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I've noticed my car's (healthy) engine will idle at a slightly higher RPM for a few moments after it's been started from cold/not run for a day.

I also noticed that my (somewhat less healthy) other car's engine which was left for over 6 months without starting idled at a much higher RPM when started and took a few minutes to reach a sensible idle RPM (and also revved itself up and down whilst trying to settle).

I'd be interested to know what's happening here mechanically or otherwise?

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  • Good observation..
    – Shobin P
    Commented Jun 25, 2015 at 15:09

2 Answers 2

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When the engine is cold, the engine does not run as well, so it counteracts this by running at higher RPM to avoid stalling.

The main issue is fuel vaporization. Fuel vaporizes at 196*F (91*c). Unvaporized (liquid) fuel does not burn. To keep the engine running while cold, the ECU adds in extra fuel. On carburetor engines, they used a choke. The rich mixture requires ignition timing to be restarted. This causes the engine to make less power. The higher idle allows the engine to run while the air fuel mixture and timing are less than optimal.

On the car with the idle that was going up and down, the ECU was likely trying to calibrate the inputs and output to get the idle where it needs to be. There is a small solenoid in the throttle body that allows air through to raise the RPM while the engine is cold.

Another issue is that your sensors may be out of range or not working while the engine is cold. The oxygen sensor needs to heat up before it can start working. Until the oxygen sensor is ready, your ECU is in a open loop state and must guess how much fuel to add. This can cause an up and down in idle.

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  • Doesn't anyone remember the choke!? It's not even like they went away (thinking of lawn care machines).
    – Eric Fossum
    Commented Jun 25, 2015 at 15:58
  • Sure do remember the choke. Just used it last weekend on my motorcycle. :-)
    – Brian Knoblauch
    Commented Jun 25, 2015 at 19:40
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    Not a bad answer, but there are a couple of details. The "small solenoid" you are talking about is the Idle Air Control (IAC) valve. It controls the idle no matter what state the engine is in, hot or cold. Second, when the engine is cold, the computer is in open loop and goes to closed loop once the engine has met all of its presets. Can take up to two minutes on most modern fuel injected vehicles. Thirdly, gasoline may boil at 196°F (depends on the additives), but will evaporate at 32°F (and even lower). Boiling means a much quicker conversion to vapor. Even water evaporates below 212°F.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jun 25, 2015 at 20:03
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I had the same issue with my Toyota Corolla 2003 car. Cold start RPM was sky rocketing 2600 and above. However, I managed to sort this out after investigating for a long period (4 years ++ with spending lot of money). Clean your MAF sensor and the throttle body including the IAC valve openings. If your issue is not sorted, check short term and long term fuel trim. If they are too high for a long period (above 6-7% for a long period) then there is a vacuum leak(buy a cheap ECU scanner and test this). If no vacuum leak you should remove the power socket from your IAC. If you see a difference in RPM (if it goes up or down) you better change your IAC valve. Please note that some IAC allows you to manually adjust the idle RPM up to some extend by rotating the IAC valve anti-clockwise(some Toyota allows this). Therefore, before replacing your IAC valve you better check for this adjustment. This will definitely sort your issue.

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