Cool down period - Diesels

Hi Banjo,
My current car was my first turbo diesel so I spent a long time reading all I could about them. From my reserch I concluded that driving around town 60km/h ect, the time it takes for me to grab my wallet and phone and get my daughter out of the car is pleanty of time for a cool down 30-45 sec max. If say I pull off the fwy after doing 120km/h I will let the car run at idle for a few mins around 3-5 mins. Turbos generate incredible heat and their cooling is done by oil so if you stop the car after high speed running straight away you have one very hot turbo and no cooling....what happens to hot metal? it expands, not a good thing in such a device which has such close tolerances...plus they cost a fortune to replace....this is what I was told so its my opinion only, different people will have different ideas to how long to cool them down ect, so it comes down to what you want to do as its your car and your the one who has to fix it

cheers
Stefan

Hi Banjo
I usually drive turboed vehicles (large & small) and as the previous reply indicated it is always good practise to idle down,to allow the big build up of heat to slowly dissipate, to allow different metals of different grades to contract steadily as they cool down .
some one will pop up and say they have never had a problem,great, that's there personal choice
I will generally allow time for some cool down,turbo or not(It can't do any harm) particularly if coming of highway/freeway speeds.
You are not alone by not allowing cool down,and I never heard of a mechanic diagnose an engine failure because of lack of cool down(NON turbo)
Its just personal choice ,like what brand of oil do you use

I have this in my truck

www.dieselmanor.com/isspro/ttm.asp

the EGT is only an indication of the turbo temp but close enough to be safe.
This is a must especially with high power tunes or boost controller ..
you can drive a turbo red hot ..

good luck
gmd

Hi Banjo, cars will generally run hotter after idling for a couple of minutes after a drive, than when you first stop. The airflow when you are driving keeps them cool, sitting and idling = = no airflow = higher under bonnet temps. And unless you are driving 'flat out' a turbo won't be spinning fast enough to be hot enough to require any cooling down period. So by the time you have been through the couple of side streets off the highway to your house the turbo/engine will be cool enough to turn off straight away.

Hi Banjo
When on highway runs, I usually let it cool down for a few minutes, but when in the desert and the vehicle has been working hard, I extend the cool down period to about 10 minutes. This way you know that the turbo has cooled down correctly.

Cheers

Stephen

When all else fails........Follow the instructions in your owner's manual. :-)

Hopefully someone with a exhaust gas temp guage will reply with how little time it takes for the turbo to cool down. I know with heavy machinery (which has been working at max RPM and max load) will be safe enough to turn off after about 2-3 minutes idle. Therefore even after you've been cruising on the highway you'll only need a minute, 2 tops. Around town - don't bother.

Even if you havn't been doing it I wouldn't be too worried. I've seen a 350hp remotely operated loader used for underground that had the full exhaust glowing red hot from turbo to the very end of the exhaust. These loaders get switched off from this state up to anything like 200 times a day (each bucket of dirt)....and there is no real measurable shortening of engine life.

As previously said, it comes down to a personal thing. One of my first 4x4's was a TD Rodeo and I used to idle it heaps but now I know more about diesels etc and realize that I was just wasting fuel.

Hi All,

As usual with this topic, a lot of urban myth and misguided assumption. First a couple of clarifications: A cool-down or idle-down has little to do with the engine itself - it's about preventing premature failure of the turbo-charger bearings. Also, it's got little to do with expansion of metals and clearances - it's about the oil flow to the turbo bearing being stopped while the bearing are still very hot - possibly far above the breakdown temperature of the oil.

When you stop after highway running, the exhaust turbine housing may well be still above 400C (based on my own EGT measurements). While the engine is still running there's a continuing flow of oil to lubricate and cool the bearing. If you let the engine idle for 3 - 4 minutes in these conditions, the exhaust gas (at idle, probably well under 200C) will continue to cool the turbine housing and the oil flow will keep the bearing temperature under control. When you then shutdown, the bearing temperature doesn't get too extreme

If the engine is turned-off immediately after pulling off the highway however, different things happen. The oil flow stops so there is no longer any cooling of the bearing. The heat of the turbine housing is conducted to the bearing housing and the oil remaining in the housing may be heated to well above it's breakdown temperature. (AFAIK, even top-shelf synthetic oils are not rated for temperatures above 130-140C.) So, the oil 'cooks', carbonises, burns, - and leaves deposits in the bearing. If this scenario is repeated often enough, the bearing fails.

My personal practice, based on seeing the time it takes for the indicated EGT to fall to close to it's typical 'idle' level, is 1 minute after 'round town driving and up to 4 minutes after pulling off the highway in hot conditions. I regard the cost of fuel for four minutes idling to be cheap insurance. Also, I've yet to hear of any heavy-duty turbo-diesel engine manufacturer who DOESN'T recommend a cool-down period after heavy load running - can anyone show me one?

Ian

Hi Banjo,
I feel that it is very important to allow the turbo to cool before shuting down. My first TD was a Disco and I questioned the cooldown period and was told not to worry about it by the dealer, well at 15K kms the turbo failed and was replaced under warranty.
I used to work in the oil & gas industry and all the turbines driving compressors etc have cool down periods when shuting down to allow the bearings to cool and after shutdown the oil pumps keep driving to further lubricate the bearings.
My new TD had a turbo timer put on as soon as I took delivery.

Hi Banjo,

I only do cool downs after long highway driving or heavy off road work.

One thing I do is to pop the bonnet to let the heat out especially if it's been along run - this was taught to me by one of my driving instructors in the Army many years ago.

Karl

Most modern turbo's are water cooled, so the problem of relying on the oil to cool the bearings down isn't as critical as it used to be.

hi there buy turbo timer and lets do it right