There are two different coolant temp sensor plugs. One is the sensor that threads into the cylinder head. That's probably the one you changed; it has 2 pins and a nice harness plug that goes to the ECU. If that is out of whack, the car will run like shit.
The gauge uses a 1-wire connection down under the upper coolant hose where it meets the engine. It's above the gearbox, kinda around where the fuel hoses meet the rail. Should be a black plastic rectangular connector with one wire coming off. It's super flimsy, so that's probably your culprit. There's a spade terminal floating around under there that it goes to. I think it's on the coolant neck/thermostat housing.
Sorry I don't have pics right now (at work). If you fish around under there, you may be able to find the terminal and/or single-wire cable. It ties into the engine/trans harness somewhere nearby above the transmission.
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