[GJ Humphrey]

What would you do next?

Since purchase of this 73 T337G (P-model) three years ago, the rear engine has underperformed the front engine at altitudes above 7000 feet unless RPM is 2500 or higher.

At take-off both engines produce full power. Even a recent take-off from Gunnison, CO, field elevation nearly 8,000 feet, produced full takeoff RPM and MAP. But at climb or cruise power, the rear engine just doesn't have the power of the front engine.

In climb, in order to maintain something like equal engine output, it's necessary to advance the rear throttle farther and farther ahead of the front. In cruise at higher attitudes, the rear engine settings won’t stay put. Bootstrapping all over the place. I have to constantly adjust RPM and MAP.

Leaks? The exhaust and induction systems have been tested multiple times for leaks. Minor leaks discovered and eliminated.

The turbo has been replaced. Ditto the wastegate, wastegate actuator, and variable absolute pressure controller. No change in the symptoms.

Recently, with the wastegate wired closed by an A&P the engine exhibited no improvement on a test flight.

Believe me, there are no leaks left in the exhaust system or induction system. The most recent leak test was accomplished using a shop air compressor (not a vacuum cleaner) attached to a special metal adaptor made for checking turbo systems (the adaptor attaches to the intake side of the compressor). The one uncertainty remaining is the integrity of the intercooler. I have no reason to suppose a leak or restricted internal airflow, but can’t rule out either since the unit is difficult to check without removal from the airplane. As for the pressurization system, we capped that off upstream of the flow limiter, with no improvement noted on a test flight.

Have we missed any remedial action?

Is it possible the problem doesn't lie on the "air side" at all, but on the "fuel side?"

Has anyone solved a problem like that described above by addressing the fuel side of the equation? The fuel pressure settings were recently checked and readjusted at idle and at full throttle. Still the underperformance problem remains.

Several really good mechanics so far have not been able to find the problem.

Does anyone have experience with this set of symptoms? All advice welcome.

GJ Humphrey
N3CU, SN P3370063
Concord, NH

P.S. Disregard my posting of two years ago (below). Adjusting the fuel pressure did not cure the problem at altitude.




.