It is interesting that the 3.5 litre P6B engines and possibly the SD1 engines too have a bad habit of discharging coolant from their respective radiators, hence the valuable contribution that an expansion tank can make. Looking back through my log book for work carried out on my Rover since 1978, the need for frequent topping up of the coolant was always ever present. Distances varied but typically 1 to 2000 Miles was the normal range. When left for longer the level down was sometimes quite significant with half the radiator empty. Coolant loss was independent of the thermostats fitted, 74, 78 and 82 were all used over the years, radiator caps all 15psi and different brands made no difference. Increasing coolant capacity by using a 3 core as opposed to the original 2 core design did not prevent the purge either nor did changing from tin to composite head gaskets. Blown head gaskets will certainly see considerable coolant loss, much more so than what is normal for these engines, but that is down to component failure rather than a possible design issue.
My 4.6 on the other hand doesn't display this problem at all. I checked my coolant level for the first time on December 15 having travelled 16,446 Miles since May 1, a new radiator I fitted at that time. The coolant level was as it had been when it was filled, engine off..right at the base of the filler neck, engine running...an inch down at most. So no change and no loss of coolant, interesting.
I wonder if the 10.5 : 1 engines like my original are the biggest offenders? Is the high compression ratio the cause or does it stem from the P6B heads? Does a P6B engine fitted with any of the later 14 bolt heads behave the same? I tend to think it stems from the high compression and the additional heat that generates as a result when the engine is switched off, but I have no proof.
Ron.