Q: Cannot hold rpm in neutral, rpm bounces in rhythm

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Recently, my car has been jerky when trying to maintain speed regardless of gear. When I try to hold the throttle to cruise, the engine would think I let off throttle completely even my foot is still on the throttle at like 5%. Then I would give little more throttle to keep going causing the engine to jerk. I now have to either keep accelerating or decelerate. I notice when I keep steady throttle in neutral, rpm bounces. I believe it is related. I cleaned the throttle body and adjusted the throttle cable already, no CEL.

My car has 75000 miles.
My car has a manual transmission.

This may be related to a couple of different possibilities. What you are describing sounds like a dirty or failing idle air control valve. The idle air control valve monitors the air intake as it is mixed with fuel prior to being injected into the engine at low speeds and at idle. This valve is controlled by the vehicle’s computer and will adjust idle speed based upon other measurements such as engine temperature, intake air temperature and electrical system load or voltage. This is also an important function when starting the motor as it allows the motor to run and idle on it’s own once the motor fires. When you accelerate, the engine RPM increases, and as you let off the gas, the RPM slowly returns to the normal idling speed with the help of the idle air control valve making the transition from a higher RPM back down to idle speed while adjusting the air/fuel ratio constantly to allow this to happen smoothly. When the engine RPM drops below the normal range of about 750-800 RPM, this often times will cause the engine to stall indicating a dirty or faulty idle air control valve.

Another possibility may be a faulty throttle position sensor that may be producing an intermittent signal. The throttle position sensor monitors the throttle position of the throttle plate inside the throttle body in relation to your foot position on the gas pedal. Based upon the inputs from the sensor reading your foot’s position on the pedal, the ECU then makes the direction to supply more or less fuel to the motor. When this sensor is not working properly, this can cause intermittent responses when pressing the gas pedal.

There are trouble codes that are typically associated with the TPS. One code will set if the TPS voltage is too high when the computer expects to see it lower. Another code will set if the TPS voltage is lower than the computer expects to see. The TPS code for low voltage is the most common and will usually set if the TPS is out of adjustment or the sensor has failed. The first thing you should do when you get a TPS code is to check adjustment and signal output of the TP sensor before replacing it. Be sure to wiggle all connections while watching scan data/voltage readout to make sure the problem is not a loose or bad connection.

There are circumstances that could occur with a failing throttle position sensor that may not set a trouble code. One of the most common symptoms of a failing TPS would be a tip-in hesitation or stumble when you apply throttle to take off from a stop. This can be caused by a dead spot in the TP sensor’s internal circuitry, which usually causes the output voltage signal to not change (or it drops out) when the throttle opens. Unfortunately this type of failure is not easy to diagnose without the proper tool – a digital waveform scope. Most digital volt meters and scan tool displays will not respond fast enough to show this type of a glitch; but some may. If you do find this fault, then the obvious fix is to replace the TP sensor. I would recommend having an expert from YourMechanic come to your location to diagnose and inspect your vehicle.

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