This issue just started several days ago, This is a 75 C3 Coupe.

After testing the gauge and the engine temp, things appear to be normal, EXCEPT when I first start the car and I’m riding down the road, the temp gauge will rise to it’s normal position of about 195, then after about 2 minutes out of nowhere it will slowly climb to 280, stay there for about 30 seconds and then slowly go back to 195 and stay there as long as you drive the car, this action happens every time you restart the car. Here’s what I know,


2 separate heat temp guns read the same, when the gauge reads 280, the engine temp with the heat guns never read higher than 200 at the sensor which is top mounted, or at the thermostat location on the engine, or anywhere else, with the exception of the manifolds.

The amount of time for this action never changes.


The car has a Chevy 602 Crate, with a dual action oil pan, not the CT Oil pan.
here is the engine model number 88869602

I have a new Temp sensor arriving today for a 75 c3 corvette.

After this new research, if any of you have any ideas of what could be causing this, I’d like to hear it, and thanks in advance for all your help.

Powe.

 

I believe I mentioned to you that I had a sensor that read close to normal at normal temp. But shot way up when the temp only went slightly up.
I would suggest trying a different sensor.

 

Yes, you did, and thank you. I just thought the new information I discovered might give an indication of another issue, being that the timing never changes. Regardless, I will be changing out the sensor, hopefully today.

Thanks again.

 

One more thing to double check would be the engine's Ground. The engine should have a good solid ground strap going from the frame to the engine. Without a good ground connection the gauges will not be very accurate as they depend on a grounded engine.

Is it more accurate to measure the "engine's temperature" on the intake manifold where you are essentially measuring the coolant temperature or where the temperature sending unit is located on the cylinder head and the sending unit is reading the temperature of the metal in the cylinder head? The temp gauge on the dashboard is reading from the cylinder head and shows the engine block's temperature.

 

Thank you,,
We’ll find out today when I swap out the old sensor for a new one, The mystery to me is why this is acting the way it is? It would seem to me that a bad sensor would continually do this Intermittently While the car is running, But this is not the case, When you start the car and after exactly 2 minutes, it will slowly rise To 195° Then exactly 30 seconds the car will begin to slowly rise into the 280 marker, Where it will stay there for another 30 seconds then slowly goes goes back down to normal. It will stay at the normal position as long as you drive the car, The moment you shut the car off and wait a while and restart the car it will do the exact same thing in the exact timeframe. It will get to 195 immediately because the car is already hot, but after about 2 minutes it will slowly rise to 280 then after 30 seconds back to normal, this time frame never changes….You do not even have to drive the car you can let it idle, and it will do the exact same thing. Meanwhile, the temp guns show the temperature to be completely normal at 195 to 197°, Never has it shown any Temperature to be above that. This would seem to be a temp sensor, but why the consistent timing???

 

I'm thinking the gauge got a look at the new gauges that do a full peg when you first start the engine, but it's 50 years old so is doing its best.

ctmccloskey, his sensor is mounted on the manifold - being a new replacement engine.

Changing the sensor is the easiest first step, after that it's got to be the gauge... They aren't expensive (~$75) but the swap is not fun.

 

There are about a half dozen threads on the same problem now, so I can't keep track. Do you have a tiny hole in your thermostat? If not, consider adding one when you are in there.

The way it rises and falls, every time, makes me think it is either:

1. A very consistent air bubble (your read location is NOT in stock location).
or
2. Something electrical in the sensor circuit that changes when the engine is warmed up.

To point 2, do you have an electric choke? Do any of the other gauges have any odd behaviors?

 

No this is the only one acting like this. This coming week I will know for sure. I’m installing a new sensor, new thermostat, flushing the cooling system.

‘thank you for your advice.