Well, the “fix” with the new thermostat failed after about 100 miles. The repeat excessive heating was less dramatic than last time. Whereas the first overheat resulted in both temperature needles stopping a needle’s width below the red segements, this one resulted in the needles rising no higher than 3/4 of the way up their respective black “normal” arcs. Not a crisis, but certainly indicative of a fault.
My friend Alun had a good look on his way home and found some noise eminating from the water pump. It sounds like a bad bearing, but could be the impeller coming loose on the shaft. There was not even a trickle of flow in the top of the rad – even though the temperature didn’t get up enough to open the thermostat, there should still be a small current from the small hole in the thermostat’s flange. He has also suggested guage faults, but I find it very unlikely that both could intermittently over-indicate and return to normal at exactly the same times, even though they are independent of each other. I also find it unlikely, though not implausible, that I have had two thermostats fail in very short order (1000 miles for the first, 100 for the second).
I’ll be fitting my spare water pump tomorrow – it’s the old one I took off when I was trying to find the cause of the engine continuously running hot before the rebuild (the only significant change during the rebuild was the 2″ exhaust, which cured the problem completely). Keep your fingers crossed for me!
If it´s not the water pump, maybe it is a dirty radiator (inside)
By the way, the mudstaff removal bar doesn´t fit a 88. Thank you, anyway
Thanks for the suggestion. The rad had been purged with caustic soda before fitting, so its interior had been thoroughly cleaned. The small mileage covered since then would not be sufficient for fresh sludge or debris to accumulate. The cleanliness of the inside of the rad was confirmed when I drained it down to replace the water pump – the drain plug didn’t have a trace of dirt in its hollow core.
Overheating can break the thermostat. Therefore once the waterpump has caused a case of overheating, you replace the waterpump and it overheats again because the thermostat is kaputt. Any of this possibly explain what you are experiencing.