starts breaking up at 4500 rpm.

[buzzinhalfdozen]

Originally Posted by partsbob67

here we go. 89 mustang 5.0 stocker. changed from the stock tank and lines to a fuel cell and an external pump. starts breaking up at 4500 ever since. the only thing I haven't replaced is the mass air flow meter and the cell. it's an 4 gallon aluminum cell. no foam. --10 line from tank to the pump. -8 line up and back. I change over to a 4bbl intake and carb and it runs great. any ideas. i'm to the point where i'm ready to put the stock tank back in.

As others have suggested, have you verified fuel pressure? Is the regulator the correct one for EFI? An -8 return seems big but should not be an issue if the regulator is working correctly (possibly returning too much fuel). What does the O2 sensor read during the event?