I have a problem with my XK150 (as usual).
When idling or at slow speed the temperature will rise and the eventually the coolant will boil. At normal road speed the temperature is ok.
Thermostat opens correctly at 78 deg centigrade. Thermostat of correct type and the “sleeve” that close the bypass moves correct way to close the bypass.
The radiator is restored (new cell package). When running with the electric fan the temperature drop was ruffly (hard to measure correctly) 30+ deg from top to bottom.
Engine running nicely, carbs set by a specialist who did not detect any problems measuring the exhaust gases.
The temperature on the exhaust side of the cylinder head varied from 109 deg (fire wall) to 102 deg front .
After running the engine to 90 deg I removed the spark plugs and cranked the engine on the starter and could not detect any steam.
Checked the compression at the same time - even and around 160 PSI.
500 revs at idling.
The water pump is a new high capacity type.
The bypass is not closed well enough – the thermostat housing is corroded and some water slips by at low revs due to a lower flow of water (symptomatic high capacity pumps?) and the resistance to flow imposed by the radiator (even if it is “new”)??
Any clue what to look for?