GM-GMC: 1988 k-1500, market gauge, temp gauge

Question
recently swapped from oem temp gauge to after market gauge,old school type from autozone.now truck will not idle,plugged up original gauge and truck runs fine what could be the problem??

Answer
The output from the temp sensor is used by more than one system on that particular year/make/model.  When you unplug it, the PCM thinks it is -40 outside and will provide the appropriate amount of fuel, which usually means flooding the engine instantly.  If you are going to use an aftermarket gauge, you will need to find the wires that come into the back of the cluster which are responsible for the engine temp, or, some of the older v8's had a plug in the rear of the cylinder head where another temp sensor could be added.
Hope this helps.