Why does the fiesta run perfect when throttle position sensor is disconnected?

When the throttle position sensor (TPS) of a Fiesta is disconnected, the engine control unit (ECU) goes into default mode. In this mode, the ECU uses a predetermined set of values for the various engine parameters, including fuel injection and ignition timing. These values are designed to allow the engine to run smoothly and safely, even though the TPS is not providing any input.

In some cases, a Fiesta may run more smoothly with the TPS disconnected than it does with the TPS connected. This is because a faulty TPS can provide incorrect data to the ECU, which can cause the engine to run poorly. Disconnecting the TPS ensures that the ECU is using the correct values for engine control, which can result in improved performance.

It is important to note, however, that disconnecting the TPS is not a long-term solution to a faulty sensor. The TPS is a critical component of the engine control system, and its failure can eventually lead to other problems. If you find that your Fiesta is running better with the TPS disconnected, you should have the sensor replaced as soon as possible.