Fuel exits from regulator. Car does not start when regulator disconnected. (Just thought I'd try. Relay ok (replaced). There had been some prior issues, but ignition repairs (cap, rotor, plugs) cleared that up. Just only starts (and runs great) on the residual gas in rail when fuel line disconnected. (Yes, I direct it into a container away from the exhaust, etc.) No codes come up.
The car only starts when the fuel line is separated from the fuel rail. on 1990 Volvo 740
You may have a bad regulator. When they fail on this car, they often create excessively high fuel pressure. So disconnecting the fuel supply is removing the high pressure condition. It's very easy to verify this wil a fuel pressure gauge. Check and if it's above 40 psi, replace the regulator.