The OPs radiator looks like the proper unit. But I bet it's been visually restored. But is old and clogged. That's what mine was like when I bought the C2.

Cooling issues on the hwy means the rad is likely toast.:yesnod:

Don't dis regard this note by Toddalin, the impellers in the pump look so much alike that it has happened many times to many people. If I find one water pump with the reverse rotation impeller installed I would ask for a pump with a different date code to replace it. In manufacturing if one is made wrong the odds are the whole run is done wrong so start with another manufacturer or a different date code produced on another day to get you away from this issue.
You can rule this out by checking flow, harder than most cars to check on a vette, you can't just open the rad cap and look in at flow. I am thinking you would probably need to create a site glass or container that could receive coolant from your heater hose feed line and somehow tie back in and complete the circuit so you could watch flow while running your engine. Maybe someone has created such a tool already and can help?
Your statement about having a hot heater core does indicate you have flow but is it enough?
On engines I have dealt with that had head gasket leaks causing overheat you had both overheat and the radiator would pressurize blowing fluid out the cap even before the temp was in the overheat zone. You could make these drive fine temporarily by drilling a hole in the Rad cap to relieve the combustion pressure in the radiator.

What happens if you park it hot while running and run cool water over the radiator, maybe a mist from a spray nozzle? If you have flow in the engine the cool mist approach would quickly pull heat out of your radiator and your temp gage should drop instantly. If it does not drop you have a flow problem or blockage.

If a reverse rotation impeller is installed but is turned in standard rotation it does not pump coolant backward, it will flow coolant but at a much slower rate with far less pressure. Rather than cupping the fluid and directing it in the right flow direction it slices through the fluid more like a knife going through water compared to a spoon going through water.
That may be an exaggeration but I hope it helps understand what happens.

Lot of good ideas being thrown out here and while this can get very old in a hurry just hang in there and work through each of the cooling system elements. Assuming this is not a timing issue which sounds like you have ruled that out it comes down to the cooling system. I will throw my .02 in here if for no other reason than to reaffirm what others have already said.

I also think you have a water flow problem not a air flow problem as Doug has stated. If I was troubleshooting this I would first rule out any internal pressure problems using a pressure tester. Next I would remove the T-Stat and drive the car to see if this made any difference. Next on my list would be to replace the non stock water pump with the correct GM pump.

Keep good detailed temp notes using the IR gun during this whole troubleshooting process and post those numbers so we can get a feel for what’s going on and when. I look forward to staying with this thread as I like troubleshooting hard problems like this. For what’s it worth I don’t think that Griffin radiator is causing your problem if working properly which as you stated it seems to be.


Coolant Flow

The coolant enters the block through the water pump at the front of the block on each side, it flows through the block to the rear then up to the heads and flows to the front where it inters the intake, both sides meet where the t-stat is and flows to the radiator.

Great input everyone.

It looks like I have my work cut out for me to check off all these new suggestions. In addition, I had a new discovery last night. After going back and double checking the timing, I noticed that the Base Timing and the Total Timing fell close to spec as before. The part that stuck out is that there is no change when the vacuum advance is connected. That's the first step in the process.

Going back on your comments, here is my action item list that I will be starting tonight:
1. Replace vacuum advance mechanism and verify functionality. Test drive.
2. I.R. Gun different areas of the radiator to look for inconsistencies.
3. Replace water pump with one from Napa (OEM pump from Zip or somewhere will replace Napa part if pump is issue). Possibly run flow pressure test in addition. Test drive.
4. Block Test for combustion gases in cooling system.

Make sure your vac advance is hooked to full manifold vacuum.

Good find. I would stay on this until it is corrected. Be sure you are connected to full time vacuum AND you vac can is doing it job. Check the springs and weights in your distributor as well. Are they working freely? I think you are on to something here.

Dennis:cheers:

Also remember that when it comes to timing;

Initial + Vacuum + Mechanical = Total Degrees

When you change out the Vac Cann also check to make sure the mechanical fly weights under the rotor cap move freely and that you have a spring on each weight. I had a dist on a 74 Vette where the mechanical fly weights were rusted in fixed position.

Never mind, Dennis beat me to this.

The timing numbers I found on the forum per Corvette News for the 327/365 are:
10-14 deg at idol with vacuum advance disconnected and plugged
34-38 deg above 2500 rpm with vacuum advance disconnected and plugged
26-30 deg at idol with vacuum advance connected

I'm searching for pictures that clearly illustrate the proper connection location for full vacuum.

simple enough, just pull hoses off, with the engine at idle, until you find one that is sucking in air; this will work for your vacuum advance.
Bill

Do a pressure test ! It is free and easy . Unless you don't want to know if you have a crack or a bad head gasket.

Bill

As noted your dist vac should be hooked up to full manifold vacuum.
People are posting this because your Carburetor will have Manifold vac ports and Ported Vac ports.

Hook a Vac gage up to the available ports, at idle Manifold vac will pull the most vacuum.
Probably a constant 15 inches or so at idle, crack the throttle while running and Man Vac drops until the engine reaches a steady speed again and then it will rise again.
Ported vac is nearly zero at idle and increases with RPM.
Make sure you hook up to Manifold Vac. It makes a huge difference.

think of this as a speed vs heat transfer test; in reverse...

set the a/c fan in your car on high and feel the outlet temp; then turn it down to successively lower speeds and feel how the air temp feels colder. this is because the air flowing through the evaporator is slower and has more time to transfer the heat.
Bill

I think of the high flow water pumps this way.

GM spent time/money developing the best flow rate pump for the SBC. It's the same pump flow rate for all of them.

Why try to re-engineer the thing?

I have also never seen the lack of vacuum advance (by itself) in a midyear Corvette make any difference in the running temperature of the engine unless it's at very low speeds. And that's assuming the cooling system is in proper working order. If something else is wrong, all bets are off.

I don't know what's wrong with the OP's engine but I'm in with the group that says low or no coolant flow.

I suggested to somebody here, may have been Toddalin, to put a clear pastic section in his top radiator hose to check flow, lack of flow or direction of flow. He did it and found his problem, I think. One of those guys from the left coast.

You could do the same thing by pulling the thermostat housing, remove the thermostat and see which way the coolant flowed. That's kinda' messy but it'd work.

Mike
OP said the temp differential inlet to outlet is 20* (outlet being lower), so if it is flow related it would have to be a velocity problem
Bill

Yes, I know. I waited until post #56 to say anything as I'm not sure we're working off exactly accurate information.

This is what I'm keying on: "I start with a 5 min drive to the highway and cruise at 60 mph. 15 min after that the needle climbs over 210 deg. This isn’t an issue of simply running hot, because it will continue over 210 if I don’t stop".

In my experience, either his coolant isn't flowing or his cooling system isn't full. You really have to work at it to get an engine that hot, that quick.

Thats fine with a factory rad. On the other hand, if you install a larger capacity rad, the higher volume pump is fine.

Another thought. What would be the temperature differential if his water pump was not circulating anything?

Just gravity flow like a Model T?

Good question. I would think the temp differential would be higher though with the coolant sitting in the radiator that long, but that's just a guess.

I was thinking maybe the system is not full to capacity, but I would think any air pocket would have been burped out after a few heat cycles. If he pulls the thermostat, the coolant level should be right there at the t-stat.

A failed water pump would go a long way to explaining it, but I doubt the shaft would break. Could the impeller fall off the shaft? Belt so loose that it's slipping?

:cheers:
Gerry