Jaguar V12 and XJ12 Engine Hot Start Problems
Why you need your AC working all the time
The aircon cooling system has two purposes on the 5.3 litre V12 engine:
- To cool and dehumidify the cabin
- To cool the fuel
Fuel cooling
The V12 engine bay runs rather hot, and can reach 120degC when the car is stationary. This is hot enough to cause the fuel to vaporise in the fuel rail on top of the engine and cause a vapour lock, which in turns prevents fuel from circulating to the fuel injectors, and so the engine can stop or be very reluctant to start.
As an aside, in a fuel-injected car like the XJS, the fuel is pumped from the tank and up to the engine’s fuel rail and then back to the tank continuously.
The fuel gets heated whilst it is in the engine bay, and then cooled again by the rest of the cool(er) fuel in the tank. However, on a hot day the fuel gradually gets hotter and hotter on this journey around the car until, especially if the tank is nearly empty, all the fuel is near vaporising point.
If this point is reached the engine will stop and vapour pressure in the tank reaches a dangerous level.
To prevent this, Jaguar fitted a fuel cooler in the engine bay, running from the aircon system which removed all the heat (from the fuel) gained in the engine bay. But if the aircon is turned off, or not working for some reason, the fuel will just get hotter and hotter. Normally this is not much of a problem in cool climates like the UK but in really hot weather it most certainly is a problem.
In convertibles, it is common practice to switch off the aircon cooling when the roof is down, but this then allows the fuel temperature to rise. So, Jaguar from about 1989 fitted a device which kept the aircon compressor running even when the cabin control was ‘Off’. Thus, the system did not attempt to cool the cabin, but did keep the fuel cool.
De-humidification
While the need for cooling the cabin in hot weather is obvious, the need to dehumidify is less so. All aircon system cool the air to such an extent that moisture in it is condensed out, and the exiting air is relatively dry. It is very much more comfortable to be sitting in a dry-air atmosphere, and this dryness reduces the risk of various bacterial and fungal outbreaks. This is not helped by most XJSs being somewhat damp inside due to imperfect door and screen seals. This situation is vastly worsened when the cooling side of the aircon is not working. Up to about 1991 the system did not have a fresh air over-ride, and automatically switched to recirculating the cabin air if the inside temperature gets a lot hotter than the temp demanded at the control knob. In this state if the cooling side is not working, the cabin air is circulated round and round getting hotter and damper ad infinitum – very unpleasant. One’s only option then is to turn off the ac and open the windows! So, it is important to have the aircon cooling side working, even if one does not need a great deal of cooling at the time.
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