Andreas, what you describe is a leak, The wastegate is sealing and might have a worn hinge and comes loose and allows pressure to escape, leading to normal thrust pressure. My experience is that when this happens you get a drop in pressure, not a return to ambient pressure, but it is possible that the system could go to total leakage.

The difficulty for your mechanic will be in replicating the problem, but parts are available.

The STC for Rajay is owned by Hartzell Engines and you try calling their tech support and hear what they suggest, if anything.

Rajay Parts in Texas has the approvals for the support items, like gaskets etc. Between the two you will be able to repair this issue.

My email is 26981@att.net and I invite you to communicate directly ..... let me have a phone number.

Pat Barry

@jk53 

Hi Pat, thanks for your reply. I'm about to leave country for a couple of weeks, but will certainly get back to you. 

I completely share your perception that this is about a leak. What bugs me: I cannot imagine/explain what happens in that engine bay to keep the pressure stable and above ambient for some minutes and then to drop back. 

I imagine it working like this: when you close the wastegate initially, the lid in the airduct is still open, allowing the Ram Air from the cowling inlets flowing in. 

When the turbo starts spinning, it builds up some pressure on the turbo outlet side which eventually overcomes the Ram pressure and closes the lid in the airduct. Only then MP can actually rise, correct? 

Air is now only flowing through the turbo to the engine. 

I'm sure that actually happens, as when I operate the wastegate, pressure builds up and I can adjust for a higher MP. Just from the vernier positions, I would say left and right are identical (i.e. same wastegate position) at that time. 

It's only after about 5 minutes or so, that one can watch MP dropping again. Why only then? What's different at minute 7 from minute 3 if the wastegate hasn't been touched at all? 

Thanks in advance and I certainly reach out once I'm back. 

All the engine air goes through the turbocharger. You close the wastegate and that energizes the system, so you get pressure air going into the engine.

There is a lot of vibration. I've actually held these parts and found looseness. I can see where your turbo will hold pressure when first engaged and the vibration can make the flapper move and allow leakage. 

The turbo is supposed to be inspected at every annual --- on a PA30 I was doing this and found RTV that an owner had used on his rubber induction tube which had torn. I've found screws, staple, inside the turbo. I believe that many mechanics are not opening up the turbo(s) at annual time since they don't know how they are supposed to inspect them.

As I mentioned, Hartzell Engines builds the Rajay units -- Hartzell Engines holds the STC. The rubber portions, gaskets etc are sold by Rajay Parts in Texas. All the things that you need are available. I still say that you might call Hartzell -- they have tech people. If you go to Oshkosh you can ask at their booth.

Since the unit performs well initially I can use the term "intermittent" and that can make it hard to diagnose. The job is to use low pressure air (the vent from a vacuum cleaner is low pressure) and tape that up to the induction and see if a leak can be found. Once the leak is found then the repair is straightforward. 

Call me when you get back .......  (949) 362.1600

Pat