The only place to drain coolant is the tap at the bottom rt. side of the radiator.
Does this give a complete drain of the system assuming that the heator valve is also open?

 

Pretty close.

You will NEVER drain the engine block 100% unles you tip it upside down, its just how it is with any engine, always a pool as such left inside. The V12 will have about 3ltrs held in and around the liners. Lots of mud around the rear 3 on each side.

 

Thanks. I thought that might be the case since there was no block drain plug.

 

it is important to get all the mud out from the around the outside of the cylinder liners, that you can. it is a jag V12 problem area.

what have done on some of them is remove the water rails, and anything that will restrict pressure washing down into the holes in the heads,of course cover the dizzy and electrics).

just blast away trying to move the sludge loose and blow it out the open holes!
yup dirty job and messy engine compartment.

only other alternative is remove the heads, much more work and cost!

 

I may well have to go to that extreme, but for now I'm going to fill with water and Prestone coolant system flush and run for a while, drain, and fill with coolant and then see where things are temp wise. Do you think I should leave thermostats out while running flush to get a non restricted flow?

 

do what you can , but the Jag V12 is along length engine, and the sludge/mud at the rear of the cylinder liners builds up around the rear liners(not the front ones), and the temp sensor is at the front, it will not show hi temps at the rear , a local overheat, in that area MAY cause piston drag and liner scoring,and loss of compression!

it ocurrs because water flow to the back of engine is almost non-exsistent, most goes thru the front leaving sludge to settle at back, while you have the rails off would be a good time to modify the flow to force the coolant to the rear!

its kinda a miss and hit affair, but try it, DO NOT overheat a Jag V12 engine!

 

I was going to mention running it with just water to flush it, but you're on top of that already, another thing to do is to add some form of cooling system flush to it, stuff that can break down scale deposits.

What is the reason behind this? overheating?

 

The car is new to me. According to PO there was no overheating problem. I drove it for a few miles and experinced no overheating. I then Brought it in the garage and started to go over it. I like to replace old hoses, belts, fluids and any thing that doesen't look good. I deceided to pull one of the water rails to further look at the cooling situation as suggested.
I blasted water down all the water passages and probed down about 8-10 inches and found no evidence of any silting. I have now got everything back together and the car shows no sign of overheating. I am running 88C thermostats and the temp gage needle barely gets to the "N" in normal.

 

SWEET.