Hello together,

a few days ago I changed the old broken temp sensor at my ´63 390cui engine.

I used an ACDELCO part-no.2131140.

With the old sensor, the gauge didn´t move at all.

With the new ACDELCO, during normal driving with a cold engine, the gauge moves in the HOT direction within the first 5 minutes until it reaches HOT and higher. Then suddenly falls back to the center again and shows normal temperature levels during the whole rest of the test drive.

This behaviour of the temp gauge can be reproduced everytime, I start the car with a cold engine.

Since I don´t trust the gauge, I ordered an infrared laser gun tester to be able to measure the engine temp directly. But the tester has not yet been delivered.

My guess here at the moment would be a stuck thermostat or a thermostat with a wrong temperature value, that won´t open until the gauge hits HOT.

Have you ever had a similar problem with your temp gauge or sender unit?

thanks in advance


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A sticking thermostat is a possibility. Had it like that in my old truck. 

Thank you Russ :-)

As a precaution, I would order a couple of thermostats from Rockauto in case one should be dead-on-arrival.

The shop manual says: 180°F so I will choose some from the Premium category from Rockauto.


Get the better STANT thermostat

When they Fail they stay in the open position preventing overheating  and getting you stuck someplace 


180 degrees

If you are in a cold environment I would use the 195 degree  to stay warmer in the car 

Both work well in our year Cadillacs 


Just to add: I had a similar problem. In my case it was air in the cooling system. I had a minor leak at one of the heater hose connections. Even though the radiator looked full, there was an air bubble in the block. My guess is that the air bubble affected the opening of the thermostat. As a result the temp gauge would go to "H" then quickly go back to normal when the thermostat actually opened. After this happened I again checked the radiator and noticed it was low. After topping off the radiator the condition did not repeat. When I flushed the cooling system I saw this behavior again. The point is sometimes it's tricky to burp all the air from the cooling system.

You can easily test the thermostat by dropping it in hot water (boiling) and observing whether it opens or not. 

Thank you Kurt,

yes, a stubburn air bubble could also be the problem. So I´m going to drill a small "steam hole" into the case of the new thermostat to prevent new air bubbles in the thermostat housing.

The half way mark on a proper working temp gauge   is 200 degrees 

You should be running a bit past the first quarter mark ( between a quarter and half way mark ) on the gauge with a 180 degree thermostat in the engine at operating temp 

Anything over 200 degrees is starting to get  HOT 


Hello everybody,

yesterday I changed the thermostat. As I suspected it was broken. Rusty and stuck.

While I was removing it from the case, there was a small "pffft" and then some cooling fluid sloshed out of the engine. So it seems that there was an air bubble under the thermostat.

the old one was a 160°F no name thermostat.

Before I installed the new thermostat, I checked it in boiling water with my Laser Temp Gun (love this toy):

It starts opening at 80°C (176°F) and ist 100% open at 90°C (194°F)

I also drilled two "steam holes" into the case of the thermostat to prevent air bubbles in the cooling system.

After I had it installed, I filled up the radiator and started the engine. The gauge moved slowly towards the middle while the radiator hoses get warmed up. So I started the first test drive.

During this test drive of about an hour, the gauge sometimes moved into the direction of HOT so I stopped and checked the engine temperatur with my temp gun for several times and got always normal temperature values: hoses 74°C (165°F), Waterpump 84°C (183°F), heads 84°C (183°F) .

The gauge seems to jump back from HOT to the center when there are bumps or potholes on the street. So it seems to be now an electric problem with bad grounding of the gauge or a maybe a bad (new) sensor?

Always clean the grounds First when trouble shooting an electrical issue on these cars and go from there 

So very often just cleaning the ground will Fix the problem and a  super easy  cheep Fix 


Thank you, yes this is my plan now :-)

Checking the ground connections in the engine compartment will be no problem. But are there any special ground connections to the instrument cluster, that have to be checked?

Or do the screws only form the ground connection?

I want to avoid having to remove the upper dash pad because I´m afraid of cracks.

Some of the temp sending units on the market are not good and give incorrect readings — Hot 

I think Jason or Russ have the good ones 

Do not use plumbers tape or a sealant when installing that sending unit 


Yesterday I cleaned up the groundings.

Primarily I cleaned the battery cable to frame and radiator support.

Both straps from cylinder head to firewall were loose and corroded. Therefore, I cleaned these straps specially and fastened them with new screws.

 During the 1,5h test drive, the gauge then only moved to the center and showed no longer jumps towards HOT



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