If the oil is what cools the turbo, why do we idle until the exhaust gas temp is 350 or lower before shutting down? Is that just a good referance point as to when the oil will be cool enough to shut it down or what? I don't get it. :'( Please explain! <br><br>Tony

 

The cool down lets the temperature of the turbo lower so that the oil will not carbonize on the bearings.

 

Depending on the oil viscosity and makeup, oil will start to coke or flash at around 400°F, synthetics will go higher. With the turbocharger outlet temps below the 350 mark, you are pretty sure that the components are not much hotter and the bearing temps are low enough not to burn the oil in place. If the oil burns onto the shaft or bearings, the turbo will sieze.<br><br>You may also notice that the tube feeding the turbo is a long curve, this is typically for a small amount of oil to trickle through right at shutdown to avoid any oil to be left in place just after the shaft comes to rest.<br><br>I hope this helps out a bit..<br><br>J-eh

 

Ok... that makes more sense now... ;D Thanks guys!<br><br>Tony

 

[quote author=Lil Dog link=board=8;threadid=6381;start=0#58865 date=1035913514]<br>With the turbocharger outlet temps below the 350 mark, you are pretty sure that the components are not much hotter and the bearing temps are low enough not to burn the oil in place. <br>J-eh<br>[/quote]<br><br>For those that are running stock (no extra gauges), what is a reasonable amount of idle time before shutting down to allow the turbo to cool?

 

If your just cruising around town, it only needs about a minute or so.... not much... after a heavy load... it can take a couple minutes!<br><br>Tony

 

I have noticed that if you are just putzing around town, you really don't have to worry alot about cool down. A minute or so should be ample.<br><br>However if I have been blasting around on the highway and just pull off, it can take as much as 3 minutes to get it cooled down to a respectable temp.<br><br>J-eh

 

[quote author=Lil Dog link=board=8;threadid=6381;start=0#58865 date=1035913514]<br>Depending on the oil viscosity and makeup, oil will start to coke or flash at around 400°F, synthetics will go higher. With the turbocharger outlet temps below the 350 mark, you are pretty sure that the components are not much hotter and the bearing temps are low enough not to burn the oil in place. If the oil burns onto the shaft or bearings, the turbo will sieze.<br><br>You may also notice that the tube feeding the turbo is a long curve, this is typically for a small amount of oil to trickle through right at shutdown to avoid any oil to be left in place just after the shaft comes to rest.<br><br>I hope this helps out a bit..<br><br>J-eh<br>[/quote]<br>You mentioned &quot;turbo outlet temperature&quot;. Most recommendations I've gotten have been for a pre-turbo pyrometer. If this cool down is mainly for the turbo's health, then why not get a post-turbo pyro instead? Still tryin' to learn.

 

While turbine outlet temps are important for shut-down, what is much more important is exhaust gas temps while the truck is running. High exhaust gas temps can cause internal engine damage, and exhaust turbine damage.<br><br>So the pre-turbo spot is recommended. If you let pre-turbo temps drop to 300F or so, your outlet temps will quickly follow suit. The inverse is not necessarily true when the truck is running. 1200F post-turbo temps could mean pre-turbo temps up to 300F higher, depending on a whole host of variables.<br><br>Rod

 

[quote author=TPilaske link=board=8;threadid=6381;start=0#58855 date=1035912795]<br>If the oil is what cools the turbo, why do we idle until the exhaust gas temp is 350 or lower before shutting down? [/quote]<br><br>Oil not only cools, it also lubricates those tiny, high speed bearings. When you shut down, the oil stops flowing to the bearings, but the turbo continues to spin. It is very important you idle long enough to get the turbo to minimal RPM. On aircraft we try to get about 5 minutes at minimal idle. If we even touch the throttle, the 5 minutes starts all over again.<br><br>Just think of that turbo spinning at many tens of thousands of RPM and then you shut off the oil supply!<br><br>Gene

 

GSchlegel explained this best. This is exactly why you need to let your turbo cool.

 

Turbo spin on CTD is not a real issue pull the intake hose off the intake side of the turbo, clear all loose objects away from the engine compartment. You will see the intake turbo impeller easily from the front of truck have an other person start truck watch and learn. The turbo will immediately begin to spin -rev up engine you will hear and see increased turbo rpm-drop to idle for 15-30 seconds, have the other person stop engine note time in seconds for turbo to stop turning from dead idle. Heat soaking kills a turbo-the intake impeller observed is directly connected to exhaust impeller, the exhaust housing is what transmits waste out of engine. When a hot engine stops turbo also stops in the observed amount of time with oil flow stopping just before turbo and rpm drops to 0. Heat rises- but also transfers from heavy &quot;exhaust&quot; casting to the tiny shaft inside, if the casting has not been allowed to cool (3-5 min after heavy pull) the minute oil flow is disrupted cooling to turbo bearings is stopped heat is transferring to shaft across to cool intake side. If the temperature in the exhaust housing transferred to the little shaft exceeds the flash point of the oil it &quot;cokes&quot; producing a hardened black tar-like substance (coke-manmade or altered coal). Now when the engine is started small coke particles may be attached to the shaft,bearing surfaces or completely siezed to all of them slowing or stalling the turbo which promotes more coking and stops oil flow even if engine is running= new turbo- impatience= new turbo. I hope this explanation although not technicall perfect explains the problem to prevent problems, please take the time to do experiment with buddies to know the real truth and expell myths (takes about 15-25 minutes). PK

 

Great information P Kennedy.<br><br>I don't have any gages but do idle down after a highway run of 2 minutes and 3 to 5 after pulling the 7K lbs. I look at the temp gages for a drop then wait for another minute.<br><br>I did the same for my turbo cars and one is 170k miles on origional turbo and other I sold with 202k on origional turbo.