Puzzling over this one.

Vehicle specifics are 97 Explorer with 4.0 pushrod engine. Well neglected and trying to bring everything back into spec.

Reading codes with a Bluetooth OBD II dongle and Torque Lite app.

Only codes are for O2 heater circuit, (4 sensors). There is some damage to the O2 harness and repairs will be done. Live data shows no output on any of the 4 sensors, but the Torque app claims the engine is running in closed loop mode. Also noted the idle is rather high at 1200 rpm. TPS sensor reads about 1.3 V at idle and appears to operate smoothly. I suspect the high idle is yet to be discovered vacuum leaks, but the throttle body is verified to be fully closed, yet the Torque app reports that the throttle is at about 25%. Pulling the TPS plug drops the reading to 0% and when reconnected, the reading does increase if the throttle is opened.

What I find puzzling is that I normally see the closed throttle reading to be at least close to 0%. I understand the EECV strategy should sample the TPS voltage to calibrate 0% when the ignition is turned on. There are only 3 wires in this circuit and they run directly from the ECM to the TPS. Wire integrity and 5V supply checks fine. A faulty ECM may be a possibility, but I am uncertain if the remaining problem of the high idle speed may somehow be fooling the ECM into thinking the throttle must be partially open? I know there is a certain amount of mechanical preload on the TPS sensor and I am familiar with the operation of a potentiometer, so I understand why the idle position will produce a small voltage on the TPS output. I also understand that the ECM's ability to calibrate this signal as 0% on powerup does away with any critical adjustment of the TPS voltage and that the TPS is not adjustable.

One interesting observation had me disconnect the TPS on a running engine and plug in another TPS that was not mounted to the throttle body. The TPS output dropped to around 0.8 V and the engine idle speed decreased. The Torque app reported a lower % for the TPS sensor as well. Without changing the throttle body opening, I turned the loose TPS sensor to simulate opening the throttle and noted the indicated % climbed and the engine RPM increased. All this holds that there must still be some undiscovered source of combustion air in the intake system and that the ECM does respond to TPS input, but I remain baffled about the inability to reduce the TPS indication on a properly installed sensor to less than 25%.

Any thoughts?

 

Could the throttle shaft holding the TPS from moving all the way to where it needs to be?