All good information, but you probably have a bad radiator cap. BUT>>>once you change the cap and the system holds pressure you may find that the hoses or heater core may start to leak. Another issue is that the original owner was putting straight water in it which may cause more rust to form and corrode the radiator and the heater core....If it were me, and it was, I would...

• buy some prestone radiator flush and a new 15 psi cap and a new 165* thermostat
• add the flush, thermostat and the new cap and drive it around for a couple of days, drain the hole system from the block drains and the radiator petcock. look at the color of the fluid. it shouldn't be orange but probably will be(from the rust and crud)
• Refill with distilled water and do it again until the water is coming out clearish.
• pull the radiator out and back flush it until only clear water comes out. You can use a garden hose and some rags stuffed into the lower hose outlet. If you cant get the radiator out you can do a somewhat decent job in the car but your going to get wet.
• or drop some coin and have it done professionally buy a shop that specializes in radiator and cooling system back flushing
• Once the radiator system is straightened out Add a good 50/50 coolant, either bought premixed or concentrate, its your buck
• No test the clutch fan, it should spin freely when cold and be stiff with the engine hot. replace the clutch if it is the opposite in either instance......
• If you are prone to drive in stop and go traffic like congested highways, downtown traffic or cruising the beach, consider a new electric fan system.

1. It will require a good controller
2. wiring from the battery to supply it...I added a new wiring block on the fire wall and came off the starter lug for main power and a relay from a keyed power source for the trigger
3. a higher output alternator
4. wiring from the alternator to the starter lug for the alternator output if you dont already have a minimum 8 gauge wire
5. a temp sensor, depending on the controller make is where the sensor would be mounted. My Derale adjustable controller has a sensor that is mounted to the outside of the radiator tank surface while others use a wet sensor in the block or intake.

the horsepower savings is about 20 hp but the cooling difference at low speed is incredible. You have full fan power on the electric fan while the stock clutch fan would only be spinning at idle speed...I saw incredible cooling differences

 

I have a 73 big block 454 and an auto trans.
A couple of yours ago I was coming home from a car show and got stuck in bumper to bumper traffic on Hwy.
The outside air temperature was 112 degrees and I watched the temperature gauge stay in the red and it even pegged a couple of times.
As long as I was moving, even 10 mph the temp stayed around 190-200.
Check every thing in the cooling system as suggested above and one more thing is to make sure there is a spring in the lower radiator hose.
Also, get a copy of forum member “LARS” timing papers and verify your timing is set correctly.
Timing not set correctly can greatly effect the operating temperature.
When you refill the coolant, it is mandatory to burp the system and make sure you get any trapped air out because any air in the system will cause the system to overheat.
I’ll be updating everything in my cooling system this year so I can sit in traffic if I am ever stuck again.
One thing I would also like to mention is after a long drive sometime, just pop open the hood and you can get an idea of how much heat builds up under the hood of a big block Corvette.
There is little area for air to circulate so everything must be installed correctly and working properly to keep the engine coolant temperature at normal operating temps.
Will it get hot in traffic, yes.
Will it overheat, not if everything is correctly installed and working properly.
Did you hurt your engine, probably not.

 

Many of the guys touched on these things but I’m also going to iterate it again.
1. This is a 50 year old car and your gonna have 50 year old car problems.
2. Trust nothing the previous owner said. Things break on these cars just because. See item 1.
3. If the radiator is original you need to find an old school radiator shop and have them boil it out and check it out. It’s very possible it’s plugged up with various debris/scale. It’s also very possible when they clean it out you’ll find it’s junk.
4. You need the proper shrouds to pull air through the radiator. If you don’t it won’t cool right.
5. It’s very possible you have a bunch of debris is the block as well particularly if it’s had just plain water in it for a significant amount of time. If you pull the radiator pull the pump too and see how much debris you can flush out of the block with pressurized water and compressed air (wear glasses). You can try to pull the block drains but I’m not that brave TBH. How bad can the build up be? I’m rebuilding a 1950 Ford 8n my grand dad bought new. When I put a new head gasket on the motor I decided to pull the petcock out of the block to flush everything. After I pulled it no water flowed at all and I had to knock loose probably 1/4” of crud to finally get things moving. There was a lot and it was thick.
6. Thermostats are cheap if in doubt replace it.

 

7. If the water pump is really old, the vanes on the impeller may be rotted off and it won't pump much water anymore.

 

I saw the same differences at low speed and idle. Coincidentally this is also where I saw the temps creep up to the highest point before the electric fans...

Also derale now uses a wet sensor instead of the one that pushes between the rad fins and you can put the new style /head temp sensor on the old controllers. I just did this myself. Its a $17 ebay upgrade for the new derale sensor if you have an older controller. https://www.ebay.com/itm/313390006282 You do have to cut the old sensor wire and splice the 2 wires to the new sensor as the plug is missing on older models.
I went farther and used the derale controller to control a pair of external 40a relays for my fans rather than put the full fan load on the internal controller relays which from the online reviews, are very prone to failure that way. As ive mentioned in other threeads I used a cheap chrome plated 105a 10si alternator and with the beefed up wire you mentioned I still see 13.5-13.8v on my volt meter at idle with both fans (30amps total), high beam headlights and wipers going and stereo amp going which tells me the alternator is still charging the battery with the reserve being made at ilde in this high load situation.

 

you can buy a 100 amp stator to install in an existing 10si. no brackets, no wiring plug mods. keep your numbers matching alt frame...

 

Ironically I just saw this pop up in my facebook feed today... They even sell 140 amp and beefed up 140a kstator kits for 10 and 12si alternators. They say the heavy duty kits provide more power at idle. kits I saw were between $80-$130 though.

