losing manifold pressure at altitude

[GJ Humphrey]

Thanks, Roger and Jerry,

I'll have the tachs checked with one of those devices that counts the blades of the prop. Certainly worth checking.

Honestly, I don't think it's the gauges. I synch the props by ear, and when they're in synch the RPMs read the same, front and rear. If the gauges are the problem, both gauges must be wrong by the same amount, and that seems improbable.

And the problem is not just underboost of the rear engine, it's also frequent bootstrapping. The rear engine power won't stay set for more than a few minutes before, totally on its own, the RPM will either rise or fall and the MAP will follow suit.

Swapping parts is a great idea, but on our weird bird the front and rear turbo plumbing is different. I've just replaced the rear turbo, so there's no point in swapping turbos. I'll look into the practicality of swapping the intercoolers.

Elsewhere on other threads, I've seen reference to leaking fuel strainer gaskets introducing air into the fuel and causing engine surging and/or underperformance. Ditto, reference to malfunctioning fuel tank vents and/or filler cap vents. Could one of those cause the underperformance and bootstrapping of the the rear engine?

And I continue to wonder about possbile leaks in the air reference lines that, as I understand matters, tell the fuel pump how much output is needed.

There's something wrong somewhere, and it has eluded the best mechanics I can find. I just know it's some darned thing we're overlooking. Wish I were an A&P and AI, maybe I could solve the problem on my own!

Thanks for your concern.

GJ Humphrey
N3CU
Concord, NH

[Joes]

Was this issue ever resolved????? I currently have something similar going on. Any help would be appreciated.

[B2C2]

None of these replies mentions examining the air induction path to the engine. If there is blockage there the air delivery will be at reduced pressure to the turbo. If this is large enough it will prevent the turbo from developing full inlet pressure. A clogged inlet path or airfilter could cause this. During the bad fire seasons we had on the west coast the air filter on my turbo was clogged significantly with smoke particles. I did not see a turbo pressure issue but didn't take it to high altitude much either.

As a side note I had a problem where I could not get to full inlet pressure on my front engine a few years ago. It turned out the magnet that holds the emergency air inlet closed had fallen off and damaged the impeller on the compressor side. For the posts where the turbo was replaced this obviously isn't the issue but if its a new observation make sure to get a look at the impeller to make sure its not damaged.

[Joes]

Found the leak! After pressurizing the compresor and the exhaust system I found a massive leak at the collector/ball assembly. Also just a few minor leaks on the injector o-rings. Thanks for all the input from the past and present. Joe

[mshac]

Would you mind posting a photo of the exact location of the leak? As close-up as possible? TY!

[Joes]

Here ya go

[mshac]

Originally Posted by Joes

Here ya go

Great pictures!!!

[Dr.Dan]

Originally Posted by Joes

Found the leak! After pressurizing the compresor and the exhaust system I found a massive leak at the collector/ball assembly. Also just a few minor leaks on the injector o-rings. Thanks for all the input from the past and present. Joe

How did you pressurize the system?

Cheers,
DD