Overheating 420

Hi All, I’ve rebuilt my 420 from top to bottom mechanically. Every component is either new or rebuilt.
My issue now is when the car is started it warms up quickly and comes off choke, from there the temperature gauge climbs fast to 100c. The radiator and hoses are cold and the intake manifold is screaming hot. The thermostat has been tested and changed twice. It feels like air trapped in the system.
Does anyone have any feedback or experience with this?
Dan

I’m no expert but sounds more like the coolant is not circulating; the pump/impeller, or belt or blockage.

does it boil over, or stop at 100C ?

an IR thermometer is useful

when the thermostat opens, one rad pipe should go from cold to hot

if you have air, park on a slope, place a fill tube from a cut-off drink bottle into rad neck and slowly feed the water in with engine running. On some vehicles I know, you MUST do this

The first question is always “Can I trust the gauge?” The second is “Am I holding pressure?”

If steam or coolant isn’t coming out, and your gauge is pegged, then it’s the gauge.

"The radiator and hoses are cold and the intake manifold is screaming hot. "

That’s not the gauge.

Justa thought but are you sure the stat can open when installed in its housing? I have seen incorrect gaskets impede the stat operation (other cars).
its difficult to understand how the manifold can be hot and the top hose closed IF the stat is open. Are you losing any coolant? Lastly can you advise how extensive the mechanical rebuild was? i.e did it include the entire engine and if so what replacement parts did you use (water pump?)

I know it’s a nuisance pulling a thermostat out of a 4.2, but by now in this experience if the water pump is turning I would run without a thermostat to see if I could get it to behave that way, then go from there? Paul

Thanks for all the input guys . After using a lot of different advice I changed the relay going to the gauge and voila it’s running perfectly in the middle… a infrared thermometer helped determine the engine was at normal temperature.

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