Maybe it's a bad sender?
Just how low does it idle (RPMs, not a guess)?
Maybe you could remove the sender and replace it for a test with a mechanical gauge.
Pic of mine below, you could even use a under dash aftermarket gauge (maybe $20). Temporarily run it out of the hood and into your window. This will give you a Real Pressure reading at different RPMs and at operating temperature. Maybe just for peace of mind, then you might install a new sender. Some just replace it, and carry on without any testing.
The Medic
Jul 9, 2016 at 9:24 PM