Engine problem - Seneca II

We've been having a problem with the engines on our Seneca II for awhile now
and I wonder if anyone else has any experience with it. The engines are
TSIO-360s and if we lean at cruise with about 75% power (typically around
32-in MP) or less to anything over about 1400 EGT (which is still ROP), the
manifold pressure will begin to drop off after awhile - sometimes after only
5 minutes or so, sometimes after 30 minutes or more.

It drops eventually to what would appear to be ambient pressure, as if the
turbo suddenly stopped providing boost. If the mixture is richened up the
MP will eventually return to normal - usually you have to go to full rich
until things return to normal and then lean back but not as far. Fuel flow
rates at the mixture setting that keeps this from happening are well over
what the book says they should be - as an example at 8,000 feet and 2300 rpm
with 32-in MP we need about 14 gph to stay below 1400 EGT. Lean the mixture
to below 14 gph, even to 13.5 and allow the EGT to rise to 1425, and the
problem will show up. The engines run fine when this happens - no
roughness, etc.

We have tried lots of adjusting and changing of parts in the fuel system -
about the only thing we haven't tried is changing the turbos because that is
prohibitively expensive - and we have always allowed 3 min of turbo spin
down time before shutdown. At first only one engine was doing this but now
both do it and have been for the last 1,000 hrs or so. Both engines are
still healthy with good compression in all cyls, so whatever this is it does
not seem to be causing any damage.

Anyone else ever seen anything like this?

BDS

"BDS" wrote in message
. net...
We've been having a problem with the engines on our Seneca II for awhile
now
and I wonder if anyone else has any experience with it. The engines are
TSIO-360s and if we lean at cruise with about 75% power (typically around
32-in MP) or less to anything over about 1400 EGT (which is still ROP),
the
manifold pressure will begin to drop off after awhile - sometimes after
only
5 minutes or so, sometimes after 30 minutes or more.

It drops eventually to what would appear to be ambient pressure, as if the
turbo suddenly stopped providing boost. If the mixture is richened up the
MP will eventually return to normal - usually you have to go to full rich
until things return to normal and then lean back but not as far. Fuel
flow
rates at the mixture setting that keeps this from happening are well over
what the book says they should be - as an example at 8,000 feet and 2300
rpm
with 32-in MP we need about 14 gph to stay below 1400 EGT. Lean the
mixture
to below 14 gph, even to 13.5 and allow the EGT to rise to 1425, and the
problem will show up. The engines run fine when this happens - no
roughness, etc.

We have tried lots of adjusting and changing of parts in the fuel system -
about the only thing we haven't tried is changing the turbos because that
is
prohibitively expensive - and we have always allowed 3 min of turbo spin
down time before shutdown. At first only one engine was doing this but
now
both do it and have been for the last 1,000 hrs or so. Both engines are
still healthy with good compression in all cyls, so whatever this is it
does
not seem to be causing any damage.

Anyone else ever seen anything like this?

BDS



I'm assuming that turbos on planes have a popoff boost valve. Ever had those
checked to make sure they hold pressure properly? Maybe they are getting hot
and losing their resistance. I really only have experience with turbos in
cars, I've never flown a turbo charged plane or seen how the system works on
them (if they are even different).

-------------------------------------
DW

"Darkwing" wrote in message
I'm assuming that turbos on planes have a popoff boost valve.

You know what they say about assuming....

The stock Seneca 2 does not have automatic wastegates.

D.

"BDS" wrote in message
We've been having a problem with the engines on our Seneca II for awhile
now
and I wonder if anyone else has any experience with it. The engines are
TSIO-360s and if we lean at cruise with about 75% power (typically around
32-in MP) or less to anything over about 1400 EGT (which is still ROP),
the
manifold pressure will begin to drop off after awhile - sometimes after
only
5 minutes or so, sometimes after 30 minutes or more.

I would suspect that the turbocharger bearings are lightly coked up and
squeeze the shaft when they expand from heat. This can happen even with
proper operation and cool down.

D.

"Capt.Doug" wrote in message
...
"Darkwing" wrote in message
I'm assuming that turbos on planes have a popoff boost valve.

You know what they say about assuming....

