Fuel pressure drop

Steve,

Injected engines reference fuel pressure to airbox pressure, not ambient pressure. This is done so the injectors deliver the right dose of fuel at all manifold pressures.  If you are not using a differential type fuel pressure sender that is referencing your airbox pressure, your fuel pressure reading will vary with manifold pressure. At idle it will read low because manifold pressure is lower than ambient air pressure, and the fuel pressure regulator is reacting to manifold pressure. However, as you increase throttle and the manifold pressure approaches ambient, the fuel pressure reading becomes more accurate.

if you are experiencing this behavior, you have a couple choices.  You can install a differential type pressure sensor with a reference tube to your airbox.  This is the most accurate.  You can also use a standard gauge pressure sender and set up most EFIS to compensate. This will give you a more accurate reading, but not perfect because most EFIS don’t compensate for pressure changes with altitude. A lot of manufacturers had this problem early on with the injected engines, now most use differential pressure type fuel pressure senders.  

The sender I like the best for the 912iS is the UMA N1EU70D.  This requires a reference tube to the airbox.

https://www.aircraftspruce.com/catalog/inpages/umasender10-06223.php

double post, deleted