Idling A Turbo Engine Before Shutdown

[Dwight_Frye]

I just purchased a new Mazda CX 5 with the 2.5L turbocharged engine. The manual says that after sustained high speeds, as in just getting off the freeway that you should idle the engine for about 30 seconds before shutting it down or it may damage the turbocharger. Is this to prevent coking ?
If I drive a mile or two through residential areas at speeds of 35-40 MPH after a freeway run and then stop and shut down the engine, is that sufficient to cool down the turbo ? I can't imagine too many scenarios where I would get off the freeway after doing 75 MPH for several miles and then pull over and park and shut off the engine. My commute to and from work involves a couple of miles of surface streets with some stop & go for stop signs or traffic lights before parking.

 

[CharBaby]

I thought that was old school turbos. I guess it still goes on.

 

[atikovi]

It's more so the bearing doesn't scuff. Turbo wheel may be spinning 100,000 rpm after sustained high speeds. Shut the engine off and it's still spinning but with no oil pressure. There is still a film of oil on the parts so not a big deal especially if driven a few miles at city speeds.

 

[Donald]

I think it pertains to a rest stop on a highway. Drive at 70 MPH for hours then a few hundred feet into rest stop then turn engine off. It will get hotter after shutoff then begin to cool down.

I idle my pickup for 2-3 minutes at rest stop.

 

[AC1DD]

Originally Posted by Donald
I think it pertains to a rest stop on a highway. Drive at 70 MPH for hours then a few hundred feet into rest stop then turn engine off. It will get hotter after shutoff then begin to cool down.

I idle my pickup for 2-3 minutes at rest stop.



Yes. This scenario is very stressful on the bearings.

 

[PimTac]

Originally Posted by Donald
I think it pertains to a rest stop on a highway. Drive at 70 MPH for hours then a few hundred feet into rest stop then turn engine off. It will get hotter after shutoff then begin to cool down.

I idle my pickup for 2-3 minutes at rest stop.




I agree. This is a scenario where a couple minutes of idling will help. I do it on my naturally aspirated engines too.

 

[mbacfp]

Agree with Donald...after a long run and towing with my 6.7 powerstroke...I let idle for a good 5-6 minutes to get egts down before shutoff...usually between 350-400 degrees farenheit. Maybe owners manual of car may say something about it. 6.7 diesel supplement says to let cool down.

 

[demarpaint]

They claim you no longer have to idle a diesel after the driving conditions described. I disagree, I would do what Donald suggests.

 

[ragtoplvr]

It can also help to open the hood with the engine idling. Let out the hot air directly. It will be real hot if towing hard.

Rod

 

[mbacfp]

Isn't the cool off due to oil coking in the turbo due to high temps if shut off...at least in the old days. Newer synthetics have eliminated that problem? Ford mentions cooling down in their supplement.

 

[maxdustington]

I was under the impression you did that in oil cooled turbos, but it was unnecessary in water cooled. My car is a 2003 and has an auxiliary water pump so I assume that a lot of other more modern turbo cars and trucks have them as well.
Originally Posted by ragtoplvr
It can also help to open the hood with the engine idling. Let out the hot air directly. It will be real hot if towing hard.
I've read that the fan system on modern cars is engineered to the point where this might be determental. Unless is a Polara cop car with a 440!

 

[Eddie]

I always let my engines idle before a hot shutdown. 1-2 minutes for turbo engines and 1/2 minute non turbo. Ed

 

[Andrew1972]

Proper modern turbo design includes a high point in coolant system and natural convection to move coolant post shutdown.

Subaru does this. My wife would have laughed if you asked her to cool down car. She drove a 2005 Subaru turbo legacy wagon for 240,000 miles since new and never a turbo issue.

 

[carl2591]

When i am towing with my 2003 7.3L turbo diesel and know there is a rest stop coming up I will get out of the throttle and at the exit lane, or earlier is no traffic behind, put in neutral and let it coast into the rest area. The allow the engine to cool a bit while moving when forces the hot air out. Also sit and let it cool down till EGT gauge goes back to 200 degrees which take about 30 to 45 seconds with the idle time coming into the rest area.

