I have a '72 Cutlass with a rebuilt 350 engine. The aftermarket temp gauge sender is in the intake manifold where the original vacuum tower was located. I have a new aluminum cross flow radiator that's the same size as the original. Using an IR temp gun, there is at least a 40* temp difference between the reading at the top radiator hose and the gauge fitting at the intake manifold. The gauge itself is reading within a few degrees of the intake reading. I'm trying to set a new fan temp switch, and I'm not sure which reading I should use. Before I got the fans to come on, the gauge reading went to 240* and it didn't boil over and I didn't hear any gurgeling in the radiator with 50/50 mixture and a a 12lb cap. My biggest surprise was the temp difference between the radiator and the intake. Thoughts please.................

 

Where is the temp sensor located?

 

Your question made me realize that the first sentence in the original post is wrong. When the engine was rebuilt, they put on an Edelbrock intake manifold so the temp sensor is now in a port that is right beside the thermostat housing. Before I said it was in the old vacuum tower port which is now wrong. Thanks for your question. It made me realize my mistake.

 

You can read temp in different places around the engine, and get different readings. I would set the fans to come on at 195*. Did the engine cool down after the fans came on?

 

I agree on getting different readings at different places around the engine and setting the fans to come on around 195*. My question is which temp reading should be the one to use ? If the radiator temp says 195* but the engine water temp says 240*, which one should be used to set the fans ?

 

I would use the water temp at the gauge if its reads pretty close to what your IR tester reads.