Hello everyone, my first post here. Let me quickly give you the basics and then describe the problem. Sorry the post is so long but I wanted to make sure I gave you all of the details.

Four months ago (October 2018) I purchased a 1982 Corvette with 20,201 miles on it. It was carefully stored in a climate controlled garage for a considerable period of time (my guess is at least 25 years). This car is bone stock. There have been absolutely no modifications to this car whatsoever (it was Bloomington Gold certified in 1990 and was probably placed in storage shortly thereafter). I live in North Florida.

After purchasing the car, I found that it has its share of issues (power antenna won't work [not due to broken nylon cable since motor does not run when power applied to it], cruise control won't work [now fixed-bad transducer], oil pressure gauge reads too low [probably due to bad resistor on back of gauge], and bouncing fuel gauge needle [probably due to loss of dampening oil in gauge]). But there is a significant problem and I am baffled and could use some good advice from you.

As many of you probably know, the late C3 Corvettes (probably from 78 to 82) have a vacuum operated coolant flow shut off valve. When the temperature lever is moved all the way to cold, a vacuum valve is actuated to turn off the coolant flow to the heater core. Moving the temperature lever just off of cold opens the flow of coolant to the heater core.

I found that when driving (engine at operating temperature) and the temperature lever is moved all the way to cold (turning off coolant flow to the heater core), my temperature gauge starts climbing quickly (in about 60 seconds) into the red range on the temperature gauge. However, if I move my temperature lever just off of cold (so that there is flow of coolant to the heater core) the temperature returns to normal (just under 200 degrees - based on the temperature gauge in the car) and stays there. As long as the temperature lever is not all the way to cold, the coolant temperature (according to the gauge) remains normal, even when I let it idle for long periods of time or I drive it in stop and go traffic. But, as soon as I move the temperature lever all the way to cold, the temperature starts to climb to the red range.

My first thought was that there was not enough flow of coolant through the radiator and the heater core was acting as a surrogate radiator.

I found that the radiator is original but had a significant amount of scale build up. I removed the radiator and had a local reputable radiator shop "rod out" the radiator. I installed the refurbished radiator and added new, clean 50/50 mixture of antifreeze (green) and water (not distilled though). This did not fix the problem.

I replaced the thermostat with a brand new AC Delco thermostat but this did not fix the problem.

The water pump is the original water pump. After discussing this with Bill Mock (mockauto.com) he told me that water pumps either work or they don't. Because the temperature remains normal with coolant flow to the heater core, I conclude the water pump is still good (despite its 37 year old age).

It has been suggested to me that my problem is with the gauge itself. I suppose that could be but I have my doubts. I plan to test the temperature with an infrared gun just to make sure it is not the temperature gauge.

I did not change the radiator hoses when I had the radiator out of the car (I know, foolish). I suppose the bottom radiator hose may no longer have the spring in it and perhaps that lower radiator hose is collapsing and restricting flow - but I have my doubts about this as well. I will replace both radiator hoses.

So, other than the radiator hose issue and a possible fault with the temperature gauge (both of which I doubt is causing the problem), can any one offer an explanation why the positive shut off valve to the heater core would be causing this problem? Any other tests I should perform? I welcome all of your comments.

 

 

that is curious.
barely cracked heater valve opens the valve but limits the temp blend door in
the cabin.
so not much air is flowing across the heater core.
while i have used the heater on various cars to increase cooling, in your case i don't see a great deal of "cooling" happening in the radiator system.
i guess your temp sender and or gauge have a real narrow temp window and
just a few degrees can send the gauge to a fake reading.
a good idea to verify with an IR reader.
good luck and let us know what you find.

as far as pumps, they fail by leaking mainly. they can get corrosion and
lose fins but that is extreme.
valve open could release an air bubble and cause an accurate reading but
once the air is gone it should not recur.
guess there could be a slight gasket leak causing air to be pumped into system
and the open valve releases the air blockage.
hope it is a faulty gauge.
good luck

 

If your are were in my shop for this problem.I would remove the temperature sending unit and put in my racing temperature gauge set-up that does not use resistance to ground sender to make it work. Then run the car and see what happens.

Knowing that your hot water shut off valve for a 1982 is getting the coolant supply from the hose fitting on the back of the right side of the intake area by cylinder 8. I cannot see how that shutting off the coolant supply to the heater core would cause to the temperature sender which is mounted in the cylinder head by cylinders #1 & #3 would cause the coolant temp to rise that fast.

DUB

 

IR thermometer, before you do anything else, see if that reading is real.

I'd try the suggestion in Post 2. I did this in both cars, after eliminating the leaking heating cores. The easy way is to add a 5/8 to 3/4" adaptor, and cut the two hoses short, and link them behind the AC compressor. This provides a path for coolant to flow when the thermostat is closed. This does something similar to drilling a hole in the thermostat, but perhaps gets around an air bubble that is stalling the water pump, or causing a locally hot reading on your gauge.

One other thing happens when you move the selector all the way to "cold". A vacuum is applied to the coolant valve, shutting off flow through the heater core (at least, I think it is a normally open valve, and vacuum is applied to close it). Is it possible that there is a vacuum leak in that system that is causing other issues? If the vacuum sensor for the Crossfire sees a vacuum leak, could that also cause retarded timing and overheating? I don't actually know, but I'm equally hard-pressed to imagine that your garage would incorrectly fill your engine with coolant, but they didn't add de-ionized water, so anything is possible.