Had a local shop that was working on the Evo’s exhaust also do a coolant transfusion (full flush) today. It turns out that the RED Asian Zerex Vehicle coolant that I’ve been using for a few years has had an update that no longer includes Mitsubishis, obviously including the Evo. They released a new fluid, Zerex Asian Vehicle Blue, that is more in line for Mitsubishi, which likely means minor different anti-corrosion additives in the coolant. The plan is to empty out the overflow tank next, make a black line under the full mark, and fill the overflow with water wetter to let it mix in the system.
The second part was doing a much needed boost leak test. I had not been expecting any, but they found 2 – one in the air intake pipe before the turbo, and one in a different location – both related to the 2-step boost control system I have set up. With this fixed, it has corrected the long standing lean condition on idle that I’ve been seeing, and the car overall is running much better and feels more high strung again.