This is really starting to drive me nuts!!! Every now and again, like maybe every two or three weeks, my 97 PI will crank and crank, but won't start. In the past, I've gotten past this problem by disconnecting the battery, letting the car sit for ten minutes or so, then hooking the battery back up. This normally fixes the problem for a little while. Today it wouldn't start, and not even disconnecting the battery is working.
The only other clue I can offer is usually before this happens, I'll be starting the car and I'll hear something like a weak backfire or cough from the engine. It's really brief, and sometimes when it happens the car will start just fine, but I do think I hear it everytime the car fails to start. It's got gas, it's got air, it's got everything but ignition.
1997 Crown Victoria P71
2004 Police Interceptor