Sounds like its firing off the CSV (cold start valve) which indicates there is at least an initial spark to generate that few second bang.

Have you checked fuel pressure at the injectors, metering head or WUR? If you've replaced the metering unit with a known good unit, I'd be checking the smaller components. The WUR (warm up regulator) could easily be at fault, they love to gunk up and go out of calibration over time, but no point in guessing when a pressure test will rule out immediately. (The CSV bypasses the WUR so doesnt care about what pressure, if any, its seeing).

Injectors are a bit of an arse to test, unless you have access to a flow bench, or are happy enough whipping up a crude little test harness (which isn't difficult, just takes a bit of time).