Cleaning the engine bay
Most would love to have a nice clean engine bay but are intimidated at the idea, afraid of doing damage to something. Well, fear no more as it can be achieved at home. However, I do recommend taking the car to a self-service car wash off hours when not too busy so you can take your time. And not to mention saving your driveway from being stained and then the only thing that will clean the concrete is acid.
I would not use brake cleaner or anything harsh as that as it will ruin the finish on all of your aluminum parts and paint. I do use water and have for years with engine cleaner, the same as professional detailers do. You will want to first use electrical parts cleaner on all such items and clean by hand. Then run the engine for no more than five minutes just to get it warm but not hot. Therefore, if you go to a car wash you need to let the engine cool down and depending how far you drive to get there, it could take 30 minutes or more to cool. Hence going there on-off and less busy hours. Cover all electrical parts you cleaned by hand with plastic and rubber bands will help for sealing the base of the part you want to keep dry.
Then spray the engine cleaner on everything you want to be cleaned, however, on the XJS cars 95 and 96 be very careful spraying too much water around the edge of the coil cover with Jaguar written on it as water can get in there and sit at the base of the plugs and if it does you must blow it out or the engine will shake rattle and roll and foam at the mouth like a wild sick animal taking its last breath. There should be a rubber seal around the cover but if not then for sure water will get in there. So there is no need to panic if this happens. No need to call the Jaguar coroner's office.
Follow the time instructions for the cleaner which are normally around 10 minutes but look on the can of which product you choose then hose it off. A pressure hose works best and you may want to switch to wax on the final rise if at a self-serve car wash, as this will give the area a nice sheen. If you can air hose it all dry before wiping it dry is even better. And do not be concerned about the wires as they are fine unless they are old and most likely cracking as found on too many V12 XJS cars. In such a case, the wires will need a long time to dry but if you had a fire and are cleaning up after, Then you have already changed them. If the car is a V12 and the plug wires and fuel injector hoses are old, then you had better change them now or you will have a fire. It is just a matter of time.