I began driving my 76 2wd w/ 390 on an errand yesterday. The truck has a relatively new radiator (new about 5 years ago, no more than a couple thousand miles). It has no shroud as there have been two engine swaps over the years of different type motors and the old shroud broke a long time ago. Instead I've been running an electric fan that is hooked up to a Honda thermostat on the bottom of the radiator. I have an Isspro electric temp gauge in the 4V aluminum intake manifold. The truck in recent months of occasional use has been pretty stable with temp, generally running 190.

So yesterday after running for about 20 min and on the freeway, I smell a coolant smell. I look at my gauge and it reads above 200 and climbing. By the time I get it off the freeway and somewhere I can park it, it's above 230 and the engine is smoking. I never saw any water come out of the radiator cap and the fan never came on. Upon looking at things, I see that a freeze plug that had been installed two years ago by the carb shop that installed my intake manifold was popped out.

So today I clean up the hole, coat the new freeze plug with a thin layer of black RTV and attempt to tap it in. I can't get any real good whacks on it (It's the plug just forward of the starter) however it looks like it's maybe a 1/16" recessed into the block and even. I cross my fingers and fill it with coolant/water to see what happens and begin warming up the truck. While I was eyeballing the freeze plug I see the coolant beginning to overflow the radiator so I cap it. I look at my gauge to see that the temp is about 210 and climbing. I shut it down to investigate. The fan isn't on, and the radiator doesn't seem hot to the touch.

So my theory at this point is that the thermastat didn't open and the coolant is getting hot in the block but not getting cooled by the radiator. The fan isn't coming on because the coolant in the radiator isn't getting hot. The freeze plug blew out from the heat pressure.

Sound right? Also I notice some tiny drops of coolant making out the edge of the freeze plug? Inadequate, or run some RTV around the outside margin between freeze plug and block?

 

It sounds like it may have an air pocket in it... that can cause it to heat quickly. Also possible the T stat is stuck. I't let it cool off then top up the coolant and try again. If that doesn't do it, pull the stat and try another time to see if it helps... but don't run long without a stat.

As far as the freeze plug, I don't think RTV on the outside will help... won't hold the pressure. I believe the hardening type sealers like Permatex #1 work better with freeze plugs vs RTV... need to try another new plug.

 

I pulled apart the T stat housing. I forgot what temp stat I used. Recommendations?

Also, a mechanic neighbor was telling me about expandable freeze plugs that have some kind of screw in the middle that causes rubber to expand into the hole. He says he used one and had no leaks for several years. I do have good access to tighten such a thing.

The system had been working fine for a couple years, so I'm inclined to believe stuck T stat over air pocket, in any case I've already pulled the T-stat housing apart.

 

If you want to see if yours is working correctly, you can heat it in a pot of water with a known good thermometer. It should open completely near the rated temperature.
The Ford parts catalog shows 3 possibilities on thermostat temperatures for the 1976 390 engine: 160, 180, 190. I've got the 460 engine in my '76. It came with the 180; but I've used the 160 successfully for years. There might be a bit of efficiency loss, but it's easier on the engine & oil.

 

T-stat rating is usually stamped on the t-stat flange.

Use red loctite for core plugs. You neighbor's recommendation to use a rubber plug is valid but only as a temporary fix. A long-reach tool with a ball end and properly sized core plates are available to drive in a core plug.