Volvo: 740 gl wagon just wont start, mass air flow sensor, mass air flow

Question
My son has a 1981 740 gl or gle wagon. It has no turbo or anything like that. He drove it to work and ran fine after work car wouldn't start. It cranks over but just won't catch to start. He replaced the fuel pump and when u turn the key u can hear it running. He has replaced the dist cap and rotor button and spark plugs. Even with 300 thousand miles on it the car ran like a gem til the other day. Oh he has replaced the water pump and radator cause the pump went and the radator was just flaking apart.  Like I said it ran like a charm and we have had the alternater tested and it is fine. I could really use some help here we are both frustrated. I have worked on cars since I was 5 guess my dad wanted a boy, but I am of no use to him I have always worked on American cars and feel this is just out of my league. So any suggestions would be greatly appreciated. Thanks

Answer
Lisa,
Pull a plug wire and attach a plug to it lay it on the valve cover crank the engine over. Does it spark? Test the fuel pressure at the fuel rail should be 42psi. Are the injectors pulsing? Using a noide injector tester light plug into the wire for any injector and see if the light flashes. Your car may have a fuel pump in the tank that is not working. Remove the line to the main pump and crank the engine to test for fuel. Look at the flywheel pick up and if the wire is frayed replace it. You may even have some fault codes that will lead to a fix. When the engine is cranking over is the camshaft also turning? Timing belt could have broken. Common faults are Mass air flow sensor, flywheel pick up, fuel relay, radio supperssion relay, fuses, t/belt, power stage, not enough fuel pressure, ignition coil, broken wires at the battery positive.