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The 311 code won't be set until you've reached and sustained RPM and load minimums the ECU is programmed to deem as driving, not idling in your driveway. Then, without the VSS input from the speedometer, the idle valve no longer regulates idle speed. If you shut the car off and start it again, idle will be regulated until it thinks you're driving again. When driving, the idle air path is opened up so, as the service literature explains "the development of unnecessarily high negative pressure in the intake manifold when the throttle shutter closes suddenly during deceleration."
tldr: the ecu needs the speedometer signal to properly control the idle
Here's an example of the many posts questioning this behavior:
https://www.turbobricks.com/index.php?threads/volvo-940-smoke-coming-from-the-dashboard.374570/#post-6413939
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Art Benstein near Baltimore
An elderly father asked his son if he could borrow a newspaper.
“Father, this is the 21st century,” replied the son. “I don’t waste my money on newspapers. But if you like, you can borrow my iPad.”
The spider never knew what hit it.
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