95 RX-7, mostly stock. Started having this problem a few months ago of the car losing power around 4500 rpm. At first it was random, but now the problem is constant in all gears, weather, etc. The primary turbo is still performing perfectly, till you hit 4500rpm there is no indication there is a problem. Past 5k rpm there is a drop in boost and power, accompanied by hissing noise. I dont think its a boost leak, everything has been removed, inspected, and put back nice and tight. After reading up on autosportracetech.com's trouble shooting section I think The cause may be something in the operation of the charge control actuator, assuming the hissing noise is the secondary turbo spooling with nowhere for its boost to go but back through the intake. The actuator is open when the engine is off and close when idling. The vacuum chamber and pressure chamber and their check valve are functioning properly.

I just performed the test of actuator operation and got some interesting results. When revving the engine above 5krpm I kept an eye on the actuator. through the entire rev the actuator did not open, but after letting off the throttle I noticed the actuator opened for a second before closing again. the guides do not mention this behavior and I am hoping it will help diagnose the problem. Any ideas?

 

Troubleshooting

 

already read through it several times and there is no mention of actuator opening on the down rev. does a good job of explaining most things but not a lot of detail on the charge actuator's behavior.

 

Read through my post. I had similar issues. I ended up having hoses that were misrouted. It was keeping my charge relief valve open when it should have stayed closed. This was allowing the boost generated to bypass the intake manifold and route back into the intake box.

I would have boost until 4500rpm and then a sudden drop to 0psi all the way to red line. I would also hear a hissing sound. It was probably the boost from the second turbo routing back to the intake box.

https://www.rx7club.com/new-member-r...-hose-1049945/

 

Do you still have stock pre-cat??? Mine did same thing.... ended up being clogged cat

 

It sounds like the CRV could be stuck open on the secondary turbo or that there is a misguided vacuum hose somewhere causing the boost pressure to be released somewhere. Check the vacuum lines to the two stock "blow off valves" and verify they're working by following the instructions in the user manual. It should hold a certain pressure in one direction. Use a vacuum pressure tool to verify proper function on those components first.