Cooling question

[digitech]

So,after getting the engine running correctly, I have moved on to getting the air out of the cooling system. After much jacking of the car, parking on a steep hill, filling through Prestone T and massaging of hoses, I believe all of the air is out. My fan still did not turn on by itself - I would let the gauge get to an indicated 210 or so (halfway between 190 mark and the beginning of the red zone) and then I would turn on the fan myself until the engine cooled back to an indicated 190. I ohmed my rad switch and found it was staying open, so even though it is new, I had decided I would have to order another one.

Today, while checking once again for air (none found), I let the temp creep up to the edge of the red zone. The fan came on by itself and the engine cooled back to about the previous setting, halfway between 190 and edge of red zone. I did this several times; I found that when the gauge would be indicating the edge of the red zone and the fan would come on, if I unplugged the temp sensor closest to the firewall, the gauge reading would drop to 190. Plugging it back up and unplugging the one mounted at the front of the head on the t-stat housing made the gauge read about 175.

Does anyone know what's going on and which part needs replacing?

[vandor]

The problem is that both of your senders are the resistor type. Only one should be a resistor type, the other should be an overheat switch, which is normally open and closes at ~220F to peg the gauge.

[courtenay]

I, and others, have had the same problem with the sender that "pegs" the needle. We have disconnected that sensor. Problem solved.

[124JOE]

and at that point i would grab my IR temp gun and read it at the plug area and sencors
they sell them for $20

[digitech]

I can do that; is the overheat switch the one mounted in the t-stat housing or closer to the firewall?

[124JOE]

we cant be for sure because they mixed them up at the factory and thy dont matter witch goes where,and one goes to the gage and the other goes to the switch i think
also check the rad switch

[digitech]

Thanks, I'll run it down via the wiring. The rad switch isn't the issue, it's a gauge indicating problem - as it turns out, the cooling system is actually working fine.

[courtenay]

On my '80Fi, both temp senders are in the block. The one closest to the radiator "pegs" the needle and is now disconnected. I used an infrared temp thingy (that's a technical term) to measure the block temp as well as the surface temp of the rad itself and the hoses and all was well. The other temp sender and the temp gauge are working well.