Good morning all,
Just thought I’d post a reply to this pretty old thread in case someone else comes across it trying to fix a weak heater like I was.
I have a ‘94 Daimler Double Six (XJ81), and I was finding the the heater was weak at best, freezing cold at worst on my morning commute. I tried most of the fault finding listed (checking heater control panel operations, removing and cleaning lower blend flap motor, then replacing same motor a few weeks later, checking heater valve operation, checking temperature of heater matrix in and out piping etc), and the heater would sometimes seem ok, other times it returned to its old tricks (I.e no heat).
In the end, I realised that the heater was appearing to work ok when testing it in the driveway, and when caught in traffic, but once the car was running fast it was cold. The temperature gauge in the dash tended to read around N at all times, albeit dipping a little lower in these cold mornings (-5C). So this weekend I dove in and replaced the thermostats. A couple of genuine EBC3577 at 88C each replaced what looked like generic 82C stats, one of which was jammed open, and a few hours later (add a radiator flush, run for 30 mins, drain coolant, flush with water, drain water, add fresh coolant and demineralised water) the job was done. I managed to get 8.5L of concentrated coolant, plus around 9L of demineralised water into the old V12.
In terms of general running, I really noticed no difference, but the heater was transformed! The drive in this morning was a joy, with consistently hot air available on manual, and proper operation while in auto. So, if you are suffering low heat, it may be worth having a look at your thermostats too!
1985 XJS HE (manual conversion)
1994 Daimler Double Six (XJ81)