Everyone who has a cooling problem should at least calibrate their temperature gauge. They can be waaaaay off!

Calibrate the temperature gauge:

- Remove the sensor and put a pipe plug in the hole.
- Immerse the sensor in a vessel of water along with an accurate thermometer. I used an industrial grade thermometer and a large coffee can. Both the thermometer and sensor should be suspended so they don’t touch the walls of the vessel. Use coat hanger wire or something to make a hanger.
- The process here is to heat the water slowly and monitor the temperature gauge in you vette with the engine running. You want the engine running so that alternator voltage is the reference voltage for the gauge just like it would be if you were driving. It’s higher than battery voltage and will make a difference in your readings. I used a coleman stove on a little table next to the drivers side fender. I also made some electrical wire jumpers so that the sensor wiring would reach the sensor in the can of water.
- Heat the water up. When the vette temp gauge needle lines up with a calibration mark on the gauge, record the thermometer temperature as this is the true temperature of that calibration mark. You should take two readings. One sitting in the drivers seat normal driving position which will include some parallax error because your looking at the gauge from an angle. You may also want to record the reading looking perpendicular to the gauge for future reference and overall accuracy.

My 79 vette has the first calibration marked as 100 deg and the third calibration mark as 220 deg. There is also a second calibration mark half way between the 100 and 220 deg marks with no temperature indicated. The next mark above 220 is the start of the orange danger zone. All units of measure here are in degrees Fahrenheit.

The results were:

Gauge reads 100 Thermometer reads 130
Gauge needle 1/3 way to second mark Thermometer reads 160
Gauge needle 2/3 way to second mark Thermometer reads 170
Gauge needle at second mark Thermometer reads 180
Gauge needle 1/2 between second mark and 220 mark Thermometer reads 185
Gauge needle at 220 mark Thermometer reads 204
Gauge needle 1/4 way to danger zone mark Thermometer reads 212

I’m at sea level so 212 is as high as I could go without pressurizing the vessel. Note that my gauge reads 16 deg to high at the 220 mark. Yours may or may not be so bad. I also did a similar thing with my 160 deg thermostat and found it started opening at 160 and was fully open at 170 deg. I can now look at my temperature gauge and know when the thermostat is starting to open and when it is fully open. This can be useful especially when setting the thermostat on your electric cooling fan. With 95 deg ambient, AC on,

 

In the simplest terms for brass radiator.

Stock normal 68-72 SB system has a radiator with no fill provisions. You have a hose from the water pump to the heater core that Tees into the base of the expansion tank. You also have a hose from intake to heater core. (I may be wrong about which hose it is in but you get the idea). The expansion tank has a small hose that connects to the small port on top of the radiator and it has a small port on the filler for a vent to atmosphere. The expansion tank is pressurized.

Stock later setup has a conventional fill on the radiator, hose from intake to heater core, and heater core to water pump. The filler has a small hose that leads to an overflow or recovery bottle that is not pressurized.

So if you get a Be-Cool, Griffin, or DeWitts aluminum radiator with a filler on the radiator you can remove the expansion tank and use an overflow bottle. Just replace the big hose the tank is in with a straight hose to the heater core. The only recovery bottle that really fits a 68-72 due to inner fender contour is one for a 73 only that are very hard to find and are not being repro'd. You can use some sort of universal type or a 74-77 (which kind of fits) with the small line from the radiator fill connected to it.

Hope this clears it up a bit.

 

Joe,

The 1968 small block cars came with two different radiators.
If the car was ordered with any one the following three items, the radiator was a copper/brass model.
Air, Auto, or optional engine other than base 300hp.
This radiator was a 26" core size and it had a filler cap on the radiator. It did not use a surge tank!

If the car did not have any one of those options, then it recieved the aluminum radiator (3155316) used in the mid year cars. This radiator did not have a filler cap, and used the separate surge tank for filling. The surge tank has two 3/4" tubes on the bottom and these are spliced into one of the heater hoses. It also has a 3/8" connection that runs to the radiator. This line is pressurized, and not to be confused with an overflow from the filler cap.

Now, the first thing that jumps out at me is you seem to have two filler caps. You said you "look down the radiator" so you must have a filler cap on the radiator and the surge tank has a filler cap. Something is wrong. Either the car has the wrong radiator or the tank doesn't belong there.

So many times I see people "Add" the surge tank to cars that shouldn't have them. The tank was designed to be pressurized and allow the radiator to be completely filled. The tank was filled 1/2 full to allow for expansion, thus the term expansion tanks. If the radiator has a cap, and the drain from the cap feeds the surge tank, then the surge tank is not pressurized and it bascally doing nothing.

Now for the fluid level mistery. Since the two hoses on the bottom are cut into the heater lines, the tank is part of the system. When you fill the radiator, that fills the block, and that fills the heater hoses, and that fills the tank.

