Turbo's, to cool or not to cool.

Submitted: Wednesday, Feb 10, 2010 at 13:00
ThreadID: 75926 Views:2977 Replies:13 FollowUps:3
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I am sure this subject has been discussed before but for the benefit of the new comer's on the Forum I decided to raise the subject again. The reason for this is as a retired diesel mechanic and in my travels, this subject often comes up. It appears that these days the general belief is that it is no longer necessary to cool a turbo before switching off the engine. It also appears that dealers, when selling a vehicle, when asked about this they are telling the buyer it is no longer necessary to cool the turbo. The first question I ask people is, have you read your hand book? the answer is in most cases no. For those who may not know how a Turbocharger works I will explain the very basics. It is a very simple device made up of two chambers with a shaft running through both, the shaft runs on bearings and each chamber is kept separate by seals and on the shaft in each chamber is a turbine similar to what jet engines have. The shaft bearings and seals are lubricated by oil pressure fed by the engine oil pump. This device is driven exclusively by exhaust gas flow through one chamber while the turbine in the other chamber compresses air flow into the inlet manifold or via inter cooler to the inlet manifold. When driving, especially under heavy load the turbine spins at a very high RPM and the heat of exhaust gas is very high. It is not unusual to see turbo's on large stationary engines under load glowing cherry red at night. So what happens when the engine is not allowed to idle before shut down? Because of the high RPM of the turbine it will take a a few seconds before it stops spinning but more important is the oil delivery has stopped and the oil left in the turbo can if hot enough boil. This in turn starts to cook the seals sending them hard and shrink and in time oil loss past the seal. I am aware of all the ifs and butts with regard to all of this and don't intend to argue with anyone. The bottom line is what dose the manufacturer say in the handbook? have a look for yourself, I have yet to find a handbook of a turbo vehicle with no reference to the turbo.
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Reply By: Mudripper - Wednesday, Feb 10, 2010 at 13:38

Wednesday, Feb 10, 2010 at 13:38
In my Rodeo's handbook, it makes this one reference,

"Do not operate the engine above idle before normal engine oil pressure has been established (such as applying full throttle immediately upon initial start-up). This causes the turbocharger to operate at excessive speed before the bearings receive adequate lubrication. Operation of the turbocharger for a period as short as five seconds without sufficient oil can cause bearing failure."

However, it doesn't mention anything about letting the turbo cool before shutting down the engine. Of course, I have always allowed at least 1 min for the engine to idle in any diesel powered vehicle that I have driven, no matter what the age. I have noticed in the Rodeo that there are small hoses going to the turbo from the cooling system, perhaps this is a method of cooling the turbo?

But as I said, I always allow about 1 min before shutdown.
AnswerID: 403595

Reply By: porker - Wednesday, Feb 10, 2010 at 13:53

Wednesday, Feb 10, 2010 at 13:53
Good info thanks Kevin.
The 78 series handbook has very specific details about turbo cooling.
I hope I can get my wife to read your post.

AnswerID: 403600

Reply By: Member - Kiwi Kia - Wednesday, Feb 10, 2010 at 14:08

Wednesday, Feb 10, 2010 at 14:08
If you have been driving at motorway speed then I agree - let the engine temperatures come down gently. If you have come off the motorway and run through suburban streets for a few km then by all means just shut down.

AnswerID: 403608

Reply By: Member - Timbo - Wednesday, Feb 10, 2010 at 14:28

Wednesday, Feb 10, 2010 at 14:28
Very good point Kevin (and informative about how a turbo works - I only had a very vague idea previously). All my cars to date have been petrol so it hasn't worried me too much! But the manual for my brother's TD Prado suggests different times to 'idle down' depending on the type of driving you'd been doing.

The mechanic responsible for the coaches told us that by the time we'd driven through the 60km/h streets to the depot, then through the yard and parked in the shed, you've effectively 'idled-down' enough anyway. And then there is always so much paperwork (logbooks, tacographs, etc.) to fill out for the Gov't incl. odometer reaqdings (which often weren't visible once the engine was switched off) etc. that the turbos usually had lots of time to cool down!
AnswerID: 403616

Reply By: Member - DAZA (QLD) - Wednesday, Feb 10, 2010 at 15:51

Wednesday, Feb 10, 2010 at 15:51
G/Day All

In the Nissan Patrol Hand Book they have a Caution,
*Change your engine oil according to the recommended intervals shown in the separate maintenance booklet,Use recommended oil.

