I own a Mazdaspeed Miata running the stock ECU, I use a Blitz controller unit to increase my boost levels. The controller uses percentage to increase boost with 0% being wastegate and 100% being whatever the turbo can handle. I ran my car at 20% for many years with no problems which would give me around 9.5 PSI but I noticed my boost levels have gone down recently and I can't figure out why. I'm currently at 7.5PSI and my PSI doesn't seem to have any noticeable change when using values between 0-20%, this is what lead me to believe my controller was broken. But I decided to do a test and turned my controller up to 50% and it produced 9.5PSI... so its still working. I thought the boost controller solenoid was broken so I pulled it off and hooked it up to a 10v battery and its clicking ok so it still works. But I have no way of testing if its restricted... or if a restriction would even cause the issue I'm having. I'm a little unsure what to do next and have no idea what could be the causing the issues. Is this a sign that my turbo is on its way out? It has 129k miles on it, but if its not the problem I would prefer not to waste the money.

Things I have done so far...

- My CAT was checked for restrictions while it was in the shop, but the shop only checked the side they could see which wasn't the leading side... Not sure if its worth pulling off again and putting a periscope down it.
- Boost leak tested multiple times, no issues.
- Replaced exhaust manifold gasket and studs, thankfully I had a loose stud so wasn't a waste of money.

Thoughts? I'm lost :(

 

Could be your wastegate causing the issues. It's possible the can is having a harder time staying closed and it's blowing open which is causing you to have to increase the duty to get your desired boost.

Is your air filter clean? Maybe it's restricting flow. I was going to say boost leak but you've already checked that

 

Ever resolved the problem? Last time I had mysterious low boost problems on previous turbo projects I've worked on, it was usually the wastegate (twice) or a significant increase in exhaust backpressure (once, clogged cat).