XJS & XJ from KWE Cars
Classic Spirit Reborn

Fuel Cooling + Hot start problems on the V12

This is a short explanation of why 5.3 V12 engines sometimes have poor hot starting, and the importance of fuel cooling.

Like all fuel-injected engines, petrol is pumped from the fuel tank up to the engine’s injector rail via a pressure regulator and back to the tank in a continuous loop. This ensures that a steady, constant pressure of fuel is available for the injectors to supply to the combustion chambers. The Jaguar XJS and to a lesser extent the XJ12 has a very hot engine bay whose temperature can rise to 120degC or more when the car is idling in traffic on a hot day. This has the effect of vapourising the fuel in the fuel rail causing difficulty in starting if the engine is stopped, for example at a filling station, and sometimes causing the engine to stop in full flight. To avoid this problem the engine has a fuel cooler in the engine bay which is cooled by the aircon system. This has two effects – one to cool the fuel circulating round the system and the other is to keep the fuel in the tank cool which otherwise would get steadily hotter and hotter both from direct heating from the sun shining on the boot, and from the engine bay heating the fuel as it passes through.

If the AC is not working then the fuel temperature starts to rise. This won’t initially be a problem and on short journeys it can be ignored. But once the fuel in the tank has got rather low it gets hotter quicker and fuel vapourisation occurs, stopping the engine. This only happens in really hot weather. It goes without saying that the AC system must be kept working all year round.

The hot start problem is most commonly experienced when after a brisk journey the engine is stopped, and then re-started within around 15 minutes. During this time heat-soak from the engine and exhaust have raised the engine bay temperature (in the absence of cooling air from the car traveling along) to a point above the vapourisation temperature of petrol. When the engine is re-started the relatively low fuel pressure is insufficient to clear this vapour-lock and so the engine won’t start. Although Jaguar designed-in a device intended to increase fuel pressure if the fuel is hot, it doesn’t work well enough. After a while the engine bay cools enough for the fuel to liquify again and then the engine will start fine. One way to reduce the problem is to turn the ignition key to position 2 (i.e. on but not cranking the engine) for 3 seconds, then switch off, then on again for at least five times. This has the effect of flushing the fuel round and into the cooler fuel in the tanmk, but it’s not always successful.

The proper solution is to raise the fuel pressure from 2.5 bar to 3bar, which is what Jaguar did with the very late 5.3, and all 6 litre V12 engines. This higher pressure flushes the vapour lock successfully. One can’t simply raise the pressure however since this would then make the engine run too rich – the engine computer needs to re-programmed to suit the raised pressure. KWE can carry out this operation.

One should also be aware that some V12 engines run hotter than normal, often due to ignition timing problems.

11:26 am KWE Cars