Q: Why would my truck run better without the coolant temp sensor plugged in? 1990 Chevrolet K1500

Hello - coolant temperature sensors are variable resistor devices - the colder the coolant temperature, the greater the electrical resistance value reported to the engine computer. (The warmer the temperature, the less resistance.) An "open" condition, as in an incomplete circuit, or not plugged in for example, would indicate infinite resistance. This would represent the coldest possible condition to the computer, and it would give you a very rich fuel mixture. Some engine management systems are sophisticated enough to recognize a wiring or sensor failure that shows an "open" condition like this, and it plugs in a default value (say 50 degrees Farenheit) arbitrarily. It sounds like in your case, the coolant sensor is either incorrect, or you have two failed sensors - it happens, although not all that often. Plugged in, the sensor is sending a signal for an extremely cold setting, with a very rich mixture as the result. Disconnecting the sensor may be getting you a default setting, and fuel mixture, that is much better for the engine’s performance. I would do some checking with the wiring, and sensor value at various temperature to determine where the fault lies. I would recommend an engine sensor inspection by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, give you an accurate assessment of damage and cost estimate for repairs.