Took my C3 out today. Noticed it was getting hot. Parked it and a bunch of radiator fluid came out of the overflow.

is it the radiator? Which is aftermarket.

water pump?

thermostat?

the oil was low so I changed the oil.

do I drain and put new radiator fluid ?

The radiator fan does seem to be turning on.

anything else I should look for?

 

What is your engine?
What aftermarket radiator was used?
Stock GM cap, fan, overflow tank?
Was the system recently filled (possibly over-filled without considering thermal expansion)?
Automatic or manual trans?
Do you have an infra-red thermometer handy to check the actual temperature?

An overhead pic of your engine compartment from wipers to radiator would offer a great baseline so we know what we're dealing with.

 

Let me add the following question. And of great importance. Overheating when going down the highway? When stopped at a long light or going very slowly?
Just changing the radiator fluid, (Coolant). Isn't likely to fix any problems.
You say your fan isn't running? What fan? Factory clutch fan? Aftermarket electric fans? If so what kind? How are they then controlled?

 

If you do a quick google search you will find overheating on C3 Corvette's is a very common subject. There are a TON of threads on this very forum related to running hot or overheating.
So, when it's getting hot makes a difference in what to check. And what type of fan or fans your running, what shroud and if electric fans how they are controlled all make a difference in diagnosing your issues.

 

You need all of the following in place for the radiator to work while you are driving...
Front air dam
All the seals around and under the air box in front of the radiator
All the seals around the radiator to radiator support as well as the hood

For the radiator to function you need a spring in the lower radiator hose to keep it from collapsing under vacuum from the pump. You also need a functioning cap to keep it under pressure. Higher pressure raises the boiling point enough that it won't boil over at 212ºf

For idling with a stock motor and shitty clutch fans you need the correct fan shroud and seals....they were never really any good on high compression and big block motors anyway no matter what any purist says....thats why they don't exist on any new car
For a modern performance upgraded engine you need dual electric fans and controller that will come on at preset temps especially in slow moving beach traffic on hot pavement.
For all of the above you need to flush out your entire cooling system to get the sludge and rust out. To do this you need to add a radiator/ block flush cleaner like Prestone, drive it for an hour the drain the block through the 2 block plugs...this is imperative to get all that 50 years of sludge out. Do this until the water comes out clear.... then you need to pull the radiator and back flush it to clean all of the now accumulated sludge out of it.

If you have an old small block only radiator, just upgrade it to the full size automatic/ big block radiator with 4 rows.

For all of the above situations....
Replace your old 195 thermostat with a 165 or 180 thermostat with at least one small hole in the valve so that you have some flow if it seizes. Thermostats control the lowest temp you engine will see, the lower the better for old performance motors. Again it won't run at 165 but the thermostat will be fully open earlier allowing better flow than a 195 which is only a few degrees from boiling.

My big block 427 with a 165º Stat stays at 195ºF on hot days all the time no matter how i drive it....cooler days it will run down to 165 or cooler....

 

Knowing things like:
Year, engine, timing specs, and when the overheating was occuring (at speed on the highway, or in stop and go traffic) will help.

A few pictures will mean less typing!

Everything else is speculation. It could be as simple as missing the spring in the lower radiator hose (go squeeze it, and report back).

 

SkidMarc2,

You never mentioned an actual temp reading on the gauge.

Generally:
1. Overheating at speed is a coolant circulation issue.
2. Overheating at idle is an air-flow circulation issue.

What the first statement means is, on the highway you have a 50-60-70 MPH wind thru the Rad. You have plenty of airflow. But the coolant is not keeping up: Why?
The second statement means the pump is likely ok, you have coolant flow, but you have little or no air flow to help out. Why?

 

seems late spring will kill thermostats.
my best guess is thermostat

 

Generally a sudden issue in cooling when NOTHING AT ALL was changed previously to the issue its

1. Thermostat stuck
2. Collapsed hose due to spring failure
3. It was timed last fall or early spring and on the first hot day you discover the motor doesn't like it
4. Head gasket issue
5. Plugged radiator or coolant passage

 

1968 Corvette Overheating --- CASE STUDY. Diagnosing Overheating Always Begins with Pressure Testing.

https://www.reddit.com/user/wadenels...dy_diagnosing/

 

not sure if the radiator or cap. Manuel transmission. I noticed it was getting hot when cruising around town. Got to about 220? Right at the red line on the gauge. I have had the car a few months. Didn’t do anything with the radiator.

I checked the motor oil was about 1-2 quarts low. Did oil change.

noticed the radiator fluid in the overflow was pretty dirty.

1975 C3 with a 327.

 

thank you. I’ll check the lower radiator hose. What should I be looking for when squeezing it? Should I work the car up?

I was just cruising around town. Doing 45 ish miles an hour. But there was allot of stop and go.

 

the fan was running earlier. Not sure what type of fan. I can double check when the fan turns on. What temp should it turn on?

 

Thanks for uploading the pic, it provides loads of clues to address.
I'm not versed in electronic fans and aftermarket radiators, so will leave that to others.

PS: Do you have any idea why it was so low on oil?
Why do you believe it's a 327 and not a 350?

 

Some articles say it could just be the cap. Seems ok?

 

Update. I started the car. Let it idle. And it got to over 200 degrees and the fans didn’t turn on. So I’m thinking thermostat?

 

I'm thinking fan controller. Also those fans look second best.

 

Once again I'll ask you. How are you controlling your fans?

 

Squeeze the lower rad hose. There should be a spring inside to keep it from collapsing. You'll feel it if is there.

It seems your problem is airflow. You are missing foam to force air through the radiator, and the fan you have doesn't seem to be working. I wouldn't use that particular fan, but let's start with the obvious.

In a C3, the "thermostat" controls coolant flow leaving the engine. Whatever controls your fan (could be a temp switch and a relay) isn't working, but that isn't the thermostat.

Thanks for the photos. Please post more!

 

There are various ways to control your radiator fans. From very basic to very high tech.
knowing what is wired up to run those fans is step 1 in trouble shooting why they are not coming on.
A very basic system would involve a basic temp switch installed in the intake manifold or even a probe in the radiator or a temp switch in a cylinder head. Connected to a relay. Relay would be powered by direct battery power often taken off the starter solenoid or even off the alternator, (Not the best but it's done). A bad temp sensor, a bad relay, a poor or corroded wiring connector, a defective circuit breaker or blown fuse. .....
The list goes on.
On my car my ECU controls the fans as well as a trienary switch on the air-con high pressure line.
There are many different ways to wire these up. From soft start to direct hard start.
Your car did not come with electric fans. So we can't look at the factory manual.
It's up to you to know how your car is wired.
Bottom line is. When your temp gets up to about 195-200 your fans should be running full blast.