I checked trouble codes and got 1. P0236 boost sensor circuit range/performance.

That's telling you that you have low boost. Vacuum tubes are much cheaper at around $25. Very simple to check for vacuum at black hose at wastegate at idle. I can almost promise that wastegate isn't working due to one of the reasons I mentioned. When my shop was open, I kept 2-4 in stock at all times.

The increase in IDLE speed is a change in performance ie the timing advanced and the fuel increased. So the experiment shows the ECT and PCM are at least "communicating" but did you compare the PCMs ECT SENSOR (located in cross over) reading with the I/P gage ECT SENDER (located in drivers side head) reading? But now that those expected DTC(s) have surfaced... BTW, expect to find a DTC for Engine Coolant Temp Sensor Circuit Fault(s) as a result of unplugging the ECT sensor.. nothing to be alarmed with..

You must be THOROUGH in your INSPECTIONS as well as your DIAGNOSTICS to avoid chasing ghost and/or costly misdiagnosis.. all im sayin'


Good Luck

Download- 1998 DTC Diagnostics and Procedures Manual (PDF attached), see page 89

P0236- Turbocharger Boost System

Circuit Description
The PCM operates a solenoid to control boost. This solenoid is normally open. By providing a ground path the PCM energizes the solenoid which then allows vacuum to pass to the wastegate valve. During normal operation, the PCM compares its wastegate duty cycle signal with the boost signal and makes corrections in the duty cycle accordingly.
Conditions for Running the DTC
• The PCM performs this DTC diagnostic continuously.
• The engine speed is more than 2400 RPM.
• The fuel rate is more than 20 mm.
or
• The engine speed is more than 1800 but less than 2400 RPM.
• The fuel rate is more than 20 mm.

Conditions for Setting the DTC
• The boost pressure is more than or less than 20 kPa from desired (internal to PCM).
• All diagnostic conditions met for 10 seconds.

Action Taken When the DTC Sets
• A poor performance condition will exist.
• The PCM will reduce maximum fuel.
• The PCM will disable TCC.

Conditions for Clearing the MIL/DTC
• The PCM will turn the MIL off after three consecutive trips without a fault condition.
• A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures
• The use of a scan tool.

Diagnostic Aids
A vacuum leak or a pinched vacuum line may cause a DTC P0236. Check all vacuum lines and components connected to the hoses for leaks or sharp bends.
Check vacuum source. A possible EGR DTC will store if there is a problem with the vacuum source. Also check for proper vacuum line routing.
The turbocharger wastegate actuator uses a pulse width modulated vacuum solenoid that causes vacuum to fluctuate under normal conditions. A steady vacuum reading indicates a restricted or plugged solenoid vent filter or solenoid vent orfice.





Same as DTC 78 diagnostics, also attached