BMW E30 318i Throttle Position Sensor Typical Problems

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On the BMW 318i with the M10 engine, the throttle position sensor conveys to the fuel injection computer:

1. Whether or not the driver’s foot is off the accelerator pedal. Based on that, the fuel injection computer reacts and sends out instructions to the various components under its control.

Misleading news would, logically, include:

  • The driver’s foot not being on the accelerator pedal but the fuel injection computer is being told that it is
  • The driver’s foot being on the accelerator pedal but the fuel injection computer is being told that it is not

With the driver’s foot off the accelerator pedal, the fuel injection computer is probably optimizing things for a smooth idle, and if instead the driver is already in gear and enthused to get moving, that can’t be a good combination.

With the driver’s foot on the accelerator pedal, the fuel injection computer is probably ignoring the conditions that would make for a smooth idle, and if instead that should called for, that would probably explain a rough idle: the fuel injection computer isn’t trying to enact a smooth idle.

2. Whether or not the driver’s foot is flooring the accelerator pedal. Based on that, the fuel injection computer also reacts and sends out instructions to the various components under its control.

At wide-open throttle, the fuel injection computer optimizes for power and de-emphasizes fuel economy and emissions.  So, if the driver’s foot is trying to convey “wide open throttle” and the car never seems to reach its full power potential, that might be why. Conversely, if the car is showing bad fuel economy and higher emissions it might be due to the fuel injection computer having that agenda due to being told it should emphasize power.

* * *

The throttle position sensor is only one input of many.  The air flow and engine speed are other factors.  Misleading throttle position sensor input can probably make for some funny situations for which the fuel injection computer has a hard time adjusting, e.g.,

  • Even at a high engine speed, we’re not seeing a lot of air flow and yet allegedly we’re at wide open throttle, or
  • We’re seeing a lot of air flow and yet allegedly the driver’s foot is off the throttle.

I presume the fuel injection computer has a mode of “someone is lying to me” in which case one of the contradictory inputs gets disregarded, which can’t be nearly as good a situation as when all the inputs reconcile.

Even if we ignore how the throttle position sensor can contradict other components, it can still be problematic in its own right. As I understand the inner workings of the throttle position sensor, it has two switches: one for “throttle closed” and one for “throttle wide open.”  It’s occurring to me that one way of severely malfunctioning might be where the unit is generating contradictory input to the effect that the throttle is simultaneously closed and also wide open. That can’t make for a happy fuel injection system.

I haven’t pinpointed problem symptoms on my own cars and traced them back to a malfunctioning throttle position sensor, but I’ve read of owners complaining of hesitation during acceleration, stalling, or a rough idle.  That makes sense to me.  I’d also guess we could add: lack of full power, higher-than-expected emissions and/or worse-than-expected fuel economy.

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