XJS & XJ from KWE Cars
Classic Spirit Reborn

KWE tech note: V12 cooling system and heater problems

A Jaguar V12 cooling system in good condition is capable of dealing with all conditions, contrary to popular belief, and does not need upgrading or fancy coolants and additives. However, it is relatively complex and more difficult to keep in good condition compared to the I6 engines.

A common problem is that the heater will not blow hot even with the engine up to full temperature. As far as the source of heat is concerned, there are two main likely problems:

  • The heater matrix is fully or partially blocked
  • Coolant is not flowing into the matrix because the heater water valve is seized or the engine is not self-bleeding correctly, leading to an airlock

Use of any kind of coolant leak-sealer is a major problem, because it seals up the heater matrix where there is often trapped air. Sealants work by solidifying when they encounter air, usually at a leak. We have examined this problem previously.

Keep your V12 cooling system in tip top condition

On occasion, the heater can become clogged with rust as a result of the system not having had fresh antifreeze or corrosion inhibitor frequently enough. Although the engine has alloy heads and block, there are steel components in the coolant waterways, which rust with alacrity unless corrosion inhibitor is present and fresh.

Rust flakes travel around the system and the narrowest parts are in the heater matrix, which clogs first. This is fairly easy to fix by disconnecting the hose from the right hand water manifold to the heater valve, then disconnecting the hose from the heater matrix to the pipe leading to the radiator bottom hose union, and then attaching a garden hose to this second hose and flushing it out.

This is reverse flushing – the coolant normally travels from right to left – and should blow out any cloggage. It’s a good idea to then flush it the other way and repeat the cycle several times.

The other problem is the engine not self-bleeding. The coolant filler is not at the highest part of the engine (the heater is), and so when filling or topping up the engine, there is no guarantee that coolant will get into the matrix.

The system has a self-bleeding function, which involves a large number of small-bore pipes and hoses intend to shunt any air in the system into the radiator where it won’t do much harm. Air pockets in the engine – especially the cylinder heads – can create localized hot spots and boiling, eventually warping the heads.

This self-bleeding process is reasonably effective when all the pipes and hoses are clear, though there is a special technique needed to fill and bleed the air from a V12 (download here), but it does depends on all the bleed pipes and hoses being clear – unlike this one:

Jaguar XJS rust blockage

Jaguar XJS rust blockage

This condition will stop trapped air from being removed and will lead to the heater matrix not receiving hot coolant water and will quite possibly lead to engine overheating, which in turn can cost over £10,000 to resolve. If this system had correct coolant changes throughout its life, this clogging would not have occurred.

There is also a steel pipe like this:

Jaguar XJS steel pipe

Jaguar XJS steel pipe 2

Mounted on top of the radiator panel, it is crucial in the bleeding system. It is very commonly clogged with rust, especially the ‘banjo’ union indicated by a finger.

These are simple faults to fix, but cause a great deal of costly problems if not attended to.

Speak to the experts

At KWE we are very aware of these issues and deal with them properly. To arrange a consultation, please email sales@KWECars.com or call 01635 30030.

Alternatively, follow us on Twitter or like our Facebook page.

9:12 am KWE Cars


  1. guus van rijs-
    March 19, 2016 at 2:16 pm

    Hoi Chris,
    If you would be so kind to send me the PDF,after reading i want to flush my system immediately

    regards Guus

  2. Chris Knowles-
    March 29, 2016 at 4:34 pm

Leave A Comment