I have a 1971 Challenger and just replaced the original 318 with a 408 stroker motor. Using an Edelbrock air gap manifold with a 650 Holley double-pumper carb, new Milodon hi-volume water pump, new 26" 3-row aluminum radiator with two 12" fans. Using a 160 degree thermostat with the fans coming on around 170 degrees or so. The motor runs hot - gradually heats up to 160, then 180, then 190 and so on 'till I shut it down around 230 degrees. The motor runs fantastic - right up until the time when it overheats. Added a 3rd fan (16") in front of the radiator - no change. Milodon said to use one of their hi-volume thermostats - on back order and can't get one. So I bought a different design hi-volume thermostat - no change. Then I tried getting rid of the thermostat and using flow restrictors. The 3/4 inch restrictor worked the best, but still got too hot. Tried the 5/8 inch (the smallest size) and it was worse. Called engine manufacturer tech support and told me to use a flow restrictor on the lower radiator hose - no change. Next going to try a standard-flow water pump. Also going to replace carb. It was a used carb. and could be running too lean - also other issues with carb. that makes me want to replace it with a new Holley. Before it was shipped, the motor ran on the dyno at 191 degrees with no issues. Was told they used a standard-flow water pump and 180 degree thermostat. HELP! I am running out of ideas.

 

Can you identify any leaks? It sounds like a water pump issue...

 

Thanks. I am going to try a standard-flow water pump and change the carb. Will let you know results.

 

Almost sounds like a head gasket issue. Is the cooling system building a lot of pressure?

 

Thanks. I will check on the head gasket and pressure.

 

The water pump uses an impeller to move the water in the cooling system. If this impeller has corroded or become detached because of internal failure, you would have the exact problem that you are describing. Suggest you remove the water pump and evaluate the impellers condition. Good Luck!

 

Replaced the hi-volume Milodon water pump with a standard-volume water pump, replaced the carburetor, and re-installed the fan shroud. Seems to be running well now. On the highway, it will run right at 200 degrees, but creep up to 220 degrees in traffic. It settles back to 200 degrees on the highway. Also, I have a 3rd fan (mounted in front of the radiator) wired to a toggle switch under the dash that I can flip on in traffic. Seems to help. Thanks for the advice guys.

 

A fan shroud really helps at low speeds. It makes the fan pull most of the air thru the radiator instead of around the blades.
Is it still staying cool? What thermostat are you using now?

 

Yes, it seems to be doing okay. Still runs around 200 degrees on the highway and gets up to 220 degrees or so in traffic. The last time I was out, it had a little vapor lock problem, but I think that is fixed now. I am using a 160 degree thermostat. Thanks.

 

Did you try a 180° Tstat?
Maybe the 160 is open too long or all the time and not allowing the radiator enough time to cool what is in it.