1979 400, bone stock. Over the last two years, I've replaced the radiator, both radiator hoses, the thermostat, and the water pump. I flushed, or thought I flushed, the cooling system. No matter what I've done, I've had issues with coolant backing up and coming out the overflow. I started to think that maybe I had trapped air in the system, so I bought a radiator funnel and filled it up. I started up the engine, with the vent controls set to hot defrost to make sure water went through the heater core, and watched various points on it with an infrared thermometer to figure out if it was overheating or what was going on. A few minutes after it went to slow idle, the fluid level in the funnel suddenly increased rapidly and it started spewing nasty green-brown junky water all over the pavement. (I'm sure my neighbors must love me.) I shut it down immediately, water kept flowing for a few seconds and then it stopped. After about 30 seconds, it quickly sucked all the water out of the funnel, and took another about 3/4 gallon of fresh coolant after that. I started it up again, and once again, after about 2-3 minutes at idle, the water overflowed again. I took temperature readings, and here's what I had:

Cylinder heads 200 °F
Thermostat housing 160 °F
Upper radiator hose - cold (75 °F)
Radiator cap - 180 °F

Ordinarily I'd say that this sounds like the thermostat isn't opening properly, but it seems to me like it isn't even getting to the temperature where it should open at all. Also, from previous runs when using the radiator cap instead of the unpressurized funnel, I know the thermostat opens and the upper hose gets hot and stiff, like it should.

 

i'm afraid it sounds like a blown head gasket or a crack.

 

You do have the correct thermostat in there right? Robert Shaw 333 or a Stant 13468. If not it will circulate most of the water in the block some will overheat a lot some a little but all will overheat with a Windsor thermostat in there.

 

Are the head gaskets installed the correct direction on the engine? they have a square corner that's supposed to stick out under the bottom front corner of each head. If that's in the back on one or both banks it can cause this sort of a problem.

 

I had a 400 doing the same thing. The owner had tried all that and more. Finally I decided to knock a back freeze plug out that was all rusted. It took a while to get it out because it wouldn't spin around in it's hole or even drive through. Turned out the block was packed solid with rust and crap clear up to the front plug. After some digging and flushing it ran cool again.

 

Also a gates 33128. I bought a 180 and a 195 I put them in the pan of water an brought it up to temp an both opened as should. The 351 c and the 400 thermostats have a cone/cup on the bottom that when open closes off the water bypass. Some people plug them also. Good Luck. I have also put on an electric fan and mine runs ok. I don't have the hood on yet or the front outer fenders but seems to do alright.

 

Dave McLain, you are exactly right on that. I had the same problem and did find I had put a head gasket in upside down like you described.