When you rebuilt your engine, I bet the bank you replaced the old GM temp sending unit and bought a brand new one. Most of the replacement sending units read high. 20-30 degrees too high. I'll bet you also used some pipe sealant or tape on it because the gauge goes nuts at times.

Find that old temp switch and re-install it with nothing but anti-seaze. Buy or borrow an IR gun and check the thing out. It takes 5 minutes to confirm readings. If the radiator has a filler cap, as it sounds, take the surge tank off, it's doing nothing.

Here's the two radiators for this application for reference:

Aluminum rad with surge tank

Copper radiator with no surge tank

 

Thanks Tom, I think I had the right idea for him, but the wrong details. Thanks for straightening it out.

 

There is a reason why I go to this forum with all of my problems and that reason is because this place is jam-packed with knowledgeable, intelligent professionals that fully understand corvettes. All of the individuals that responded to my posts had an impact on the way I plan on approaching this problem.

Here is what I have absorbed from this particular post.
First of all, my car is 350/350 model (HT) corvette with a manual transmission and I have the correct radiator, which is a BE COOL model very similar to Tom’s second picture. (http://www.dewitts.com/pages/product...asp?ProdID=280) This radiator does have a filler cap, so the expansion tank is not needed. The smaller hose that goes into the expansion tank should now go into a non-pressurized overflow container and the hose that the expansion tank splits or Ts into should remain the same without the split.

As for the original temp sender, it is long gone, so I guess the only option is to buy a new one from http://www.lectriclimited.com/mainpage.htm. After getting it, and before installing it, I should calibrate it to the gauge and make mental notes.

I think I have most of it, but I still have a couple of questions. First, what thermostat should I run? Right now I have a 160, but it seems to run better with a 180. What do you guys recommend? Does running a higher thermostat make the car overheat faster? Also, should I get a high flow thermostat? Mr. Gasket, as recommended by one of my co-workers, or BE COOL, who also makes one?

Another question I have is about caps. What pressure cap should I put on the radiator? I have heard many myths about this. Some people say that a less pressurized system runs cooler. Others have said that a more pressurized system is more reliable because it boils higher. Right now I have an 11 PSI cap on the radiator (I think). I have no idea what the pressure is on the expansion tank?

Lastly, I am going to get the original hose for that radiator. Do you think it will fit? If not, what should I do? Flex hose?

Thanks for all of your past and future assistance,

Joe

 

180 stat seems to be the choice for a high performance street cars, keeps the car at a good operating temp that balances the need for cool intake charge and warm eng/oil temp for better lubricating (opinion from reading not from personal knowledge). A stat temp will affect the cars operating temp other than keeping it at the rating, once the car temp climbs above it the stat is wide open, having a lower stat might delay the car getting warm/hot but the car will reach what ever temp it will no matter if the stat is 160 or 180 as long as the final temp is above 180.

Higher pressure raises the boiling point of water/coolant which allows it to transfer more heat before turning to a gas so as long as your system is up to snuff you can run a higher pressure cap, I use 15 lbs. You can buy expensive low pressure coolant if you like, don't recall any brands off the top of my head.

 

Where "in the water pump" is the sensor? I believe that this might be where your problem is with running hot at a stop. The water pump is usually one of the coolest points in the cooling system. Change the location of the fan sensor to the top of the intake manifold by the temp guage sensor and try that.

 

Jim,
The temp sensor for the fan is located on the top of the water pump. I have a performer RPM intake and I think that it only has one inlet and that is used for the temp sensor for the guage.
I could put it in the head, but I have heard several horror stories associated with that.
Thanks,
Joe

 

Joe,
That spot on the water pump is on the "suction" side of the pump and is reading the temp of the coolant coming out of the radiator. As many "horror stories" as you have heard of putting the sensor in the head, THAT is where you would WANT to have your fan sensor. That IS the hottest part of your cooling system.

Jim

 

Put the fan sensor in the head. That is where it is for most SB chevys anyway. I would use a 180 T-stat. I like the Robertshaw brand for aftermarket. Last I checked they were discontinued from GM (service replaced by the 195). A 180 stat should not make it overheat faster. It should warm up a bit faster, but above 180 should not matter if it was 160 or 180, it should flow the same. I would use a 15 psi cap.

 

I have my electric fan sensor in the goose neck thermostat housing. Seems to work ok. Fan comes on at the correct temperatue, at least according to the stock temperature gauge.

 

Larry,
Is that a stock thermostat housing??
And I noticed that you used a perma cool fan, do you like it? Does it do the job?
Thanks,
Joe

 

DeWitts sells them (they're a supporting vendor). www.dewitts.com

 

Not the stock housing. Purchased an 'O' ring chrome unit from Summit that had a place to insert the fan temperature probe.

Fan seems to do the job. Never above 195 - 200 when in traffic or running in the garage when working on it.

 

Is it possible to send me a link to that part. I can not seem to find it in Summit.

On a separate note, what do you run for a thermostat? 160? 180?

Thanks,
Joe