hp 500 efi issue
 

I have an 04 353 Fastech with hp 500's. The engines have only 145 hours each and are well maintained. I have an issue with repeatable WOT rpms on both (either) engines. By this I mean on the same day, back to back runs, same wind, same direction, same seas, same trim, either engine will decide it only wants to top out at 48 - 4900 while the other pulls 5200 then the next time that engine will run fine at 5200. Running 89 octane on stock motors, may be knock sensor retarding timing but it is good gas and both engines pick and choose when or whether they want to run 5200 or not. Any ideas? One more thing, sometimes if I back off a bit then hit it again the lagging engine will run 5200. It seems like an ecu fuel and or timing thing?

check fuel pressure

Both engines and intermittent? Was there a problem with pumps/regulators?

i had a similar problem with a mpi engine this past summer and it was my fuel pressure regulator it wouldnt let the engine get up to speed and acted like the timing was retarding couldnt get any advance out of the timing

I was actually planning on installing some fuel pressure gauges anyway for my own peace of mind. Is there a good place on the high pressure side for a sending unit? I haven't really looked is there a port on the regulator?

+1 just had a fuel pressure regulator problem with mine last season. It is the one mounted on the pump in the plastic box, not the one on the fuel rail, Merc calls that a "damper".