Hello, I own a 2013 Mustang GT automatic with just over 60,000 miles on it. My car sometimes accelerates on it’s own. It has done this 3 times since I’ve had the car. I’ve had it checked out by two different mechanics who have not been able to figure out what the problem is. The car has been scanned and there are no trouble codes. What could be the problem? ECM? Something else?
Maybe Mustang specialist @mod_man has experience with this?
Anyone who's ever tried to wrap their head around the reason for Unintended Acceleration has a different theory.
The car makers like to blame "human error" or loose carpeting pushing on the accelerator pedal.
But anyone willing to entertain the possibility that neither of those are the cause have to blame the computer.
Afterall, the computer controls the TAC (throttle actuator control) motor in the throttle body, and the fuel injectors.
Some people blame sensors in the ETC (electronic throttle control) circuits. (APP, TPS, TAC, sensors) providing erroneous voltage signals to the computer.
So we all blame the computer but we don't agree on HOW or WHY the computer is doing it.
Here's my thoughts on WHY. Voltage. Computers don't like Voltage Spikes or Voltage "Brownouts".
It makes them wonky.
So reasons that could happen are numerous. Battery, alternator, wiring shorts to power or ground, corrosion in wiring, intermittent "opens" in wiring, poor grounds.
Here's my thoughts on HOW. I don't agree that the computer is opening the throttle plate by itself. There's too many fail safes and redundancies built into the ETC circuit.
The weak point is the computer's control of the IAC (idle air control) valve. The same redundancies don't exist there and whether the IAC valve is controlled by a stepper motor or a magnetic solenoid, its opening percentage is controlled by voltage signals from the computer.
So what would happen if your IAC valve suddenly went to wide open under low load?
Your rpms would suddenly increase and if you were in gear you'd have Unintended Acceleration.
((well, ...... it's as good as anyone else's theory))
I appreciate you taking the time to write such a detailed and helpful reply. I’m going to replace the throttle body and see if that helps. If not, I’ll look into possible computer issues relative to the IAC valve.
Just don't misunderstand my post. This issue isn't isolated to your ETC (electronic throttle control) circuits and it isn't a failed part. It's an intermittent issue with power or ground going to your computer
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So you may want to make sure your positive battery cable connections to the alternator and to the underhood fuse box are corrosion free and tight. Also, be sure your battery ground connections to the engine and to the body are also clean and corrosion free.
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Maybe examine the wiring connector on your alternator
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Maybe have your battery and alternator tested
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You may want to get a wiring diagram of your ECM to determine where the computer is getting its power and grounds and make sure there's no corrosion issues at those points
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If you look around your engine compartment you'll notice several ground connections to the frame and on the engine
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Some are more important than others. (wiper motor ground compared to an ECM grounds for example)
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To illustrate I pulled up an ECM wiring diagram for a Nissan (it was handy)
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The computer controls many components by "switching ground signals" to them. But where is the ECM getting its ground to do that?
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In the diagram we see it's getting its ground at connection F14 on the front of the engine
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You'd want to locate that and make sure it isn't damaged/corroded. Then you'd run down any other body/engine grounds for the ECM by looking for ground points on the diagram
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Same goes for its power supply.
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I'm not the best at this, but since it only has 60k miles I would check the basic things first. There are a few possible causes of this but most of the time it is caused by a failing throttle body. If the throttle plate sticks and does not return to the correct position then the vehicle will accelerate. In some cases it can also be caused by the floor mats in the vehicle, (They can get stuck to where they'll be sitting on the gas pedal lol).
Thank you! I’ll definitely look into getting a throttle body replacement.
A sticking or hanging throttle in those cars usually is a throttle body problem.
I would look into upgrading to a BBK throttle body as the OEM is known to have problems as it ages. You can swap the same size throttle body for aftermarket without having to program anything. Just make sure you get the same size.
If you want to save money, you can also unstick it when it hangs. Just pump the gas really fast and it should free the throttle plates. I drove my 1999 GT with occasional throttle hangs for years that way. It is a common occurrence for those cars.
Thank you! I’m going to replace to throttle body and see if that helps.
You're welcome! 🙂