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I currently live in Japan where I bought a 1997 Subaru Impreza WRX STi. Since I don't completely speak Japanese, I wasn't able to ask anybody correctly about one thing that the previous owner had installed, which is a small timer under the steering wheel that does, it seems, different things. Here is the pdf I could find on it

One of the things it does is to prevent my car from shutting down after I turn off the ignition. I can turn towards me and remove the key, and when I do, nothing happens. However, the timer starts, and after 60 seconds, the engine turns off. There is a button that allows me to turn off the engine before the end of the countdown, and when I disconnect the battery to change something, and reconnect it after, the timer's default behaviour is to not activate itself, and in such case the engine will turn off as soon as I turn the ignition off.

My question is, why would somebody want to delay the engine shutdown like this ?

  • Is this a gasoline engine? – Acccumulation May 29 '18 at 18:32
  • It is a gasoline engine yes – P1kachu May 30 '18 at 4:27
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It is a turbo timer.

The theory is that an engine with hot turbocharger shouldn't be immediately turned off. The turbocharger may be damaged because there will be no oil flow. The timer ensures that the turbocharger has managed to cool down enough by idling.

Now, do you actually need the turbo timer? Probably not. The turbocharger is hot only after hard driving. Usually, the last few hundred meters of your driving are near a parking area with low speed limits, meaning there is already enough time for the turbo to cool down.

If you drive hard and turn the engine immediately off, on the other hand, you may benefit from the turbo timer.

  • Also if it is still rotating... same arguments though. But we always allowed our turbo engined tractors 5 minutes idle after heavy sustained work at full or even partial load. – Solar Mike May 29 '18 at 8:37
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    Engine fans won't cool the turbo much @P1kachu, you need coolant circulating. A habit of driving sanely in the last few minutes before you stop is all you usually need, otherwise let it idle for a few minutes. – GdD May 29 '18 at 10:21
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    A turbo timer in intended to prevent hot spots occurring in both the turbo and the engine. These are essentially due to the turbocharger and engine sharing the same oil system. Some more modern cars employ alternative systems where a small electrically driven auxiliary water pump circulates coolant until temperatures subside to safe levels after the ignition is switched off. – Steve Matthews May 29 '18 at 10:27
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    @juhist I thought it was to allow the turbo to cool down so that the excessive heat does not burn the remaining oil in the turbo bearings. Burnt oil could then block oil flow next time the engine starts. – HandyHowie May 29 '18 at 13:56
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    I'll add to Howie's response. This was an issue when turbos became more common on consumer vehicles. Oil tech. hadn't caught up with turbo tech (i.e. synth oils were less common than turbos) and the oil left in the turbo oil passages would burn and "coke" up the passages. Eventually blocking oil flow and destroying the turbo (if you're lucky! Manufacturers came up with solutions including water cooling the turbo bodies (with electric motors to run after shut-down). Fast forward to today and a lot of the urban legends from the past persist today, many of which have engineered solutions. – Tim Nevins May 29 '18 at 15:38

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