Piper Aztec Fuel Flow Puzzle

[APcommitteeCBU]

Does anyone out there have a CLUE what to chase & fix on this one...

our turbo Piper Aztec has a mysterious fuel flow 'bounce' on the left engine.
Starting four years ago, it surges up and down by 1 gph, it did a few other
nutty obviously indication-only problems, BUT, this surging thing is definitely not just indication, as we now also have electronic monitor as well as the old needle, and they're both showing the same behavior. Recently, we've noticed that when the fuel flow surges up 1 gph, the Manifold pressure goes down. and when the fuel flow surges down, the MP goes up! The mechanic(S) who have looked at this say they've tried all the major obvious causes, inlcuding switching the spiders ...nothing is settling this.

Got clues? ideas? are we missing barndoors or is this a rare unknown?
thanks...

 

[planemech669]

Sounds like you've got holes in the fuel metering unit diaphram. I had an Aztec that had mixture issues. I'd tried everything that you have for yours and ended up sending the entire system to a shop that bench checked every component. turns out that the diaphrams had dry rotted to that point that fuel pressure was overcoming map and causing the mixture to go rich then lean then rich again as the fuel pulsed through from the pump.
Remove the fuel metering unit, spider, injector lines and injectors and send them to a reputable fuel system overhaul shop. If it's been more than 10 years since you've had any work done on the fuel delivery system and you only fly the plane 2 or 3 times a month, chances are pretty good that the rubber components in the system are too old to work properly.
I'd do both engines, especially if they have the same number of hours in service as the left since the last overhaul.

Does anyone out there have a CLUE what to chase & fix on this one...

our turbo Piper Aztec has a mysterious fuel flow 'bounce' on the left engine.
Starting four years ago, it surges up and down by 1 gph, it did a few other
nutty obviously indication-only problems, BUT, this surging thing is definitely not just indication, as we now also have electronic monitor as well as the old needle, and they're both showing the same behavior. Recently, we've noticed that when the fuel flow surges up 1 gph, the Manifold pressure goes down. and when the fuel flow surges down, the MP goes up! The mechanic(S) who have looked at this say they've tried all the major obvious causes, inlcuding switching the spiders ...nothing is settling this.

Got clues? ideas? are we missing barndoors or is this a rare unknown?
thanks...

 

[APcommitteeCBU]

THANKS FOR YOUR REPLY! I'll look into this right away, I know we've sent in the spiders, fuel dividers etc for overhaul, but not sure about the part you mentioned... thanks for the info.