I've seen several posts that seem to be the same type of problem I'm having. It was like I got in the car one day a few weeks ago, and it just didn't pull anymore. At first I thought I was nuts, until in the few days following, a couple of friends noticed i without me saying a word.
The car used to pull like a frieght train, plain and simple. The boost came on clean, noticably, and strong, starting at about 2800-2900 RPMs. I could hear it start to spool and at about 3400 RPMs it would just go. That's where I'd usually be in the rev range, about 3k RPMs normally, and when I'd go to pass or merge or whatever, a slight increase in pressure on the accelerater would yeild a nice smooth powerful pull. Part throttle boost was instant and strong. If I layed into it from 2.5k-3k, hold on!
When the car was brand new, I did a moderate break-in, I didn't beat the crap out of the car, I just drove normally, not going above 4.5k-5k until I hit about 600 miles. Even then I let her rip a few times, but wasn't treating the car like ****.
About 2 weeks ago I noticed this difference, and after confirmation from a few friends, (I did try to tighten all the connections on the pressurized plumbing, the only on I could not get to was the one directly off of the turbo) I decided to make an appointment and take her in. I figured I'd just have them do my first oil change then anyway. I explained the problem to the girl @ the desk, and went on my way.
About an hour after I dropped the car off, I got a call from the girl, she told me my car was ready, and that there was no problem with the car, "it works as designed." I told her I'd spek to the tech when I arrived. When I got there, the tech informed me that he, along with the manager, took the car for a test drive, and "The turbo is working as designed". When I continued to explain that I could hear the turbo spooling, but the boost comes on late and weak, and that the car is noticably slower, he countered with..."If you put your foot in it, all the way in it, she pulls like a bastard all the way to redline." (BTW, this is the first one he's driven, this dealer allows nobody to drive the Evo's, and they basically have NO experience with working them besides one other that had the clutch replaced) So I went on to tell him that part throttle boost is NON-EXISTENT, and that I knew the car was much slower. That the car's behavior was indicative of a leak somewhere or the wastegate just pissing the pressure out. I asked him if they checked the car in any way, any kind of leak checks or any way to check operation of the wastegate/solenoid. He said no, that they don't really have any way to check it.
This is the best part:One of his statements was, "With your foot to the floor you should have 19 inches from 3200-redline." I argued that stock boost drops off to around 16psi after peaking and at first he told me that I was flat out wrong. I insisted, telling him I would bring in dyno plots with boost readings from several sources, and whatever other info I could to prove this point. He told me to bring it in, and "get a boost guage to verify." Altough I was planning on getting a boost gauge, I wasn't planning on doin it tomorrow. I guess I'll have to. There is another dealership near me, and I may bring the car there, and tell them if they help me out I'll be bringing them my business from now on even though it's 30 miles further out of my way. If anyone can help with ideas, or can shed some knowledge about the operation of the wastegate (I have a general idea) or possible problems that could cause this type problem, please help.
What bothers me is that I came to them stating the car is not performing the same, and they basically took it for a joyride, and told me I was wrong, that it's running fine. I wish we could've jumped in the brand new Blue one in the showroom to compare, but that never would've happened. Not very happy with my $30K experience so far.

 

First off, you need to get a good boost gauge. It's hard to argue with a tech when you can't prove you're not making boost. Second, it's possible you've become accustomed to the acceleration of your car. I felt like my 11 second supercharged Mustang was slow after having it a month. I feel the same about my Evo now with all the mods in the signature. Third, quit screwing around with stealerships - it's obvious they don't know an Evo from a DSM and aren't going to treat your car like anything special.

 

I don't really know anything, but there are turbo-related diagnostics in the service manual. It sounds like your dealer's "tech" hasn't even read the sales literature, let alone the service manual. Do these apply (see attachment).

I definitely agree that you need a boost guage though. But it would've been better if you had it installed before when you thought it was running good, the dealer might just say you installed it wrong and that's why it's reading low...

 

I was out on lancer-evo.net site yesterday, and was reading the section outlining frequent problems w/evo's. One thing that jumped out at me was that they mentioned that the angle of the wastegate canister sits in a manner that water can splash up and run down into the cannister where it sits and corrodes the metal causing premature rusting. This in turn keeps the actuator from working properly, and can cause problems with either low boost, or even fuel cut and high boost if the rod doesn't release properly.

The Evo VII and VIII were both included in this potential problem area. I went back to look at the details again and found that the site is down today for reconstruction. This is something to keep in mind for sure!

N10S