Following info come from an issue with my 1.6...... When my engine was cold it idled and ran at other fixed RPM points fine. With warm engine when I would get to IIRC fixed point of 2,200 RPM the engine would surge back and forth between 2,200 RPM and 1,500 RPM. I checked everything out electrical, vacuum leaks, whet ever, no improvement. The engine at the time had a new thermosensor at rear of head which sends outputs to ECU.
When I followed TEN & GRD shorted with engine warm, no surging. Below is a different issue, but still took some ECU inputs out of the game.
"However, when the DC (Diagnostic Connector) TEN & GND contacts are shorted together for Self Diagnosis and troubleshooting the ECU control loop is opened.� With the control loop open, the ECU ignores inputs from Ignition rpm, Temperature and Throttle Position sensors and sends a fixed signal to the ISCV."
Even though I had electrically tested the TPS and it showed within test specifications IIRC with the engine warm I unplugged the TPS and no surging from 2,200 RPM TO 1,500 RPM. Bought a new TPS sensor, issue solved. Don't know if this supports a 1999 or not.