The stock Seneca 2 does not have automatic wastegates.

D.



That is why I stuck the disclaimer in there. So what happens on a turbo
plane if you pull the power back real fast without a wastegate? On a car if
the wastegate fails you can damage the engine.

I have had the distinct pleasure of driving a Lingenfelter Twin Turbo 01'
vette on more than one occasion, 650hp, 6 speed, BAD BAD BAD!!!

--------------------------------------
DW

Capt.Doug wrote:


You know what they say about assuming....

The stock Seneca 2 does not have automatic wastegates.

D.


That's correct, but it *does* have pop-off boost valves.

Happy Flying!
Scott Skylane

"Darkwing" theducksmail"AT"yahoo.com wrote in message
...

I'm assuming that turbos on planes have a popoff boost valve. Ever had
those
checked to make sure they hold pressure properly? Maybe they are getting
hot
and losing their resistance.

Thanks - I can't say for certain whether that has been considered or checked
at this point. We have mainly suspected that there is some coking despite
being careful about allowing proper spin down time.

BDS

"Capt.Doug" wrote in message
...

I would suspect that the turbocharger bearings are lightly coked up and
squeeze the shaft when they expand from heat. This can happen even with
proper operation and cool down.

This is what we have suspected as well, although it hasn't gotten any better
or worse in the last several years.

The first time it happened I was on a trip from the east coast out to
Colorado and had it looked at while I was in the Denver area. The mechanics
at the time suggested that coking might be the problem but that the only fix
was to replace the turbo to the tune of $900 plus labor.

I wonder if the bearings could be changed out as opposed to replacing the
entire turbo.

BDS

BDS wrote:

I wonder if the bearings could be changed out as opposed to replacing
the entire turbo.

For automotive turbochargers specialized companies or the manufacturer
itself used to do this, they were then sold again as "reworked" for the
aftermarket. With mass production of turbocharged (mainly diesel-)engines I
don't think it's worth the labour any more for the automotive market. But
for aviation turbochargers, it might be an option, maybe best to check with
the manufacturer of the t/c directly. Though I don't know for sure for
aviation engines, I strongly doubt the local AP would be able to change
bearings on a t/c. The main obstacle would be to re-balance the rotor after
reassembly.

regards,
Friedrich

BDS wrote:
We've been having a problem with the engines on our Seneca II for awhile now
and I wonder if anyone else has any experience with it. The engines are
TSIO-360s and if we lean at cruise with about 75% power (typically around
32-in MP) or less to anything over about 1400 EGT (which is still ROP), the
manifold pressure will begin to drop off after awhile - sometimes after only
5 minutes or so, sometimes after 30 minutes or more.

It drops eventually to what would appear to be ambient pressure, as if the
turbo suddenly stopped providing boost. If the mixture is richened up the
MP will eventually return to normal - usually you have to go to full rich
until things return to normal and then lean back but not as far. Fuel flow
rates at the mixture setting that keeps this from happening are well over
what the book says they should be - as an example at 8,000 feet and 2300 rpm
with 32-in MP we need about 14 gph to stay below 1400 EGT. Lean the mixture
to below 14 gph, even to 13.5 and allow the EGT to rise to 1425, and the
problem will show up. The engines run fine when this happens - no
roughness, etc.

We have tried lots of adjusting and changing of parts in the fuel system -
about the only thing we haven't tried is changing the turbos because that is
prohibitively expensive - and we have always allowed 3 min of turbo spin
down time before shutdown. At first only one engine was doing this but now
both do it and have been for the last 1,000 hrs or so. Both engines are
still healthy with good compression in all cyls, so whatever this is it does
not seem to be causing any damage.

Anyone else ever seen anything like this?

BDS

There were a few good suggestions from others already - Turbo bearing
coking does make sense, but you clearly know to let the turbo unspool
before shutdown so that might not be as likely. (IIRC there is a way to
clean up the turbo bearings without turbo removal if that's what it
turns out to be) My guess would be the fuel control unit gone wonky;
the fact that power returns after richening the mixture might back up
that theory, but what are the chances of both acting up simultaneously?
Maybe the alternate air diverter is partially open? Out of curiousity,
what CHTs are you seeing at 1425 EGT?