The reason the POST turbo readings stay higher longer then the PRE turbo readings once you are idleing is because the Exhaust side of the Turbo is made of cast iron and holds heat longer then the Pre turbo side since the exhaust coming out of the cylinders and passing through the heads cools quicker from the aluminum heads and coolant. Once your engine is idleing it does not produce alot of EGT heat so this cooler exhaust goes through the turbo and picks up heat from it (Like a heat exchanger) then it goes from the turbo into the exhaust and past your POST turbo Thermocouple. Which in turn is giving you the higher EGT reading from the heat exchange it has performed. It is still ok to shutdown as long as yoru Pre Turbo EGT is 300 or less since your turbo has actually been cooled enough at this point.
This is another of the many reasons why if you are only going to install one Thermocouple it should be PRE turbo. No matter what you do, post turbo readings are innacurate as to the true EGT of your engine. Bigger exhaust will cool your turbo faster by letting more volume of hot gasses get away from the turbo, which in turn leads to even more innaccurate readings from the Post turbo Thermocouple.
The main purpose on an EGT gauge is not for telling you when it is ok to shutdown your engine. It is to let you know if your engine cylinders are reaching a dangerous tempature due to too much fuel or when you are pushing it too far. Aluminum pistons WILL melt, i have seen it happen more then once on diesels. The EGT gauge is there to let you know when to back off the throttle. Its secondary function is to let you know that the EGT has dropped to a cool enough level that you can shutdown without any "Coking" problems. Synthetic oil is great for that problem since it has a much higher thermal stability.
Personally i think 99% of the people who are against Pre turbo Thermocouples are scared because somewhere somebody started a rumor that the Thermocouples can break off or melt and trash your turbo or they are afraid that a few metal chips you may miss when drilling and tapping for it will wreck the turbo. I have said before, Thermocouples wont melt untill about 2000 degrees, Your EGT could NEVER reach that temp, your pistons would have melted and your engine siezed long before then. And the only way one can "Break off" is if something hits it, and if something does (Which could only be a piece of a valve) that object itself would grenade your turbo. Metal fillings damaging it from installing the Thermocouple?, Remotley possible, i have installed almost 10 of them and have never taken a turbo off to do it, what few pieces that the magnet may miss will blow out on startup at which point the Turbo is not spinning near fast enough to be damaged. Dont worry about these virtually immpossible odds of Turbo damage. Correct EGT readings are far more important for engine and turbo life.
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