1998 C230 Fuel delivery issue > fuel pump ?
 

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 !

Start with a fuel filter.
Your oxygen sensor has nothing to do with starting. ;)

There is no CE light? Probably not an O2 sensor, and I think the above statement is true, as the sensor is not in play during a rich cold-start.

I'm not familiar with the fuel delivery systems on the C230 (it's my own car, guess I better bone up) so I'm not sure if the car has a fuel pump relay (I suppose it does) and that would be a suspect.

Things to check:

Ignition: Plugs, plug wires.

Fuel: Pump, pump relay, filter.

There might be some stored codes even without a CE light, you might want to have them read.

The beating noise would have me suspecting the fuel pump.

How does it idle when not in gear?

I would say ....unless u have a particular code stored in there...that tells u that a component is wrong.............

then ...

Drive the car until whatever is not operating well actually breaks down.....it will cost u 50$ to tow it. At least u will not be guestimating and replacing parts that don;t need replacement.

I had a similar problem....I spent 200$ changing minor things....plugs, wires, fuel filter, cleaning throlttle body....then I gave up and decided to drive the car until it broke down......

but hey what do u know......the next day the car was normal ....it's been a week ......no symptoms whatsoever yet......knock on wood.

good luck

Replacing a fuel filter on a 5 year old car, with 76 k miles on it, is hardly considered a waste of money. ;)

I had a similar problem- dipping idle at stop lights. It started with a mild dip on startup, which got progressively worse. I changed the fuel filter just as a part of routine maintenance (It had about 30.5K on it, and had seen some tough duty cross country at slightly shady looking gas stations), and this cleared the problem right up. I still get an very occasional and mild dip at a stop, but is extremely rare (I think I've noticed it once in the past month or so).

Fuel filter/service/siren sound ???
 

Ok, I am going to replace the fuel filter, air filter, spark plugs, and change the oil. We will see if this corrects the idling issue. On another note, I have gotten a loud siren noise while starting the car twice in the past week. Literally sounds like reverb or the sound of the emergency broadcast system. Seems to be coming from under the hood with radio and ac/heat off ?? The most bizarre sound I have ever heard a car make...high pitched whine.....freaky....almost as if the noise was coming from the speakers !!! ??? No mechanical issues accompany the sound except for the idling issue. Will post after work is done.

I don't know if your vehicle has an air pump for polution control, but they have been know to 'cry', particullarly if the by-pass / diverter valve gets stuck. But suspect anything with a bearing that rotates. Idler wheels, alternator, water pump, power steering pump, or the belt itself.