I have a 1996 sunsation with a 502 mag m.p.i.The fuel pressure on the fuel rail is only 32 at idle but drops to 28 26 at 1500-1800.Have checked mechanical fuel pump 4-7 .Changed vst out same rail pressure.Checked fuel lines and gas tank pick up ok.Just oreded a adjustable regulator,What would cause this problem.Mercruiser wants 37 psi. ??????

Have you taken the VST tank apart? There is a small filter/screen at the pickup for the high pressure pump. I had this problem and ended up not being able to even get on plane as the screen was clogged almost 100% and allowed just enough fuel at idle. Be careful when taking the VST tank apart and you can reuse the gaskets. The filter/screen is also before each of the injectors. I took both apart and was extremely surprised at how dirty they were. Keep us posted!!!!

Yes I pulled apart vst ,filter was clean,also check filter at regulator clean.I have heard to check ecu ,they said the voltage should change when the rpm,s go up?