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Fixing the soot on my prop: a manifold pressure tale
I recently purchased a Defiant, and one of the things I noticed on the prebuy was a _lot_ of soot on the rear prop. We're talking absolutely _caked_, picture attached below. At first I chalked it up to using an extremely rich mixture too often (perhaps the owner had cooling issues, we've all been there), but there really was something amiss...
I ran it on long cross-countries with extra care to run lean -- helped reduce the caking, but it was still quite black. I'd get deposits even after a short taxi run across the airport, fully leaned. I double checked the clocking on the prop, and it was A-OK, in the recommended 1 o'clock position. Heck, I even tried resetting the curves on the electronic ignition/fuel injection units (a legacy FlyEFII system) back to default, figuring that maybe this was just due to a set of bad curves...
Of course, there were signs that pointed to something more: the engine would run rough at initial takeoff, then smooth out. The Dynon display MAP pressure gauge seemed to be "off", we thought perhaps it was dead -- at full throttle it read 14" inHg. We were making power (full static RPM), so clearly the Dynon display was faulty. The previous owner noted that paying attention to mixture was critical, the engine could be a bit touchy.
The clincher was twofold: the ECU head unit, once I checked, was also displaying an abnormally low MAP reading at times. And the Dynon data logs were quite telling (picture attached); this was very clearly some sort of low-pass filter on what should be a very responsive signal that tracked throttle motion. Not a common sensor failure mode.
I found that the MAP ports at the throttle body itself were blocked (some tiny bit of dirt or FOD had gotten in there, the MAP holes on the FlyEFII throttle body are maybe only 0.040" wide). The install had also made the situation somewhat worse by placing an _additional_ orifice in line with the MAP ports; double orificing seems like too much. For systems like those provided by SDS and FlyEFII, a correct MAP value is _critical_ for figuring out appropriate fuel metering -- it's the way the ECU figures out the volume of air being consumed by the engine. (The fuel-vs-RPM curve is fairly flat, while the fuel-vs-MAP curve has quite a bit of slope!)
Cleaning the ports out fixed everything: the prop looks good, the engine now responds as expected, and I'm seeing a nice grey/white on the inside of my exhausts!
Anyway, if you run real sooty and have a fuel injected setup: try cleaning your MAP ports and inspect those lines!
Defiant N431RA / VariEze N27GM
A sooty propeller at prebuy
A MAP trace (red) from the front engine, showing good response to throttle inputs
A MAP trace (red) from the _rear_ engine on the same flight, showing serious low-pass behavior, almost entirely blocked
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Interesting read of diagnosis and cure - thanks for taking the time to share :^)
On Mon, 8 Aug 2022 at 08:46, Raymond Jimenez <cyanoacry@...> wrote:
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