Ok, this is odd.. I have replaced the stat with a Robert shaw, have a new water pump, hoses, fluids.. Have a 77 250 with a 400.
Here is what happens: When I start it up and let it warm up or idle for a bit, temp is fine. When I get in start it up and take off, over heats. Both starts are from a cold engine.
When it is warm I don't have this problem. I have burped it a couple of times, thinking there is air bubble in system..
Has anyone come across this before?

thanks!

 

Is it possible that under-way your water pump belt slips, but idling it doesn't?

First thing I thought of was "burping" also......I'm sure some one with more expertise will happen along and shed some light soon

 

I guess I never thought of that.. I am changing plugs this weekend. Will look for signs of Bad head gasket possibility?

 

Check out this thread - you may have the wrong thermostat - is it the right Robert Shaw?

https://www.ford-trucks.com/forums/1...ng-issues.html

 

Get the nose (radiator) up (higher that the tail) on some car ramps and burp it.

Have you flushed the block/heater core/radiator out?

The 351C/M/400 takes a special thermostat with a kind of collar around it. A 192-5°F thermostat is the one to use. Use only the Cooper-Robertshaw thermostat. This has been discussed many times in the 335 series engine forum.

Where to buy a 335 Series thermostat
3 options:
-Find a dealer with a RT-139 still in stock.
-Robert Shaw 333-180
-Napa THM 297 After calling a local Ford dealer, I was able to track down a dealer that has the RT-139.
It’s a dealer in Kansas but they will ship it to you.
Bob Allen Ford 1(800)676-0675

 

It is normal for the temp on the gauge to go a little over normal operating temp before the stat opens the first time... but just a little. Barely noticeable and not "overheating". When it overheats how hot does it get? Boiling? Then it sounds like it comes back down as you roll.

It should be well "burped" after a few heat/cool cycles. I'm thinking you just have a sticking stat. I bet a new one from NAPA will fix it. Make/year/model/engine should get the right one with collar for your 400... which is usually special order in a few days.