I have an 82' with a B21F. Last night I drove home late, parked it, and then this morning it would not start. It's very strange however. I noticed that it Immediately fires up on the first crank, the engine doesn't even have to cycle at all, just instant ignition once it cranks. However, the second you stop cranking, the engine dies. Its as if it wont run unless the starter is spinning the engine, but that's weird because it seems like it's getting fuel and spark? It will even rev up if you are cranking it and press the throttle. I'm just a little confused as to how it seemingly is combusting air and fuel, but cant allow itself to keep running without the starter. I looked around the intake a bit for any large vacuum leaks, maybe one of the hoses popped off (as they often do on this car if they aren't zip tied), but that wasn't the case.

Would this be an ignition related issue perhaps? Any hints would be great because I'm totally lost.

 

I'd be suspcious of the ignition switch and whether the fuel pump is getting power with the key in position 2. You can try jumpering the fuel pump relay to see if that allows the car to run after cranking as a quick test

 

There are some older Volvos (not sure if yours is one of them) that the current to the coil goes thru a ballast resister, EXCEPT when the starter is engaged when "full" power is applied. So a poor connection at the ballast resister could cause the engine to only run with the starter engaged. (I seem to remember (but could be wrong) a ballast resister mounted near the wiper motor on a 240 - i know - a long way from the usual coil mounting spot)