Q: 1997 GMC Safari SLX 4.3L I Just install a coolant temp sending unit. now my motor wont start. but dose when unplug. what could be

The coolant temperature sensor sends a signal to the ECM to enrich the mixture when the engine is cold (cold engines need more fuel at initial start up), otherwise you will have hard starting or perhaps no starting. Something is wrong in that sensor circuit such that when the sensor is in the circuit, as it should be of course, the computer is either receiving, or generating, the wrong signal. It could be a wiring fault issue. If you have the factory wiring diagram, you can look at the logic on the diagram, measure for voltage and continuity, check the ground(s) and so forth and possibly track it down. If you do not have the factory wiring diagram simply request a sensor circuit diagnostic and the responding certified mechanic will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.