Hot Rods cooling issue

I've been helping with a '39 ford pickup with a SBC . Truck ran perfect temp for a couple of months , just lately on initial firing the temp goes up to 240deg , then will settle at normal temp . T-stat and water pump replaced , and has always had a Walker radiator . We have "burped" all of the air out of the system , at least we believe so . Our little group is stumped , so I figured I would reach out to the Hambers for help . There were no changes to vehicle before the issue appeared . Thanks in advance for any help . Bill

 

How long does it read 240 before settling?

 

Photo's help,does it have a electric fan or mechanical? HRP

 

Collapsed radiator hose?

Blown Head Gasket??

 

Make shure you get all the air out , With thermostat & hose removed , I fill the intake up , let it set about 20 mints ,top off intake again , then install thermostat /hose ,
then top off Radiator ,

 

Try another gauge to rule that out first.

Then check for a sticky thermostat

 

I have the same issue in my truck. I'm running a 250 straight six. It'll get pretty warm right after start up. Then the thermostat seems to open and it'll go down to where it should be and stay.

I figured that I was developing an air bubble somewhere in the system when it cools and once it burps out, it's good?

 

Now that you mention it, the family car did that to, swing higher than normal shortly after startup, then drop when the thermostat opened.

Then one day the thermostat quit opening

With the new thermostat I redrilled the small "vent" hole in the thermostat a little larger, takes a couple min longer to warm up, but it doesn't do the swing up down thing any more.

Swap out the thermostat again (may want a lower temp one too)

 

Got it cured ! Opened up the bypass hole in t-stat . Thanks for all the input everyone . Bill

 

What do you mean by "Opened up the bypass hole in t-stat"? Please explain.
Thanks.

 

Well then, I think I'll be doing this as well!

 

I've run 4 SBCs in the 39 Ford pickup that I have driven daily for 17 years and never had any over heating. Didnt drill any holes in the thermostat either.