The other day I drained my coolant to back flush the heater core; cut off the heater hoses from the pump & block; back flushed into a bucket with water and compressed air; put on new heater hoses; flushed the rad w/ clean water; installed a new drain plug on the rad; refilled with 1 1/2 gal antifreeze concentrate + 1 1/2 gal water.

I only ran the truck long enough to check the heater output and then started in on my "Nightmare Scenario" with the fuel filter in another post.

Now that I have the carb situation temporarily band-aided, I've been running the truck and the coolant is overflowing out the vent tube by the rad cap.

I'm not sure if it's overheating because my temp gauge doesnt work. The PO had an aftermarket temp gauge installed that didn't work either and I removed it (although the wiring is still attached to the temp sending unit on top of the water elbow going in the block).

Anyway, my question is: why would I all of a sudden be overflowing coolant? Would changing the coolant cause a problem w/ the thermostat? Is overflow normal after a rad flush?

BTW...I didn't mix the coolant & water. I poured in a gallon of coolant followed by a gallon of water, then 1/2 gal coolant, then 1/2 gal water. I figured the water pump would mix it. I hope that didn't cause a problem.

 

Its very possible your airlocked. Meaning you have an air bubble so the water pump is not circulating the way it should.

 

You mean that from me blowing air through the core? I didn't think of that. When I filled the rad I assumed coolant would run through everything and fill it back up.

So, what to do? Will it correct itself?

 

You do not fill the radiator all the way to the top. Unless you have an overflow reservoir. Leave coolant level 1" below the seating surface for the cap.

 

Maybe that's all it is...I hope. I don't have a reservoir and I did fill it to the top. I have to get my temp gauge working to see what's going on.

 

You typically get an air lock just from draining and refilling. There are many crevises (sp?) in the cooling system that will trap air. Idle the engine with the rad cap on but not tight. When the engine warms up check to see that the thermostat is opening. Once it is hot the thermostat should be open, you can SLOWLY take off the radiator cap, I do this with the engine off and a towel so I don't burn myself with hot fluid. Then with the engine running you can look into the radiator and see if there is turbulence on the top which means the thermostat is open.

 

I would take a hard look at the cap. I am kinda **** about guages and would install a working guage before I drove it though.

 

I followed your advice...ran up to Pepboys and picked up a Sunpro temp gauge. After warming up, a couple laps around the block and 10 minutes of idling in park, the temp never went above 150. In fact, when I drove it the temp went down to 110-120.

I did what you said and it looks like a river flowing past the rad cap opening. I assume this means water is flowing through the pump and block.

What's the normal operating temp? I thought the t-stats for our trucks were 180 degree or 195. Doesn't that mean the t-stat OPENS at that temp to allow flow? Since my coolant seems to be flowing at 150 I'm not sure what the 180-195 t-stat means.

 

Could be you have a 160 deg. thermostat. The temp is stamped on the thermostat.
Without an overflow tank, if you fill the radiator to the top cold, when it warms up the coolant expands and the excess goes out on the ground.
Cars and trucks now days have coolant recovery systems. The same thing happens but the coolant recovery tank captures the excess, then when the coolant cools down and contracts, it gets sucked back into the radiator.

 

I would install a 195 degree thermostat. They are cheap and easy to install. It wears an engine rapidly if it runs to cool.
I am slightly suspicious if the gauge is correct.

 

I would be cautious of that gauge as well. IF the gauge is correct you have no thermostat or it is stuck open. Many people have their own opinions about what t-stat to run, some choose 160 some 180 some 195, I myself go the 160 route and don't care what anybody else says. However there is no way you have a functioning thermostat and your engine is running at 110 deg, not even at 10 below.

 

So, if the thermostat is operating correctly, then when cold it is closed allowing no coolant to flow through the block.

When the engine temp reaches 160, 180, or 195 (depending upon t-stat) then it opens to allow coolant to flow...correct?

The gauge's probe is inserted in the top of the t-stat housing, so the water temp is being measured there...would that make any diff?

IF the t-stat is stuck open and the water temp reading is accurate, then obviously it's not overheating. But what temp is considered over heating?

 

I would be cautious of any temperature over 230 degrees.

I would get a 190-195 degree t-stat and install it in your truck.

When you do you will know if the old one is stuck open or not there at all.

The best location for the temp probe for the gauge is in the intake manifold or the cylinder head.

Dan.

 

I'll do that.

Where exactly is the stock temp sending unit on a 78 400M? I replaced the old non-working probe that the PO had installed on the t-stat housing.

 

Yeah, the thermostat stays closed until the minimum temp is reached. The thermostat controls the minimum operating temp.