UK Car Repair: Jaguar XJ-S problems, jaguar xj, 6 cylinders

Question
I own an XJ-S that was passed down to me from my mom. It is an 89' 5.3 litre v12 and I know little to nothing about how to go about repairing it. It was purchased at an auction and has sat for a few years without running. It has no problem starting, but it runs poorly in that it idles unusually low and it can rarely go over 30-40 mph.I had someone look at it and they told me that it gets "bogged down and only fires on 6 cylinders" I haven't tried to have it fixed by a mechanic because I would rather learn how to fix it on my own. Can you tell me what's going wrong?

Answer
This is a complicated car and quite difficult to work on. If it is only firing on six cylinders it is usually the fault of a bad resistor block, these are on the inner wing, to the right of the radiator.  You need to start from scratch by pulling the spark plugs, looking at them to see if they are working and then doing a compression test. If the cylinders are OK change the oil and filter. Now you can start looking for the problem.  You can get it running and then, one-at-a-time, pull off a spark plug lead or an injector connector to see which cylinders are working.  When you have done this come back to me with a result!