tb42e misfiring

[GQ Bear]

My 4.2 l efi's running like shit on petrol but ok on gas. I've recently replaced a few bits such as Crank Angle Sensor, Coil, Leads, Dissy cap and rotor.

On gas it runs fine, idles a little ordinary sometimes, but pretty much ok.

On petrol though it runs like a dog. It backfires occasionally, stutters, has flatspots and is generally reluctant to rev over 2500rpm unless you really give it to it. Generally a PITA to drive. But it idles well, maybe a little high (900rpm constant) and revs no worries stationary from idle.

Any suggestions? Anyone had similar problems? It's frustrating with so many sensors, ecu, and vacuum hoses it's hard to know where to begin.

[GQ Bear]

Yesterday i clamped the return fuel line and it drove a lot better. So fuel pressure's down. Fuel Pressure Regulator ya reckon? Injectors? Or fuel pump? Something else?

Any way of testing regulator without buying a new one only to find it's not the cause of the fault?