QuestionWhen the ignition key is turned to start, the fuel pump starts and runs for 5 to 10 seconds then shuts off. The engine will run fine until that first shot of gas is used up, then the engine will shut off. The fuel pump will not deliver another shot of gas when the key is turned off and back on - that is until usually the next day. All vacuum lines have been checked, replaced the throttle position sensor, crankshaft position sensor, installed new plugs. What causes the fuel pump to shut down and remain down for so long? ECU? Pump itself? Purge valve?
AnswerGeorge:
Since as my profile notes, my hands on experience is limited to the earlier (1990 and older) 900s, but I think similar designs apply to your model as well. So I can only respond in a general way.
Upon turning on the key, the the ecu would energize the pump to precharge the fuel lines enabling timely ignition. Once the engine is running, a crankshaft revolution sensor (which also enabled ignition sparks) signal would keep the fuel pump relay energized by employing a "retriggerable one-shot multivibrator". Thus, under the conditions you describe, the most likely cause would be loss of the crankshaft revolution (or position as its now called) sensor. Since you replaced it without a solution, its time the check the physical wiring connections.
Tedious, but necessary!
Next in order of probability is the ECU itself.
Another possibility is the fuel pump relay itself. On the older SAABS, it is found behind the carpeted wall of the passenger side footwell.
And finally, if you are so inclined, I would recommend subscribing to <alldatadiy.com> at $25/year/vehicle for the most complete info on your vehicle I have ever found! Its well worth it.
Hope this helps,
Jerry
