Vehicle is a 1974 Corvette, SB with stock water pump, 190 t-stat and aftermarket elec. fan. All parts/hoses are correct GM and new.

Question is...after the vehicle reaches operating temperature, the temp guage will hover around 200 and then shoot rapidly up to "red". As a test, I've driven the car for appx. 1 hr. with no apparent ill-effect. Radiator didn't boil over, no "overheating smell", no drop in oil-pressure.

If my car was overheating so severely, wouldn't I know it? I'm hoping the problem lays with the temp guage, but I'm not sure. (Temp sender has been replaced with the correct model designed to work with the stock guage).

Thanks in advance,

D. Ocean
Miami, FLA

If the temperature readings are correct...

First choice would be radiator needs rodded out or replaced...

Second choice would be that engine compression is making it to the coolant passageways [no matter how slight] causing hotter operating temperatures...

B. Martin
Anytown, USA

Sounds like the gauge, but go to local cheapy auto store and get a temp gauge, with thermocouple if you can find one. Install it and try your test. If all is OK the problem is your gauge, if not, you definitely have an overheat problem.

Thanks for the replies.

Yes, there is some blow-by when I really stomp on it. Haven't checked the compression yet.

The mechanical guage is a good idea. That's probably the cheapest way to find out if the guage is defective.

D. Ocean
Miami, FLA

Remove the thermostat and perform the following test...

Drive say 20 miles with normal summer driving techniques passing, hill climbing etc...

If the car engine overheats [for a fact] the radiator probably needs replacement...

There are other tests to see if the heads/gaskets are leaking into the coolant... problem is if these leaks are slight or only with engine under power... you may not be able to know for sure until the engine gets a whole lot worse...

Post results...

Yes, I'm going to remove the thermostat and see if that changes anything.

Thanks,

D. Ocean
Miami, FLA

You can check the stat by pulling it. If the engine is cold when doing so,it should be fully closed at the time. Place it in a pan of boiling water and see if it opens fully.
As far as running the car without it,I'm not sure that's going to tell you what you need to know. Some cars with oversized rads will run ok without them but normally the stat needs to be there in order for the coolant be in the radiator for a sustained period of time to cool down before re-entering the water jacket of the engine.
Without the stat in place,the coolant will constantly be free-flowing throughout the system,never getting the chance to cool down in the rad before re-entering the engine.
You said you have an aftermarket fan on it. How is that set up? Does it run all the time or is it supposed to come at a certain temp?

Yeah Danny, I agree that removing the t-stat is not the best test. IMO replace the T-stat regardless, just to be sure. If you were overheating real bad you'd have the typical symptoms. Ping, noise, smell, steam, bucking sometimes if it's real bad, fire, nuclear meltdown, etc...you know.
Having the rad boiled and cleaned by a pro is never a bad idea either. The small cost is worth the effort, especially down there.

I hate old stock guages, especially GM. If it were mine and a resto I'd hide good guages to keep track of a $$$ motor. At least oil press and water temp. If not a resto then several quality guages in plain sight.

I too would get a good mechanical water temp guage to verify the actual temp. If anything for peace of mind.