I recently noticed that the Coolant Overflow Tank Pressure Cap on my 2012 XK X150 is labeled 200 kPa. Looking at the Workshop Manual, the
relief pressure is shown as 140 kPa. When I go online I find that AutoZone carries both caps, but the $9 140 kPa cap isn’t recommended, while the $80 200 kPa cap is. I’m much more concerned about running a 200 kPa cap when the recommended value is 140 kPa. Is the 140 kPa for Naturally Aspirated motors and the 200 kPa for supercharged?

 

Check out post #16 from a related thread... Coolant Reservoir Cap? 120 kPa vs 140 kPa - Jaguar Forums - Jaguar Enthusiasts Forum

 

This won't help you a bit, but I use a 3x3 inch piece of duct tape. Works perfect.

 

show off

A pressure cap from Autozone…. I’ll pass, thanks.

 

After searching the Owners and Build Number correct Workshop Manual, I’ve decided to go with the 140 kPa cap and change my coolant to water mix to 55/45.

Both the Owners and Workshop manual say that 140 is correct. That’s 9 psi lower and much closer to what is typically recommended for a naturally aspirated motor.

This should give the pressurized PA12 tubing in the cooling system some much needed relief (literally).

 

I recently uncovered something distressing about my naturally aspirated 2012 Jaguar XK X150 which I feel obligated to share with the group, as it may also affect other owners.

While replacing a damaged coolant overflow hose, I noticed that the reservoir pressure cap was rated at 200 kPa (2.0 bar or ~29 psi). This is significantly higher than the 140 kPa (1.4 bar or ~20 psi) cap specified in both the Owner’s Manual and the Factory Workshop Manual. The discrepancy raised concerns about my cooling system’s longevity and reliability. I also wondered if other XK X150’s might also have an incorrectly fitted higher-pressure cap also.

The cooling system of the XK X150 was designed to operate at a pressure of 140 kPa. That ensures:

• Optimal thermal management: By maintaining a balance between boiling point elevation and component durability.
• Protection of system components: The pressure cap regulates the system to prevent overpressurisation, protecting hoses, the radiator, expansion tank, and other key parts.
• System reliability: All components are tested to function efficiently within this pressure range under both normal and spirited driving conditions. A cap rated at 200 kPa far exceeds this specification, potentially placing undue stress on components. Over time, this could result in:
• Accelerated wear and failure: Hoses may crack or burst, plastic tank’s seams could split, and seals may degrade prematurely.
• Masked overheating: A higher-pressure cap raises the boiling point of the coolant as high as 280 °F, which might conceal overheating issues, potentially causing unseen damage to the engine. Recall that the engine overheat warning comes on when the temperature exceeds 230 °F for over 30 seconds.

I’ve replaced my cap with the specified 140 kPa cap and inspected the cooling system for any signs of damage or wear, such as leaks, cracks, or swelling in hoses, including conducting a 15 psi, ten minute leak-down test, which it passed with flying colors.

If you haven’t checked the pressure cap on your NA XK X150, I encourage you to do so.

1. Open the bonnet and locate the coolant overflow expansion tank.
2. Examine the pressure cap for a marking indicating its pressure rating.
3. If the rating exceeds 140 kPa (1.4 bar), I strongly recommend replacing it with the correct cap, readily available through Jaguar parts suppliers or reputable aftermarket sources. While you’re inspecting, take a moment to visually assess the condition of the cooling system. Look for any signs of wear, such as bulging hoses, cracks in the expansion tank, or traces of coolant residue around connections.

An incorrect pressure cap could have been fitted during maintenance or by a previous owner or technician unfamiliar with the specifications of the Naturally Aspirated XK X150. Although it may seem like a minor oversight, the consequences of operating the cooling system beyond its intended pressure are far from trivial.

I share this information to inform. The XK X150 is a remarkable machine, blending performance and elegance in true Jaguar fashion. Verifying the correct pressure cap is a simple step in preserving the car’s reliability and safeguarding its cooling system.

 

Unlike the X100 engine which uses a pressure release vent to maintain the coolant system pressure and vent excess pressure/coolant to the atmospheric tank, I believe that the X150 engines used a sealed pressurised system with no atmospheric tank. The system is designed to run at a certain pressure, clearly below 20psi, and, for most owners, during the entire life of the car, the cap will never be required to vent any pressure/coolant as it will always be below the maximum operating pressure. The vent in the cap only opens when the engine is faulty and is operating outside its design parameters.

Fitting a cap, or having a faulty cap, which opens at a lower pressure than 20psi, would be a serious mistake and would be likely to cause coolant loss and consequent overheating. Fitting a cap with an opening pressure of 29psi is obviously not recommended but, for the vast majority of owners, is not going to cause a problem as the engine will be operating at its design pressure below 20psi.

If my analysis is correct, it is, however, worth fitting the correct cap just in case something goes wrong.

Richard