OK well if your MP and rpm stayed steady (and I think ordinary instruments are responsive enough to display any significant variations -- this is not an aliasing problem), then I'll stick to my assessment that you have irregular fuel flow causing mixture variations causing power variations that are being soaked up by prop pitch adjustments and appear as fluctuating thrust.  It sure sounds like your boost pump is producing irregular flow and maybe some simple diagnostics are in order before removal/overhaul:

1.  Do your fuel flow and fuel pressure (panel instrumentation) fluctuate when you turn it on, with a period similar to the surging period?

2.  Dumb but very fundamental question:  does fuel press go UP like it should or does it go DOWN when you turn on the pump?  (wild thought, but worth verifying).

3.  Ground test -- check the boost pump fuel flow by pumping through the system and out of the fuel filter, with the filter outlet disconnected from the engine and streaming into a bucket/catchbasin -- is it steady?

4.  Other quick check -- is the fuel filter clean?  (is there anything that could get stirred up, like water, even, at higher flows driven by the boost pump?

Those are all my ideas.  If you do these diagnostics you may find a smoking gun.  If not, I'd swap the pump out.  And, here's a free opinion, I'd regard a boost pump that causes surging as an unreliable backup and consider that you're now flying with a single point of failure (the mechanical pump) that could force a landing any time.  Strictly speaking your airplane is airworthy, but you've peeled off a big safety layer.

On Oct 1, 2013, at 1:57 PM, Art Jensen wrote:

Charley,
The surging was first noticed after take-off with low boost ON at about 800 feet AGL leaned for altitude 2500 RPM WOT. Being under the KFTG TCA climbs were in steps to 9500 MSL where I leveled off to try to understand what's going on and see how changes in operation affected what I was seeing and feeling. I reduced power and went LOP same results. Then turned off the low boost and the surging stopped. The engine ran very smooth. Turned the low boost back on and the surging came back. Again I turned it off and it went away. I determined I had found the cause (fuel pump) but still don't understand why (what is beginning to fail) or the connection (how it plays a roll to create the results I experienced).

Still looking for the expert who can put it all together so that I can come up with a plan of action for repair. Called Lancair to price a new Dukes Fuel Pump, $3350, might be a bargain tomorrow but at that price I'm not going to R&R as a test. Haven't checked a price for overhaul assuming my problem is in fact the pump. Any and all advice appreciated. Art

Sent from my iPhone; Art Jensen

On Oct 1, 2013, at 1:30 AM, Charles Brown <browncc1@verizon.net> wrote:

Just a thought ... if there wasn't any rpm change, and you heard or felt surging, it could be a torque increase that causes a pitch increase, and increased thrust.  And if MP was constant, then the only thing *that* leaves is mixture.  If you're running lean of peak, power depends significantly on mixture.

The thing is, this sounds backwards.  If it's worse when you turn the boost pump ON, then it sorta seems that you were running so far rich of peak that additional fuel pressure degraded power?  I'm not aware that the engine can even produce a mixture that overrich, except at very low power settings.
Charley

On Sep 30, 2013, at 7:58 AM, Art Jensen wrote:

Looking for thoughts from the collective minds of the LML.  Last flight in my IO-550 normally aspirated legacy, I noticed surging in the engine with the low boost pump on. I did not notice any surging when I turned the boost pump off. Also did not notice any fluctuation in RPM. Nor did I see any fluctuation in RPM when I downloaded the engine data. The obvious question would be what are the possible causes and does it make the airplane unsafe for flight as there is no surge when the boost pump is off. I don't understand how I could feel and hear surging in the engine yet see no fluctuation in RPM, so I'm stumped. Any ideas would be appreciated, thank you. Art

Sent from my iPhone; Art Jensen
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