Ovation 3 fuel flow jumps 1 gph back and forth

[Ed de C.]

Good evening, folks, I thought I would run this odd fuel flow situation by the group in case anyone has seen this before.  It seems to be different than what others have reported in fuel flow fluctuation posts in the past.

After leveling off for cruise and setting LOP mixture, I'm getting significant fuel flow fluctuations of up to 1gph.  When I first noticed it on one of the first flights since I owned the airplane in September, the fluctuations tended to by "spiky" downward of varying amounts up to 1 gph.  Now, the traces are not spiky but step changes upwards and downwards by 1gph.  The dwell after the step change is variable.  (pictures of engine data are attached).  It's definitely not instrumentation since I can feel it and the EGTs and RPMs react in each case.  The step changes have gone from one or two in a 2-hour flight a month ago to quite frequent 2 weeks ago (seems to be getting more frequent).  Disconcerting.

There was some shop work done between the "spiky" excursions and the now step function excursions:  Adjusted max rpm up to 2,700, adjusted idle low speed down to 650 rpm, installed set of GAMI injectors, lubricated all control cable ends.

It's a 2008 Ovation 3 (G1000) with IO-550-G with 310 hp STC and 150 hours on the overhauled engine.  2,000 hours on the airframe and control cables.

The mixture knob has some play in it, meaning I can push/pull on it maybe 1/16" to 1/8" (throttle and prop controls have zero play).  I tried that in flight and the gph change per engine data was minor, maybe 0.4 gph).  There's also some lash at the fuel control unit at the other end (unfortunately, I cannot attach the video of the movement).  We also notice the bracket holding the mixture cable end at the fuel control unit is surprisingly flexible (much more flexible than the setup in the Cirrus models in the shop.)  It's a 2-piece bracket assembly and it's the piece attached to the accessory case (not holding the cable) that flexes.  That flex appears to cause movement at the fuel control unit lever arm, and when combined with the cable lash, the lever arm can move quite a bit (maybe 1/8" or more) with no movement in the cockpit.  

Savvy looked the data over and said they've never seen this sort of trace before on a Continental.  It's as if something is bouncing from one limit to another.  Our combined best thinking is to replace the mixture control cable and go from there.  We'll look at the attaching bracket carefully for any cracks that might explain its flexibility.  If that doesn't fix it, maybe IRAN the fuel control unit next.  

Have any of you seen something like this?

Ed

Pictures 1 and 2 are of the complete 3-hour flight, the step changes are apparent and uncommanded.

Pictures 3 and 4 blow up one portion to show the moves in more detail.  RPM overshoot is evident along with EGT reactions.  All uncommanded.

[Ed de C.]

I see the images did not load in order.  Hopefully you can tell that #1 and #3 go together and #2 and #4 go together.

Ed

[Ed de C.]

Looks like we have this on solved.  

The Savvy guys and John Paul at GAMI said they've never seen this before.  We swapped in a new mixture control cable to no effect.  Upon advice from Savvy and GAMI, in mid-February I had the shop pull the fuel pump, fuel metering unit at the throttle body, and the flow divider and send out for IRAN.  I didn't want to sneak up on this one given the downtime. The fuel shop reported that they found a "bad spring" in the fuel metering unit.  All parts went back in and the fuel system was adjusted.

First thing I noticed on the test flight was that the "Check Gear" aural warning (and red light at the gear switch) came on at 16 inches of MAP.  That's no good since we spend plenty of time at 13 inches with the gear up descending into the traffic pattern or slowing before FAF.  I was pretty sure the A/P neglected to set the throttle cable switch for the gear warning correctly.  The shop owner suggested I carry a sharpie next time I fly and mark the throttle control where I want the gear warning to activate.  I set it for 10 inches MAP, the A/P made the adjustment and we're set.  There's still more travel to go so the gear warning will sound as I pull power off to start the flair.

Good news is that after 15 hours of flying, the fuel flow anomaly has not returned.  

Best,

Ed