Technical 39 Chevy overheat

I have a stock 216 that has an overheating problem. I back flushed the radiator and did clean out a lot of dirt/rust. The thermostat opens at 170 degree when tested. The car will sit idling and remains at 180 degrees, this is indicated on the gauge and an infrared thermometer.
I can drive at 15-20 MPH without overheating heating. When accelerated to 40-45 MPH it immediately overheads and quickly goes to 212 degrees. I will try a total engine flush, and a test for bad head gasket.
I am looking for any ideas or like experience.
Thanks, Tom

 

I'd say you need to have the radiator rodded. You have sufficient capacity to cool the engine when there is little load but not enough when you put a load on it. I had a 47 Ford that was good to 40/45 mph but would overheat if you pushed it any faster. It's very indicative of a plugged core. It may be hard to find an old fashioned radiator shop that will pull a tank and rod a radiator.

 

^ Wondering if the impeller in the water pump is rotted away and no longer able to move coolant as it should...?

 

I will second @jaracer comment. If you can't find a radiator shop, I have had good luck removing the radiator, turn it upside down and flush it. I have also laid it flat with two short hoses on it and filled it with a 50% solution of white vinegar and let it sit overnight, then back flush.

 

I have read where a lot of the 216/235 engines have a bad rap for filling the lower part of the blocks with trash. Pull the core plugs and dig around. A high pressure power washer does wonders when you need clean block passages.

 

Had a pump impeller come loose once on a "rebuilt" pump. Be worth a look ....

 

Retarded timing and lean fuel mixture also cause heat. Make sure the vacuum advance isn't slowly leaking. It should be able to hold advance indefinitely. Engine timing should reach 32-36 degrees at full centrifugal advance without the vac unit hooked up. If its not advancing the engine WILL overheat!

Use your infrared thermometer to verify you're getting about a 40 degree temp drop from the stat to the bottom radiator hose. That indicates the radiator is good. A pressurized upper hose with the radiator cap in place should indicate both circulation and a higher boiling point than 212. A new cap may be in order if there is no pressure building.

 

Chevrolet didn't pressurize cooling until 1948 - it's open to atmosphere.

 

Thanks for all the replies. I have also heard that removing the freeze plugs and cleaning out is a good idea. I will start at the top and try all remedies.

 

I did the vinegar on a running engine and radiator. Also did the panty-hose filter trick. Immediately learned to take a nut driver for the hose clamps on any little test drive. Filter clogs up quick!

 

I’ve flushed with CLR

and with cascade dish washing tablets.