Having recently purchased an E320 cdi I have noticed on several ocasions reference to turbos and the cooling of them.
One common thing to do is to allow the engine to run for a while after stopping to allow the turbo to cool down.
I have looked through the handbook and can find no reference to this so I have listed a few questions for you.
Is this necessary.
How long should it be left to idle.
What will happen if the engine is turned off as soon as I stop.
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In the early days of turbocharging,the quality of oil was not what it is today and after a good run,the turbo could get very hot.If the engine was then just switched off,the heat soak could,cook the oil and block up the oil galleries in the turbo with subsequent damage to the turbo bearings.
I had one of the first Saab 99 turbo's and the turbo went.On striping it down the oil had gone coal like.Later Saab turbo's had water cooling to the turbo and some had an electric pump to keep water circulating for a while after switching off.
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If you are stopping after normal street driving,just turn it off.You only need to idle it for a while if you have been running at max power,max speed.
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If you intend to keep this vehicle a while, it is advisable to let the vehicle idle at the end of your journey, to prevent carbon being deposited on the turbo bearings. If you ar running locally on B roads, I would suggest that 1 minute is adequate, but if you are on a long motorway journey, you should extend tickover to at least 3 minutes. This is the routine that I have got into, as I intend to keep my 320d for at least 5 years.
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In the past probably-but with modern oils and turbos-if you have been trundling around at 30 mph., you can turn it off immediately and I worked on the engine side of a major motor manufacturer(and I mean major).
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When you start up (cold or warm) allow the engine to idle for 20 seconds before driving off - this is good for the whole engine, but especially so the turbo. The handbook for my car says to do this for 10 seconds.
If I've just been into town and back I just allow the motor to settle at idle and then turn off.
If I've had a bit of a blast on the way home from work (25 miles thro' country) i allow it to idle for about 2 minutes or so.
If I've had the car loaded (family + luggage, say) and have done a fair distance then I leave the car idling untill the fan cuts in and goes off again.
Again, my handbook says to allow the engine to idle for just 10 seconds and that the fan will cut in if the temp underbonnet is high. I prefer to cool the turbo properly with oil & water running through it rather than just blowing air over it.
Also, keep your cooling system in tip-top condition (I use a 2/3 dose of water wetter, long life antifreeze at 25% and deionised water flushed & replenished every 2 years) and follow HJs advice about oil changes (ie every 6mths / 6k, not forgetting the filter too) and use the best quality fully synthetic oil you can afford.
Some would say this is OTT, but I bought my Rover Turbo at just 3 yrs old directly off the previous owner (a car nut like myself) who maintained it no expense spared on a company account. It's now 6 yrs old with nearly 130 k up - I have every confidence I will be able to keep it till it is 12 yrs old with 250 k or so.
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Motorway services are a challenge. You suddenly decide a service stop is required and wind down from boost to park and the turbo is dull pretty hot, always best to idle to 20 to secs then shut it off. This stops the oil from boiling in the turbo bearings poor things. I have fitted several turbo overrun fan coolers that run for 3 minutes afer ignition off. You have to override the alarm but most alarms have a link for that. Also safes the bonnet paint work. Demo Tweeks used to do a kit for this. Regards Peter
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The main reason you should allow a turbo-engine to idle after a motorway bash is to enable the turbo to slow down whilst it is still being lubricated by the engines lubrication system.
As a turbo can revolve at 100,000rev/min, it can take quite a while to stop even after the engine is switched off, during which time it could be starved of oil.
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The Land Rover Defender Tdi (turbo diesel) driver's handbook (1996) states: "To avoid the possibility of damaging the turbo-charger bearings through inadequate lubrication, ALWAYS allow the engine to idle for 10 seconds before switching off."
CMark
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Mate of a mate was an engineer at Garrett. He told me the T25 on my Rover can hit 425,000rpm at full chat and idles at over 50,000.
He also told me that the turbine accelerates and decelerates very quickly - it is extremely responsive to throttle position. If it weren't then turbo lag would be really bad.
Once your engine has settled at idle the turbo is only fractions of a second behind. It is heat soak that is the big enemy of turbos (that and 'cyclic fatigue' apparently).
I gleaned this from a half hour treatise after asking if dump valves were actually any good!!! The short answer to my query was 'no'.
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