Randy, you are getting the variation only when the boost pump is on? And it goes haywire? Quite probably an electrical ground. A couple of things:
1) how long, has this been going on; anything changed that brought this on?
2) have you verified the fuel pressure with a mechanical gauge?
Since things seem to be fine in ground tests, I feel that there is an electrical issue, maybe ground, maybe power at the sender. MIght also be that in flight there is enough movement in the pressure manifold to distrupt any additional grounding of the sender. Try this---short piece of grouning wire, clamped to the sender, and terminated on one of the mounting bolts on the pressure manifold. Probably a long shot, and its probably a sender issue itself, but worth the 10 minutes to find out.
UH---any other electrical anomalies in flight?
Tom Swearengen, TS Flightlines LLC, AS Flightlines
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