I have just completed an engine & gearbox change using a donor car of exactly the same specification. I discovered the following which may help others.
Left hand side engine mounting that screws to the crankcase had fractured leaving the sump sitting on the power steering rack. It had been like this for some time. I hadn't spotted it nor had the annual inspection. Even when the engine, gearbox and sub frame was removed and was sitting on the floor it was not easily visible. Only when the engine was lifted off the subframe did it become apparent when the main part of the engine mounting fell off.
Steel heater pipe that runs under the air flow monitor was so badly corroded on both cars, tapping the crusty corrosion on the outside of the pipes revealed a hole in both cases. I have replaced the pipe with a straight run of 15mm copper pipe. It doesn't need all those bends!
The pipe from the condenser to the bulkhead had worn through where it passes through the small gap between the condenser and the body shell. This was caused by the condenser rubber mounts perishing allowing the condenser to drop and trap the pipe and the relative movement causing the pipe to wear through.
I replaced; engine mounting rubbers, sub frame mounting rubbers, radiator mounting rubbers, condenser mounting rubbers, clutch and air con pipe.
The recipient car has done 205,000 miles. The donor car had only done 92,000 miles but was completely rotten underneath. The donor car is the same colour as the recipient and the interior is the same colour as my manual XJR so there is not much left!
The reason for the change, recipient car jumped out of fourth gear and had done so for 100,000 miles but had become worse also on its last run it blew out all the coolant after instantly overheating. I suspect the water pump impeller has fallen off but i have not had time to check.
The only fault remaining is the alternator is not charging even though the red light is going out. The donor car did have a charging problem so i will fit the original alternator and see if that solves the problem. Anyone any other thoughts on this one?
Thanks
Charlie