What can cause black smoke from the exhaust under all engine conditions?

Black smoke from the exhaust typically indicates that the engine is running excessively rich, meaning there is too much fuel relative to the amount of air in the combustion process. A fixed Engine Coolant Temperature (ECT) sensor at a reading of 4.5 volts would suggest that the engine control module (ECM) believes the engine is either running very cold or is providing inaccurate temperature data. This faulty input can lead the ECM to incorrectly adjust the fuel delivery to compensate for what it perceives as a cold engine, resulting in an overly rich mixture and consequently black smoke from the exhaust.

On the other hand, a fixed Throttle Position Sensor (TPS) at 4.5 volts typically indicates a certain throttle position that may not reflect the actual position of the throttle plate. Depending on the engine design, a stuck or fixed TPS could lead to incorrect air-fuel mixture adjustments, but it's primarily the inaccurate temperature reading from the ECT that plays a significant role in causing consistently rich conditions across all engine operation modes.

Thus, the ECT sensor being fixed contributes greatly to the condition of black smoke due to its influence on fuel enrichment.