Hallo Jim & Heather,
> So we're pretty confident the problem is not MOGAS.
Ok, I got multiple messages like this.
Because of these messages I set aside my vapor lock assumption and I did
some additional investigations.
Fuel filters look fine.
Fuel flow looks OK (on both individual pumps).
Fuel pressure holds ok on full power.
However...
During flight testing I could reproduce the problem (@4000 ft, 25C), if
I go to idle, wait a few seconds, and then (smoothly) open the throttle
up to 115% power. What happens then is that the fuel pressure drops for
half a second, the engine falls back in power, and then the pressure
recovers and the engine runs fine, and keeps running fine at full power
as long as I want, even on just one fuel pump. Pressure is then rock
stable at 3.8 psi, exactly the Rotax recommended value.
The short drop in fuel pressure is odd. I can't recall that I have seen
that before. (Well, not as much. I have always seen a slight movement of
the needle with significant power changes, but can't remember having
seen such a sharp drop, but maybe I don't remember it correctly).
It doesn't matter whether I have both pumps on or only one. On idle it
is 3.8 psi. on full power it is 3.8 psi, but during the transition I see
it fall back to 2 psi and then come up again.
If it would be a blocked filter then the fuel pressure should
permanently drop I think. And it should make a difference with one or
both pumps on. But it doesn't.
For the 914, the fuel pumps always have full flow. What the engine
doesn't consume is returned to the tank. From the pumps perspective,
nothing changes with different power settings.
The only plausible thing I can come up with, is that the fuel pressure
regulator is "sticky", i.e. needs to overcome some friction befor its
valve adapts to a changed pressure demand. So, if the turbo kicks in,
the airbox pressure increases so the fuel pressure has to follow. If the
fuel pressure doesn't follow I see a drop on my differential gauge and
the carbs starve for fuel. Once the pressure is restored the engine runs
fine on full power.
What I would expect though is a similar (but reversed) excursion from
full power to idle, i.e. a momentary overshoot of fuel pressure. But
this doesn't happen. I don't understand this.
So, what I would like to know:
1) Does anyone recognise these symptoms?
2) Has anyone ever had a faulty fuel pressure regulator?
3) Is it normal do see a momentary drop in fuel pressure when going from
idle to full power? (I'm not slamming the throttle open, but take one
second or so, to give carbs, prop, etc. some time to adapt).
4) If indeed this is not normal but it is not the pressure regulator,
what else could it be? Any ideas?
At least the problem seems altitude/temperature related, getting worse
with higher altitudes/higher temperatures.
Thanks everyone for convincing me that it could be something else than
vapor lock!
Frans
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