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

asked by on January 13, 2017

I just replaced my stock 5.7L 350 with a 383 Stroker from Blueprint motors. It came complete with the throttle body intake and so on. I swapped water pump and all electronics over to the new motor, replacing only the coolant temp sensor, which the plastic was broke, so the clip wasn't present.

The motor runs very rich when cold and floods immedialty. This immediately stops if I unplug the new coolant temp sensor. I will leave it unplugged for about 10 min, or until the motor is at normal operating temp, and re plug the senor back, and it runs like a new motor. I took out the new coolant temp sensor and put the original back, and the symptoms are the same. Could I have 2 bad coolant temp sensors, or is something else I should be troubleshooting?
My car has an automatic transmission.

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.

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