Slant Six overheating

1966wgn said
Show my 1964 dodge dart Slant Six overheated today. I only drove it about 7 miles in 50° weather, and it buried the needle. I open the radiator cap when it’s cold, and I can see that cooling is flowing through when it’s running. You can let it sit there and idle for an hour and the needle will only go halfway, but the second you start driving it starts to bury the needle to H. The coolant is relatively new, however there is some brown showing. Could it be sludge inside the block? What is the best way to remove it? Removing the freeze plugs?

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When was the last time the anti freeze and thermostat were serviced?
A thermostat stuck closed won’t affect a slant at idle because it will get plenty of coolant through the by pass. Once the motor starts making power during driving, it needs flow through the radiator. An inexpensive non contact thermometer like those sold at Harbor Freight can help diagnosing problems like this. For example you could check to see if the radiator and the top radiator hose are getting hot. If not the thermostat is stuck closed.