible bubbles in the coolant are a tell-tale sign that exhaust gases are being forced into the cooling system.

Buying a Used Cylinder Head: A Detailed Inspection Guide

Due to the high cost and limited availability of new OEM cylinder heads, a quality used head is often the most practical solution. However, careful inspection is critical to avoid buying a part with the same problems you're trying to fix. Take the head to a professional automotive machine shop for evaluation if possible. If you are inspecting it yourself, here’s what to look for:

1. Check for Warpage (The Most Critical Step)

A cylinder head that has overheated will often warp, losing its perfectly flat mating surface. A warped head will not seal properly, even with a new gasket.

• Tools Needed: A high-quality machinist's straightedge and a set of feeler gauges.
• Procedure: Lay the straightedge across the head's deck surface (the part that mates with the block). Check it lengthwise, crosswise, and diagonally. Try to slide the thinnest feeler gauge (typically 0.002 inches or 0.05mm) under the straightedge. If the gauge slides under at any point, the head is warped and will need to be professionally resurfaced (milled) by a machine shop.
• Red Flag: Forum users report that the maximum amount that can be safely removed from an AJ6 head is very small, around 0.010 inches (0.25mm) from its original height of approximately 5.016 inches. Excessive warpage may render the head unusable.

2. Inspect for Cracks

Cracks can be difficult to spot with the naked eye. A machine shop will use dye penetrant or magnetic particle inspection to find them.

• Common Locations: Pay close attention to the areas between the valve seats and between the valve seats and the spark plug holes. These are high-heat areas and common points of failure.
• Visual Clues: Look for any hairline fractures. Also, check for signs of previous repairs, like pinning or welding, which may indicate a pre-existing issue.

3. Examine for Corrosion and Erosion

Look at the coolant passages on the deck surface. Years of coolant flow can erode the aluminum, especially if improper coolant types were used.

• What to Look For: Pitting or deep channels eaten into the aluminum surface, particularly around the water jackets.
• Significance: Minor pitting can sometimes be cleaned up during resurfacing, but severe erosion can compromise the head gasket's ability to seal the coolant passages, leading to a repeat failure. Some mechanics have successfully used metal-filled epoxy like J-B Weld to fill small eroded areas before resurfacing.

4. Check Camshaft Journals and Bearing Caps

Ensure the camshafts, bearing caps, and the journals they sit in are smooth and free from deep scoring or signs of oil starvation. When the bearing caps are torqued down (without the cams installed), they should feel smooth. Any binding or roughness is a major concern.

Known Issues, TSBs, and Recalls

For a vehicle of this age, formal recalls or Technical Service Bulletins (TSBs) directly addressing cylinder head failure are unlikely to be active. Searches on the National Highway Traffic Safety Administration (NHTSA) database for the 1993 Jaguar XJ6 do not show any recalls related to the cylinder head or engine internals. The most common issues are age-related failures of the head gasket. Some early AJ6 head gaskets had a known weakness in sealing an oil supply gallery, and later-design gaskets are recommended for all replacements.

Cost Comparison

Replacing a cylinder head is a labor-intensive job. The following costs are estimates and can vary significantly based on location, shop, and the extent of any related damage found.

Frequently Asked Questions (FAQ)