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Car have not idle for two years now. Yesterday I disconnected the computer, and reconnected again. Suddenly the car idle's very smoothly. Keeps on idling, but if ignition is switched off, and restarts again not idling. Only idles when computer is disconnected and reconnected.

Can this be a key problem ?

  • A sensor is likely reading incorrectly and reporting bad data to the ECU, causing the ECU to alter the fuel mixture. You will need a code scanner to watch these values and see if one of them is not reporting correctly. Another possibility that jumps to mind is the Intake Air Control Valve (IACV), used to set the idle speed. – rpmerf Jan 25 '17 at 13:32
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This isn't a problem with your key. It's an air-fuel mixture problem.

Here's what's happening:

  • when you disconnected the battery and reconnected it

    The fuel trims are likely reset. This doesn't happen on all cars but it is fairly common for fuel management to be reset through this action.

  • when you started the car first time

    The engine temperature is classified as "cold" (usually using engine coolant temperature) so the engine is run in cold-start enrichment. The fact that the engine idles better under cold-start mode tells me that the issue is likely to do with a lean condition.

  • turn-off, turn on worsens idle

    The engine is no longer running under cold-start enrichment. The mixture returns back to being lean.

  • @Pieter as there are many possibilities you're going to have to go through a process of elimination. A disconnected EGR line, intake leaks, underreading MAF, etc. Use an OBD-II reader to confirm the symptoms. – Zaid Jan 24 '17 at 12:28

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