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Here is a couple TSB's that relate to your concern. You will need to start with these procedures first.
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All 1998-99 B-2500 with manual transmission (3.73 axle ratio only).
Some vehicles may exhibit a delay at 1300-2000 engine rpms before returning to normal idle. The condition occurs on closed throttle deceleration in neutral or with the clutch depressed. There may also be illumination of the Malfunction Indicator Lamp (MIL) and Diagnostic Trouble Code (DTC) P0320 may be stored in memory. These conditions may be caused by the Powertrain Control Module (PCM).
Customers having this concern should have their vehicle repaired using the following procedure.
Bulletin No.: 011/00RIssued: 06/23/00Revised: 07/28/00
Applicable Model/s1998-2000 B-2500
SubjectHIGH IDLE - MANUAL TRANSMISSION
Some vehicles may exhibit a high idle. This may be caused by a poor electrical connection at the wire harness to the Throttle Position Sensor (TPS). A service kit is available to resolve this concern. Customers having this concern should have their vehicle repaired using the following procedure.
THROTTLE POSITION SENSOR (TPS) CONNECTOR INSTALLATION INSTRUCTIONS.
According to Mazda, replacing the TSP wiring and reprogramming the computer should resolve your concern. If there is ever a TSB related to a concern, you always have to preform the TSB before any other repairs.
If the concern is still present after the repairs, then we will have to dig deeper into some diagnosis.