*If the engine operated at high rpm for an extended period of time, let it idle for a few minutes prior to shutdown.

*Do not accelerate your engine to high rpm immediately after start.

A lot of people wouldn't even read their Handbooks until something went wrong.

AnswerID: 403635

Reply By: bigfut - Wednesday, Feb 10, 2010 at 16:46

Wednesday, Feb 10, 2010 at 16:46
05+ Hilux handbook mentions the cool down periods, has 30 secs for normal driving, and 1 min for highway cool down. I always just leave it running whilst we get ourselves organised, by the time you are ready to get out of the car, the time has been.
AnswerID: 403641

Reply By: Member - Graham H (QLD) - Wednesday, Feb 10, 2010 at 17:21

Wednesday, Feb 10, 2010 at 17:21
Well as said on the many contensious threads about this. I have a turbo timer set on 90secs and use it all the time.

I dont care what others think, my theory is, It doesnt do any harm so I will use it
Yes I know about the rule of leaving motor running whilst unattended.


AnswerID: 403647

Follow Up By: Muntoo - Wednesday, Feb 10, 2010 at 17:30

Wednesday, Feb 10, 2010 at 17:30
Totally agree with that Graham. My book says normal driving 1 min, highway driving 2-3 minutes. So i always let her idle down for a bit. No harm done in that.
FollowupID: 673166

Reply By: Rossco td105 - Wednesday, Feb 10, 2010 at 19:03

Wednesday, Feb 10, 2010 at 19:03
I let my engine idle down too, probably more than required.

I have witnessed a glowing red, hot turbo/manifold after an extended freeway run, took nearly 10 minutes to turn back to normal, on one of my petrol vehicles. I believe the term used is 'coking'? of the bearings.

As always, different conditions require different solutions.

AnswerID: 403662

Reply By: Member - Nick (TAS) - Wednesday, Feb 10, 2010 at 20:20

Wednesday, Feb 10, 2010 at 20:20
I always wait for the EGT gauge to drop under 160deg C before turning off the engine.
AnswerID: 403672

Follow Up By: Member - Redbakk (WA) - Thursday, Feb 11, 2010 at 01:03

Thursday, Feb 11, 2010 at 01:03
Onya Nick....this is exactly what I do too.....especially pulling into a service station while towing my 2.5 ton van the temperature is as high as 450 degrees and so I wait for it to drop to 165 degrees before pulling up to the bowser.
FollowupID: 673257

Reply By: get outmore - Wednesday, Feb 10, 2010 at 20:56

Wednesday, Feb 10, 2010 at 20:56
I got excited when reading the cool down procedure for my 80 series turbo in the handbook
- that was until i read the (where fitted) in brackets next to it
AnswerID: 403674

Follow Up By: Member - John (Vic) - Thursday, Feb 11, 2010 at 01:32

Thursday, Feb 11, 2010 at 01:32
LOL what a bleep er!!

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FollowupID: 673259

Reply By: Member - David G (WA) - Wednesday, Feb 10, 2010 at 21:33

Wednesday, Feb 10, 2010 at 21:33
Good explanation Kevin the basics of any turbo are work the engine and the turbo will wind up to 30000 plus rpm stop the engine and you stop the oil flow to the turbo and so it needs a bit of time to slow down so a couple of mins idle achieves that and keeps the bearings and seals lubricated and the temp is the guideline to that cheers Dave
AnswerID: 403683

Reply By: Member - Bucky - Friday, Feb 12, 2010 at 05:33

Friday, Feb 12, 2010 at 05:33
Roachie (member) has a good theory on "winding down" or cooling turbo's

Don't ya Mate

AnswerID: 403849

Reply By: Wilko - Friday, Feb 12, 2010 at 07:36

Friday, Feb 12, 2010 at 07:36
Hi Kevin,
I think its a good idea to let the turbo cool even if its just 5 secs, Its better then nothing. My turbo timer is set for 30 secs and It takes care of remembering I just switch it off and lock it up.

Cheers Wilko
AnswerID: 403861

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