Pontiac Repair: 1990 Grand Prix temperature gauge issue, market gauge, pontiac grand prix

Question
Hi Todd, long question, but I want to give as much info as possible. I have a 1990 Pontiac Grand Prix 3.1 non turbo. The temperature gauge has been running higher than normal and somewhat erratic around the 220 degree range (Up and down in that range). I thought the cooling fans weren't working properly, so after troubleshooting the best I could with the factory manual I was able to get them to come on by jumpering the pins in the aldl connector. I also have an auto xray diagnostic scanner and after hooking up while running, the fan did turn on at 224 degrees on the scanner, however the gauge showed around 240. The computer did not match the gauge temperature. On another note the fans did not turn on when I turned on the A/C which I believe the manual said they should. After investigating, the clutch on the compressor is not engaging, so I think the pressure switch may be bad on the back of the a/c compressor( another separate issue?) I decided to change both coolant sensors, the one for the ECM below the thermostat housing, and the gauge sensor on the cylinder head by the firewall. After this the gauge now comes up very gradual, but reads hotter more quickly than before. With the scanner hooked up the computer shows around 195 degrees but the gauge is showing maxed around 260 degrees. I am afraid to let it run any longer than that in case it is showing accurate on the gauge. I also changed the thermostat and it is still doing the same thing. My question is could I have an air lock that would cause the new gauge sensor to not be reading accurate or is it more likely the gauge is bad? The check gauges light has been on for some time now, but the temp gauge problem is more recent. If the gauge is bad, is it possible to put an after market gauge on some where on the engine. It seems the factory sensor is a different thread than the 1/8 pipe after market sensor gauges. Any ideas on which way to proceed would be appreciated. Thanks for the patience with the long explanation.

Answer
Hi Lance:

usually when these engines start to run hot and have erratic temp readings they have head gasket issues. But a sub issue that comes along with that is you will loose heat in the car trapped air will wind up in the heater core and the car will not have heat. If this is not happening it doesn't mean you don have an issue but it's a good sign if that is not happening. if the scanner tool you have shows the engine temp in the 190-220 range and not 260 like the gauge then you just may have an issue with the temp gauge it's self is bad. If you have an infered temp sensor you can check the temp of the cylinder head where that gauge temp sensor goes in to the head to verify the temp it should be close to what the engine temp sensor for the computre reads. As far a sensors any kind of after market eletronic part you put on any car/truck SHOULD BE original equipment not aftermarket. your just asking for trouble. the market is flooded with cheap over seas parts. good luck :-}