1994 Deville 4.9 159000 miles. I originally thought I got a bad tank of gas, because I started having this problem right after a recent fill up. The car will sporadically lose power, sputter like it is either starved for gas or getting too much gas, and will backfire. If I turn the car off for a few seconds and start it back up, the problem seems to go away. It will drive fine for awhile. Sometimes a day, but then the issue comes back. I have tried fuel cleaners and fuel treatment in the event it was bad gas but that has not resolved the issue. I checked the plug wires and all appeared to be good and connections seem to be okay.
On a side note, when I turn the key without starting the car, afer a delay of about 5 seconds, I hear a motor running under the car that doesn't stop until I turn the ignition switch off. I thought it may be the fuel pump but it doesn't sound like it is coming from the tank, but the back end of the car instead.
Thats probally the IAC motor, is it loud? When was the last tune up? Throttle plates may need a cleaning, if carboned up it will throw off the TPS inwhich will cause the IAC to act up or break it (strip out the gears in it)