 

I just ordered one from Amazon...thanks

 

Thank you very much for all the very helpful suggestions!

I'll try to make sense of it all and get this sorted - in the meantime, I made some pictures, not sure what the shroud and seals should look like.

So if you see something that looks wrong below, I'd be happy to hear it.

Again thanks a bunch for all the helpful info. I'm joining some clubs here and I'm amazed how helpful the community (you guys here and also the locals) are!

As someone said, this is an almost 50 year old car, there will be trouble regularly, but that's fine. Part of the reason to buy it was to tinker, and it seems like on that account, there will be no shortage. :-)

 

thanks for the pics.
you should look for some stock original fans and housing pics for comparison. Someone replace your clutch fan with a flex fan, you want to to get rid of that, and it looks like they cut the shroud up presumably to get the fan in and out easier. ITs hard to tell with the lighting in the pics. The clutch fan would stick out further into that oopenning

The easiest way to remove the fan and the housing is to remove the hood and undo the radiator support bolts on the fenders and tile the hole thing toward the headlights....

So what is happening at low speeds is you fan is just sucking air from the side and throwing it at the motor, without a complete shroud it wont suck air through the radiator itself. So luckily some reasonable fixes to get you cool again

 

similar to this

pic from the internet

 

Those pictures really tell the story. I would bet the family ranch your problem will be solved when you install the correct fan blade, fan clutch, and shroud.

 

in your profile contry is ch. i believe that is switzerland. so, at least you have a swiss bank account! your biggest problem is the missing lower 3/4ths of the shroud. one design flaw i feel is they should have tilted the radiator forward, not back. but very little air will go thru the rad since the fan is sucking it into the big gap behind the radiator. this is a 75 shroud. different but nearly identical in dimensions. you can see the big block-off plate on the bottom of the shroud to force the fan to pull through the radiator. in your pic looking down there is just garage floor. you could possibly cobb something up to close off that air leak but getting the correct shroud, or an electric fan setup with shroud. may be cheaper to scare up a junkyard fan and shroud setup than trying to import one from the states. this is a 30 buck FB shroud BTW.

 

Some one cut up and removed a good portion of your shroud and installed a solid mount flex fan.
It was a big deal back in the 70’s to remove the clutch from the fan, replace it with a solid mount and install a light weight aluminum or fiberglass flex fan.
I can’t tell from you photo, but is the front lower air dam (chin spoiler) still in place or has it been removed?
Replacing the fan and shroud would help a lot with the cooling.
If the front air dam is missing that will also add to your low speed cooling problems.
Make a plan, save your pennies and do it right.
If you are having to replace the shroud, fan and clutch, now may be the perfect time to upgrade to dual electric fans and new shroud set up.
Look at DeWitts Radiator website to see their package deals on radiator set ups and replace the factory radiator, fan, shroud with a new and improved system.
There are cheaper packages available that may work, but DeWitts is the best.

 

Might be easier at this point by going with dual electric fans.

 

I'm running twin electric fans on my car. And yes it may very well be easier to install twin electric fans than to source the factory replacement parts.
But there is a bit to it. A larger capacity Alternator will be needed to keep up with the amperage demand, along with the upgraded wiring necessary to support the increase in current flow. And of course the proper wiring and switching for the fans.
Trust me I know. I put twin electric fans in my car 15 years ago. I think shortly before I joined this forum. I think I made every mistake in the beginning.
here's what I have now, extra panel mounted on engine side of right footwell.


The extra large looking cable is the positive Battery cable heading to the starter. the smaller heavy cable, (about the size of a Harley battery cable) is a cable from starter solenoid to strip fuse panel, cable is capable of handling over 250 amps and is only about a foot long. In panel, 100 amp strip fuse handling the charging, (Alternator, 140 amp S10 style). 60 amp strip fuse handling power to both fans run through a 60 amp relay temp controlled by my ECM. (yes I run aftermarket EFI). third fuse is power to my headlight relays. This type of electrical upgrading is necessary if you want it reliable. And remember, better to use wire that is way to heavy than to thin. grounds included.

 

I would not drive that car further than the end of the driveway until you sort out the cooling system.

 

If it were my car I'd do the following first:

• Make sure there are no leaves or debris in the radiator fins by spraying water from the back towards the front.
• Since you have air conditioning, you also have a second "radiator" mounted in front of the cooling radiator. Make sure there is nothing blocking the flow through this one or debris collected between the two.
• Buy the appropriate fan clutch and fan. This set up will place at lease 1/2 of the fan blade depth inside the shroud's opening so that it's definitely pulling air through the radiator rather than around as you have it now.
• Make sure you have clear openings below the chin of the car and the short "air dam" in place that directs air upward as you drive into the large openings in the chin.

See if these changes have a positive effect. Replacing the fan shroud is a beast of a job so if you can merely install the correct clutch and fan assembly to alleviate the issue that's the best solution and leave the shroud as is.

 

i don't usually disagree openly with people on here, but most of his shroud is gone. great big opening behind and below radiator so virtually all fan air is pulled from below the radiator. a cheap crappy dangerous flex fan will pull air through a radiator at idle. the fan has to leave because it is a hazard and power hog at rpm's. but his lack of a shroud is where the most of the overheat issue is coming from.

 

The only part of the shroud that I see missing is the chunk from the right side around the circular fan opening. The long lower section between the radiator and frame crossmember looks intact to me. There is a 2"-3" wide bolt-in extension that runs along the bottom of the '72 shroud that appears missing if this one should have had it.