Giddy! I have a 2001 420 and I'm not sure if the thermostat is working... I replaced a water pump gasket to stop a leak. And since u picked it up last week the water temp gauge in the cab hasn't worked, the cat parts guy thinks it's the switch....? The pump seems to work, but the water gets quite hot and doesn't seem to circulate... Any ideas? Thanks, Rick.
Can you get a hold of an infrared temp and measure what the temp actually is?
Ya I should be able to, should I read the hose going into the top of the rad?
Is it full of coolant or maybe air locked.
Measure several spots to see where the hot spots are. Thermostat housing should be around 180/190 degrees F. Top of the radiator will be about the same, bottom of the radiator should be cooler
Try Mobiltech's suggestion first and see if there is an air bubble in the cooling system somewhere. With the water pump having been out to fix the leak it's always a possibility. If bleeding the system doesn't work, try this. If you can get hold of an infra-red temp gun use it from when you start the engine from cold. Train the gun on the top hose coming out of the thermostat housing but at the radiator end. As the engine warms up and when you get to the point when the stat starts to open you should see a notable jump in temperature in that hose at the radiator end. If you keep jumping back and forth with the temp gun from the top hose to the cylinder head close by the thermostat housing you should see the cylinder head temp going up and up but the hose temp not changing much. That should all change when the stat opens and give you an idea if the stat is working as it should. If the increase in the temperature of the top hose pretty much exactly mirrors the increase in temperature of the cylinder head as the engine warms up I would suggest your thermostat is stuck in the open position and the engine will be overcooling as a result. Do you know if there is actually a thermostat in the housing..? It wouldn't be the first engine I've seen where someone has "solved" cooling system problems by removing the stat.