I've been flying my 79 P for 15 years now, prided myself on quality maintenance and reliability. Since OH engines 500 hours ago, its been wonderful. I baby the engines, cruise high and long and fast. I just taught myself a lesson I decided to share.
I started seeing a problem getting full boost at high alt on the front engine (lost 1-2" MP). Increasing RPM would bring it back, and so would a descent (increased airflow). I thought perhaps it was a turbo system leak or wear. Couldn't find a leak but I ignored or missed some telltale signs.
I kept flying, deciding that my RPM guage was a bit low, as we've seen this before. I set RPM to top of green, got full boost, but then started to lose that too. So I moved the RPM up a bit more.....
Kept flying.
Before the last long trip I ignored a slight film around certain points on the FE cowling. Didn't pay much attention because I'm used to seeing dark film around there, but it was a slightly different color.
On return to my shop in Kingston, NY, we did a compression and found one bad cylinder FE and two on the RE.
I concluded that I caused this damage by not paying attention to the TREND. When I first began to see loss of boost at high alt, I should have immediately taken the engines into service. It turns out that there was a small separation in the FE exchaust piece that runs under the prop from port to starboard. Not visible without the lower FE cowling off. It let exhaust leak enough to lose full boost at high alt, but with higher RPM (airflow) or descent, the exhaust gas flow over the turbo was sufficient to restore full boost.
I damaged the cylinders by running at too high a power setting which was necessary to get full power from the front engine, but ran the engines too hot.
So, the exhaust piece repaired, reinstalled, cylinders replaced, it's all going back together, less than two week down time.
Lesson learned: If the guages say something is trending off course, it probably is. Find the problem and fix it. This could have been an inexpensive repair.
It's OK because I learned something new and I'll have the plane back in two more days.
mike