mis reading temp gauge

[81v8gp]

1981 GP with 265. Just did a timing chain replacement and now my temp gauge is reading 220-230. I have changed the water pump, had the radiator boiled out, changed the thermostat and sending unit. Car really doesn't seem to be overheating. One time last week I got gas. The temp gauge was reading 220 when I shut it off, pumped gas - 10 gallons maybe 5 minutes. Started the car up and the gauge was reading 190. Also when the temp is at 220 and I turn the heater on it drops to 190.

I have owned this car since new and the temp gauge has never gone over 190.

Are there any tests I can do to the gauge before I rip the dash apart?

Any help is greatly appreciated.

 

[jmt455]

First thing I would do is verify the temperature gauge with a known good gauge or an infrared thermometer. If the gauge matches the infrared thermometer, you might have an air bubble, a bad thermostat or a bad water pump.

If the infrared thermometer shows a different temperature than you see on your gauge (make sure to check the temps with the engine at operating temperature and take your infrared readings at the sending unit location), you either have a gauge or sending unit issue. If you've recently changed the sending unit, I would suspect that it is not exactly the same as the original part. Many replacement sending units don't have the same resistance as the original sending units and that can lead to incorrect gauge readings. The proper resistance ratings are in the factory Service Manual.

 

[81v8gp]

you might have an air bubble, a bad thermostat or a bad water pump.

I ran the car for an hour in the garage last night and the temp hovered between 220 and 240. I turn the heater on and the temp stayed at 190. Could there still be an air bubble after an hour of running? How can I purge any bubbles out? I changed the thermostat and the water pump when I replaced the chain. Why would the temp go down when I turn the heater on and stay down? I know initially it would come down just from the water having another spot to cool off in, but wouldn't you think it would come back up? Another weird thing is after the car ran for 1/2 hour and the gauge was reading 230, I turned it off and the turned the key to run the temp dropped to 220, turned it off and on again and the temp dropped again.