This same issue has been reported on this forum many times over the last several years, and I think it’s mostly, if not always, on a low pressure carbureted engine. I had these same issue on my RV6 20 years ago, and couldn’t find the problem. I also experience the fuel pressure drop under certain conditions on my RV4. I spoke with a Tempest rep at Sun N Fun a couple days ago, and they were baffled at the symptoms. In my case, when I take off - full power, steep climb angle, high fuel flow - my fuel pressure will slowly drop to zero after I turn off my boost pump. Nothing changes with engine operation. Power is unchanged, as is all engine indications. When I level off, the fuel pressure slowly builds, but may take 15 minutes to do so. I’ve changed the VDO style pressure transducer, but that didn’t help, and I didn’t think it would because when I turn on my boost pump in this situation, the pressure quickly rises to normal. I just got back from SNF, 4.5 hours each way, and all fuel pressure indications were normal both ways, because I left the boost pump on until level off. The Tempest rep wanted me to collect data and give their experts a call when I’ve done that, even though it’s not their pump - it’s a Lycoming part - but very similar apparently. If I do an enroute step climb, the same thing happens - fuel pressure to zero, all other engine indications and operation normal - unless I use my boost pump, which will keep the low fuel pressure warning quiet. I’m reluctant to install a new engine driven fuel pump, because that has been done by several people here, and by me on my RV6, and it didn’t change anything. According to the Tempest website the engine driven fuel pump should last until TBO...... OR 10 years, whichever comes first. I mentioned this to the Tempest rep at the show, and he wasn’t convinced I needed to change the pump, as long as there is nothing leaking out of the witness line. I check that on every post flight.
I’m corn-fused, how about you?