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A piece of advice that seems to be widely accepted is that after a long high-speed run, when the turbocharger is thoroughly hot, you should let the engine idle for a while before stopping it. The reason given is that this allows the extremely hot turbo bearings to cool down a bit while the lubricating oil is still flowing, so that the bearings don't get contaminated by burned-on deposits.

I can understand this, and I can also understand the advice that often comes with it, namely that normally it isn't very important because at the end of a typical journey there will be quite a few miles of relatively slow driving between leaving the motorway and arriving home.

Here's my question. If it's all true, why are we not told to let the engine idle for a while before filling up at a motorway service station? I've never known any expert to give this advice; equally, I've never seen cars waiting before the pumps in a service station with their engines running.

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FWIW, I have been instructed by heavy duty mechanics trained by both John Deere and Caterpillar to allow turbochargers on large diesel powerplants to cool by idling for several minutes before shutting down. I also know the failure of turbocharger bearings to be a common problem on heavy equipment where engines are regularly shut down without a cool-down period. – alx9r Jan 23 '13 at 9:11
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You "never" see it because few people have turbos and even fewer know / care. That said, you'll see my 7.3 doing this every-time I get off the freeway. – maplemale Apr 28 '15 at 21:44
    
You see this all the time in areas where turbocharged cars are common. Here in Scotland I see it frequently. – Rory Alsop Mar 2 at 20:17

If you have a highly tuned performance car you are told, by the manufacturer, to idle for a few minutes before you roll up to the pump, otherwise you can destroy your turbo. Turbos get very hot - stopping them while hot means you don't have any way to transfer heat away from them - and so bearings die and cast components occasionally crack.

I regularly see other tuned Japanese cars idling while i do this. It's also a good opportunity to talk cars with other petrolheads.

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This probably depends on the configuration of your turbo. Modern turbo-charged Subarus do not need a cooldown period as they are oil and water cooled, which is sufficient to continue cooling the turbo when the vehicle is switched off. Also of note, there is no direction anywhere in the owner's manual for idling the car after driving. If some of these "tuned Japanese cars" you've noticed are Subarus, there's a good chance they're just wasting gas. – Ellesedil May 18 at 20:30
    
Interestingly, all the Subarus I have had so far do have this instruction in the manual, however my most recent one is a 2006, so you could well be correct about the more recent ones. Thanks. – Rory Alsop May 18 at 23:08

You should whenever possible let the engine idle for a short period of time to allow the turbo to cool in big trucks and cars alike. It will definitely add life to your turbo.

If you are keeping relatively low speeds and minimal boost in the last few minutes of your drive such as in your neighborhood or long driveway then the time needed to allow the turbo to cool will be reduced or eliminated. Coming off the freeway and shutting off the engine immediately would almost certainly reduce the life of the turbo.

They make a device to help with this called a Turbo Timer

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The owner's manual for my turbo Eclipse specifically mentions cool-down time periods, with the times listed based on the type of driving that was done.

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I've never experienced any problems with this over many years of turbocharging (all with sleeve-bearing oil-cooled turbos). The bearings should not be any hotter than the oil passing through it. For the most part, the center housing is thermally isolated from the hot turbine side. The turbine blades will dissipate heat much more quickly than they will transmit it up the shaft to the center housing. It's an enormous amount of surface area vs a tiny shaft.

I've found that I get 1400 F turbine inlet temperature cruising on the highway with a peak around 1700 at redline and full boost. Idling, the turbine inlet temps are below 1000. The temperatures change extremely quickly. Even by just lifting my foot off the gas for a couple of seconds, I can lower turbine inlet temperatures by nearly a thousand degrees. The problem with idling is that the turbine inlet temperatures quickly go down but never to any temperature that is remotely safe for oil, even after an hour of idling. By the time you're done parking the car, everything is as cool as it's going to get without shutting off the car. So I shut off the car. Nothing bad has ever happened from this.

Caveat, I have only ever run heavy weight, high temperature full synthetic motor oil. If you run crap oil, I'd raise the chances of coking and other misery.

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Once I get the car running again, I think I will put a temperature probe in the turbo oil drain line and see if I can get any interesting data about oil heat during long periods of idle or heat soak after a hard run. My expectation is that most of the damage tales are from oil starvation, not lack of pre-shutdown idling. – Jim W Mar 3 at 23:59

You're also not told about not riding on the clutch, keeping below 3000 RPM when the engine is still new (less than about 5000 miles on the odometer), and a few other things.

I wouldn't worry too much about it. If you live in a suburb, or anywhere that has a speed limit of 30mph/50km/h for that matter, chances are that by the time you get home and into your garage/parking bay your turbo would have cooled down enough for you to safely switch your car off.

This topic is also why it is a good idea to have a bigger intercooler and possibly a water/meth injection kit fitted to your car if you can afford it.

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The problem is not if the turbo has 'slowed down enough' as the rotating assembly looses speed very quickly when out of boost. By this logic the turbocharger would continue to make boost even when the throttle was closed, which we know is not true. – Nick G Jul 15 '15 at 14:40
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I was stupid back in 2013. – Juann Strauss Jul 15 '15 at 14:50
    
You can edit, @juann... – Shog9 Jul 15 '15 at 15:03
    
Didn't I just do that 22 mins ago? – Juann Strauss Jul 15 '15 at 15:13
    
Heh... Yes; but you may wish to integrate some of your advice from your newer answer as well. – Shog9 Jul 15 '15 at 15:17

The answer to this is all based on how hard the vehicle was just driven. More specifically, how heat-soaked the exhaust manifold and turbine housing are. These parts are meant to take extreme heat, however the bearings in the center of the turbocharger and the oil flowing over them are not. In fact, the oils job is not only to lubricate the bearings but to cool by having a constant flow going through them. Many turbochargers also have coolant flowing around the bearings housing for the same reason.

If the vehicle has been driven hard recently, the manifold and turbine housing are well above their 'normal' operating temperature. If the vehicle is turned off in this state the extra heat in these parts equalizes into the (relatively) cool bearing housing of the turbocharger, which no longer has any oil/coolant flowing through it to keep it cool. The oil still sitting in the bearings can 'cook' and crystallize becoming very abrasive.

If you've been driving casually staying out of boost then the manifold and turbine housing aren't hot enough to cause this problem when the car is turned off. Don't bother wasting your time and fuel idling in this situation, just turn it off.

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I love how some jacka$$ downvoted my answer but doesn't leave any comments as to why... – Nick G Sep 15 '15 at 15:19
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I just downvoted a bunch because there are no real citations or guidance from the car manufacturer or turbo manufacturer. If there is, I'll flip my downvote. Cheers. – DucatiKiller Mar 2 at 22:27

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