A friend of mine has a 94 XJ12. The car apparently has very heavy steering -- almost to the point of being un-driveable. .It starts out fine but as the engine warms up the steering gets progressively heavier.
He is using the specified power steering fluid, has replaced his tires with the specified P7s and has had the car checked several times -- reports all come back everything fine.
He also has a XJ6 which drives/steers fine.
It sounds to me as if the pump is loosing pressure as the oil heats up.but any other thought would be gratefully appreciated.
Cheers,, Hale

 

If there are no leaks from the system and the drive belt is clean and tight, it is probably going to need a new or rebuilt pump. As the fluid heats up, my guess is that some valving or the impeller of the pump (if plastic) might become too soft (due to age), causing too low pressure in the system? Likely a new pump will correct it. If not, then the rack will be to blame.

The X305 series XJ12 has a small pressure transducer device on the steering rack, to provide higher assist at low vehicle speed, and vice versa. It adjusts flow of the fluid. I don't think yours (an earlier XJ81) has this electrical device, although I am not certain. It is not on the parts diagram that I could see.

 

Many thanks Richard.
That was my take on the issue as well but as it isn't my car and I deal with it only occasionally I just don't have much knowledge of that particular model.
I have a 95 V12 XJS and it's steering is great but I wasn't sure if the parts were the same. I think the first step is to pitch in a new pump and go from there.
Cheers, Hale