Two things come to mind:
First, physically verify the feed and return paths from the tank to the rail and back again. Don’t assume they’re correct because the connectors fit or because fuel flows. Confirm pump outlet → feed hard line → rail inlet, and regulator outlet → return hard line → tank. Make sure the two paths were never swapped, even if you’re sure they weren’t.
Second, completely bypass the car’s return line for a test. Disconnect the return line after the regulator and route the regulator outlet into an empty gas can with fuel-safe hose. Then run the pump-prime test again.
This is not a “does fuel come out?” test. This removes the body return line and tank return path from the equation. If pressure changes, the problem is downstream of the regulator. If pressure does not change, we can stop chasing the body return line.
First, physically verify the feed and return paths from the tank to the rail and back again. Don’t assume they’re correct because the connectors fit or because fuel flows. Confirm pump outlet → feed hard line → rail inlet, and regulator outlet → return hard line → tank. Make sure the two paths were never swapped, even if you’re sure they weren’t.
Second, completely bypass the car’s return line for a test. Disconnect the return line after the regulator and route the regulator outlet into an empty gas can with fuel-safe hose. Then run the pump-prime test again.
This is not a “does fuel come out?” test. This removes the body return line and tank return path from the equation. If pressure changes, the problem is downstream of the regulator. If pressure does not change, we can stop chasing the body return line.