Running an XJS in high temperatures

With record temperatures expected here are some tips about running your XJS in super hot weather:
The V12 cooling system, while adequate when new, has little headroom for dealing with high temperatures, along with probably a partially clogged radiator, as is usually the case. Once the dash gauge goes over ‘N’ then one can be sure of some problems.

A common symptom is coolant being blown out of the catch tank located in the left front wing just ahead of the door. This can sometimes be rectified by replacing both radiator caps since they weaken with age and allow coolant to be vented to the catch tank too soon.

Another problem is that the steep pipe portion of the vent hose from reservoir tank to catch tank usually rusts solid and doesn’t allow coolant to expand naturally. This can cause substantial over-pressure which can cause the reservoir tank to balloon, and puts stresses on all the hose and pipe unions, and especially the reservoir tank with is usually on the point of rusting through in several places.

The coolant radiator is usually partly clogged internally with scale, and externally with road dust, hair, etc. These issues can drastically reduce the cooling efficiency of the system. While regular de-scaling treatments can work, the real solution is a re-cored radiator. Fitting one is usually a 2 hour job.

The engine can also overheat due to fan problems. The main fan is is driven by the engine via a viscous coupling. These can fail and allow the fan to run too slowly. Also, the electric auxiliary fan can seize (or its fuse blow). In this weather I would expect it to come on with 10 minutes of driving if the aircon is working, a bit longer if not.

The aircon system produces a lot of heat from its condenser radiator, and this heat is blown into the engine’s coolant radiator – not a happy state for the V12. If the engine shows over-heating, try turning the AC off – it may get you home!

Sometimes the engine can over-heat all by itself due to the fuel mixture being too lean or the ignition being too retarded, If you suspect this then it’s straightforward to measure the mixture from the exhaust, and to check the ignition timing. On the pre-Marelli V12s (single king lead from distributor cap) the mechanical advance mechanism inside the distributor can seize up, fixing the advance in an incorrect state.

It’s tempting to go for ‘water-less coolants’ to plaster over a problem, but they can’t fix an inefficient cooling system, and cost a great deal of money. See our article here.
One can improve the performance, noise and energy efficiency of the cooling system by having all electric fans fitted.

Lastly, do make sure that the cooling system is properly filled and bled. It’s tricky on the V12, and many specialists have their own methods. Ours is as follows, and works every time:

1. With a cool engine, put coolant in the reservoir tank until it is a bit over half full. Put its cap back on.
2. Fill the system via the centre cap until coolant level is about 2 inches from the top of the fill pipe. Squeezing the top hose nearby repeatedly can help disperse air bubbles, and speeds the filling operation. Put the cap back on
3. Open the bleed screw on the top of the radiator on the left hand corner.
4. Start the engine and hold it at 3000rpm with an assistant.
5. Observe the bleed screw outlet, possibly with a torch. The coolant will warm up over a period of about 5 minutes. Keep the revs at 3000.
6. When coolant bubbles out of the bleed port let it do so for a few seconds to get most of the air out. Keep the revs at 3000
7. Re-fit the bleed screw (careful, the coolant will be hot) and tighten. Then and only then release the revs to idle. Switch.

The car is then properly bled, but will need to be topped up at the central cap when the engine is stone cold

So in summary, make sure the coolant system is leak-free, and that the caps and overflow pipe are clear. Change the radiator every, say, 10 years. Replace all the coolant hoses at the same time. Make sure the coolant is correctly filled and bled.