So many truck come into our shop with high EGT's, turbo lag, excessive smoke output, and bad fuel mileage which are all symptoms of a boost leak. It is so common to devlop boost leak that we now require all trucks to have a boost test done prior to strapping them down on the dyno. What is funny, is that no one shows up knowing they had a boost leak. The reality is that even if you can hit peak boost, you may have a leak, especially if you have a variable vane turbo. Variable vane turbos will change vane position and other operations to try and achieve a disired boost number which may cover up a leak in the charged air system.
Leaks can be developed by corrosion of the intercooler pipes, blown or torn boots, loose clamps on intercooler piping, or even a cracked intercooler. Many times a leak starts small but over time becomes more prevelant. Again, most people do not know they have a leak until the charged air system is pressurized. Soapy water and looking for bubbles is strenous and time consuming, feeling each boot does not tell you anything, and running a smoke machine through the intercooler piping does not pressurize the system hard enough to expose all of the potentail leaks.
Kent Moore makes a boost test kit specifically for the Duramax. It takes less than 10 minutes to pressure test a stock truck. We use the one pictured below to pressure test nearly every Duramax that comes through our doors. More than half of them have boost leaks. You simply disconnect the boost tube from the turbo and hook this tester up, then pressurize the system with 15-30 psi depending on what you're troubleshooting.
Do not put a test plug in the hot side pipe, this takes the y-bridge out of the test loop (a common leak spot). The engine will hold the pressure.
If you don't feel like getting fancy with it, you can use something like this:
If you hear any hissing, find it and you'll find your boost leak. Fixing it will lower EGT's, improve spoolup, and improve peak power.