Temperature gauge reading low?

I just got this 87 DX with 66K miles and it looks like the radiator water was never changed. How can people be like that!!! Anyway I’ve flushed the system (including the block), replaced hoses and put a new t-stat in it. The old t-stat was actually corroded/rusted/broken on the hose side. When I drove it home, the temp gauge never got up to half way even though when I got home it bubbled for ~ 5 minutes in the overflow bottle. Doesn’t that sound like it was hotter than hell or just crappy radiator cap? Why wouldn’t the temp gauge go up?

Anyway, after replacing the hose and t-stat, I started the car with the ‘new’ radiator cap off, waited for it to get hot and open the t-stat so I could add water. Finally after holding it a 1500 rpm that fans kicked on for about 15 seconds. If it idles, the fans don’t’ kick on. Bringing it back down to idle causes water to flow out the radiator neck. I was expecting the new t-stat to open.

I’ve pulled the sender unit and resistance reads ‘close’ but not dead nuts on the manual at different temp readings. Using a Fluke VOM and a Raytek IR temp gun, I’ve heated the sender unit up in a bowl of water.

The manual shows:
122F 176
154 ohm 52

I'd like to confirm with you guys that the senser readings are high and I should probably replace it? I'm not sure how close these numbers should be so that's why I'm asking. (I also grounded the wire and temp gauge goes to H)

Temp Resistance
77 735
80 650
90 525
122 250 should be 154 +/- 20
176 70 should be 52 +/- 5
190 52
212 45

Thanks!
Pat

Yeah I would say they are a bit high. You are correct with what the manual says. That sender is only for the temp gauge, it has nothing to do with the fans. I think that if you had the cap on and let it idle for a while the fan should come on. You can tell when the t-stat opens because the top hose gets hot. If it doesn't get hot the t-stat is closed.

So you bleed the cooling system using the bleed bolt?

also doesn't the fluid temp raise above boiling since the water is under pressure? and since you removed the radiator cap that pressure was no longer there... also the natural boiling water you used for the test was not as hot

Check the gauge too, my manual says:
C ~ 154 ohms, H ~ 15 ohms.
On my car, the gauge needle is horizontal while driving and goes to a little over half way (between C .. H) when the fan kicks in. The only time i saw it toward H was when t-stat was bad (closed) and I had to replace it.