* A faulty idle air control valve. The idle air control valve is responsible for controlling the amount of air that enters the engine at idle. If the valve is faulty, it can cause the engine to run rich or lean, which can lead to a rise and fall in RPM while idling in gear.
* A dirty throttle body. A dirty throttle body can cause the engine to run rough and stall, which can lead to a rise and fall in RPM while idling in gear. This is because the throttle body is responsible for controlling the amount of air that enters the engine, and if it is dirty, it can restrict the flow of air and cause the engine to run lean.
* A faulty oxygen sensor. The oxygen sensor is responsible for monitoring the amount of oxygen in the exhaust gas. If the sensor is faulty, it can cause the engine to run rich or lean, which can lead to a rise and fall in RPM while idling in gear.
* A faulty fuel injector. A faulty fuel injector can cause the engine to run rich or lean, which can lead to a rise and fall in RPM while idling in gear. This is because the fuel injector is responsible for delivering fuel to the engine, and if it is faulty, it can deliver too much or too little fuel, which can cause the engine to run rough and stall.
