My idle has dipped from 750 to around 500 when the car is sitting at a light. This is now happening daily (with a mild thump when the idle dips). This is much different from the normal dipping of the idle when the car is started and there is some type of pump that starts to whirl on (air pump/oil pump/fuel pump ?) I am used to the initial dip in idle and it appears normal. The second dip in idle I am encountering feels as if the car will stall and it literally sounds like someone is beating a drum near the trunk. Car has also failed to start after getting fuel during my last two fill up's, (first time I closed the fuel cap and the car started right up on the second try, second time the car started on the second try).

Are these indications my fuel pump is about to go or that my fuel filter is clogged ? Oxygen sensor ? Looking for feedback from people with '97/'98 c230's who have had either a fuel pump replaced or oxygen sensor replaced and at what mileage. My car has 76.5k mileage >> never had anything replaced except mass air sensor. Looking to do something preventative before I am stranded on the side of the road ! Thanks !

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1998 C230
1994 Infiniti J30T
1984 BMW 318i
1987 VW Golf GT
1975 Buick LeSabre Conv.