Hi,
My 2005 mercruiser 5,0 MPI doesn't run as it should be.... Engine is “hesitating” so not prompt responding when accelerating and seems to run on 6 or 7 cilinders on the lower rpm's....Hard acceleration seems to be ok....Checked internal compression; perfect on all 8... switched TPS sensor, no change... tested fuel pressure...when switching on, briefly 90 (!!) psi and dropping to +- 20 psi. When running and accelerating engine, varies from 90 to 110 psi....NO fault codes found in ECU (Diacom)Fuel pump makes funny noises when switched on, I would rather believe pump is at the end of her days (so too low pressure) but isn't the case, rather the oppositeIs is old style MPI fuel cooler and 1 pump All help and ideas wanted...

Fuel pressure regulator has an issue and/or you have a restriction in the return line. Might also have leaking injectors

Whats your serial number

seen this many times. contamination in the fuel system. mostly from the fuel system itself coming apart. check the regulator for debris in the return screen. the regulator cannot return the fuel it is given. when repairing this i will always clean the lines, rail, injectors, and replace whatever failed component creating the problem. if you clean the regulator and reinstall, it will lower the fuel pressure however the engine may not run anymore because of the dirt in the injectors. hope this helps

screen under the fuel pressure regulator restricted. Or like most of the other comments above ^^^^^. Also if someone changed the fuel pressure regulator could be the wrong one installed. Have no idea how long you owned the boat here. .

Restricted fuel injectors is not uncommon. Actually a bigger problem than one would think it is.

where exactly are you taking fuel pressure readings at ? just checking as well

ALSO I WOULD CLOSELY EXAMINE the fuel rails internally, These also can be a problem for causing fuel injector contamination

If your engine is a bravo configuration you have a cool fuel 3. This is mounted on the lower front starboard side of the engine. The early cool fuel 3 was painted inside and the paint cannot withstand today's fuels. The paint washes off over time and plugs up the screen on the regulator. As already stated you can fix fuel pressure by cleaning the screen. But the fuel module needs to be cleaned out or replaced. Cleaning is a bunch of work as the paint needs to be removed completely. The injectors also need to be cleaned and flowed and the lines and rail need to be cleaned. The paint plugs everything up and a lean injector can make an easy fix something major.

Just noticed you have the old style cooler. This has a small screen under regulator. This style doesn't have a paint issue but the system still needs to be cleaned

Model year 2006 started gen III cool fuel. So around Aug of 2005 the changed was made but all of it goes by serial number to determine what parts and set ups one has for their application(s).

Volvo Penta Carter cool fuel cells since 1999 has the same issue with contaminating the fuel injectors.

Mercruiser certain years fuel rails (internally ) would cause fuel injector contamination as well. Also they can have pins holes that start to take place. 4.3 and 5.0 and 5.7 MPI engines / ECM 555 years