Remember to get the pressure when it's 'stumbling' not when its running ok....but I bet its the same problem i had with two of thes 300 HPDI engines....when running hard the vacuum rises above 4.5 inches of mercury....due to the length/diameter of the fuel hose....maybe one engine has a longer run or a hard bend....put a vac gauge(cheap everywhere) inline just before engine filter(after fuel water separator) and run it hard....they can't run much above 4.5" Edit: meant to point out these are very very thirsty engines at full throttle, can require larger hose than other engines same hp