When I was OTR driver on a 290 cummins tractor the shop guys told me to let it idle for about 1-2 mins and this was in the early '80's.

 

[RamFan]

Even with a post-shutdown pump, Alfa recommends the following for the 2.0T.

Originally Posted by 2018 Giulia Manual

It is recommended before switching the vehicle off, to keep the engine idling for a few minutes so that the turbocharger can be suitably lubricated. This procedure is particularly recommended after severe driving.

After a full load operation, keep the engine idling for three to five minutes before switching it off.

This time allows the lubricating oil and the engine coolant to eliminate the excessive heat from combustion chamber, bearings, inner components and turbocharger.


I think the gist of it is that they don't want you going all out then immediately shutting down. In my particular circumstance, I have 3-4 miles of off highway driving between my exit and my house. 60-70mph on highway, 40-55mph on back roads for 2-3 miles then 25-30mph in the neighborhood for the last mile; I consider this my cool down.

 

[Cujet]

Originally Posted by Dwight_Frye
The manual says that after sustained high speeds, as in just getting off the freeway that you should idle the engine for about 30 seconds before shutting it down or it may damage the turbocharger. Is this to prevent coking ?


1) Modern, liquid cooled turbochargers still coke up.
2) A high quality synthetic oil significantly reduces/eliminates coking.
3) A modern turbo that has some coking will still perform perfectly, they are quite tolerant by design.
4) The turbine wheel gets hot, transfers heat to the shaft, which is lubricated by engine oil, which can then fry. This has NOTHING to do with the liquid cooled housing.

This leads many to erroneously believe that liquid cooled housings have eliminated the coking issue.

As always, the best thing you can do for a turbocharged engine is to choose a quality synthetic oil of sufficient viscosity and change it regularly. I use 5000 miles as a safe interval.

 

[Andrew1972]

Originally Posted by Cujet
Originally Posted by Dwight_Frye
The manual says that after sustained high speeds, as in just getting off the freeway that you should idle the engine for about 30 seconds before shutting it down or it may damage the turbocharger. Is this to prevent coking ?


1) Modern, liquid cooled turbochargers still coke up.
2) A high quality synthetic oil significantly reduces/eliminates coking.
3) A modern turbo that has some coking will still perform perfectly, they are quite tolerant by design.
4) The turbine wheel gets hot, transfers heat to the shaft, which is lubricated by engine oil, which can then fry. This has NOTHING to do with the liquid cooled housing.

This leads many to erroneously believe that liquid cooled housings have eliminated the coking issue.

As always, the best thing you can do for a turbocharged engine is to choose a quality synthetic oil of sufficient viscosity and change it regularly. I use 5000 miles as a safe interval.




Interestingly we choose 4K OCI with conventional oil in wife's old car. Synthetic definitely better but my wife read manual of car and followed that.

 

[benjy]

using the highest quality real synthetic oil that resists heat is a smart move + that would be an Ester based oil like redline or similar + and since you did not mention recommended viscosity its well known a higher viscosity protects best. my traded 2001 1,8T jetta with 300 ft lb + 275 hp boosting to 25 lb after tuning went 200,000 without issue with real synthetic 10-30 oils. even before VW changed recommendations i dumped + saved the free dealer changes right after i got it home + put in my real synthetic, at first in 2001 + a few years on i used Amsoil but when they quit with the PAO + Ester base oils i switched to redline until i traded it, still running great!

 

[bobdoo]

I've owned two turbo cars. After normal driving, with a few WFO stoplight accels, the turbos would be glowing red hot. Hard to see in daylight, but real obvious after sundown.

Running the engine forces cooling oil and water thru the turbo. Open the hood if you were on it within ~4 minutes of home, or it's 80*f or hotter, if you want the turbo to last.

Remember, there's about 10 pounds of cast iron there, it takes it's own sweet time to cool down. And its thermal coefficient is low, so the heat doesn't travel fast like in copper.

 

[DriveHard]

I have about 1 mile of gravel after I turn off the highway, which I normally idle down to my house at about 12-15 mph, so I'm not concerned. Both my turbo cars have an electric circulation pump that keeps coolant flowing through the turbo after shutdown as well.