I am working on a 99 E320 with a 166,000 miles and a 3.2L V6. the problem I am having is only at an idle, mainly when the vehicle is in gear at a stop, otherwise the car seems to run fine while going down the road. The check engine light was on when I first started to look at the car and I pulled codes to find a P0300, P0302, P0304, I had replaced the plugs and wires on the according cylinders(Cyl 2&4) with no change. I have been told that the plugs were changed less than a year ago, and I know that doesnt mean they are good, but they appear to be oe style plugs and I would not expect them to be causing a running condition due to how well the car runs off idle. I am also wondering, what is reason there are two plugs per cyl, are they there for emission reasons? well any help would be greatly appreciated,thanks. I have already cleaned the MAF with no change in the running condition. Also as I have been running further checks on the car I have found that i may have a leaking fuel injector/injectors. Fuel pressure checks good, but I am losing pressure at rest after I pressurize the system and close off the rail from the rest of the system(this is done by having a shut off valve between the rail and the feed line and closing the valve after pressurizing the system) I will lose almost all pressure after an hour or so. Due to the fact there is no return line on the rail the loss must be through an injector/injectors, but as I am watching data on the scan tool I dont see a fuel correction for a possible problem. I do have a long term fuel correction of 10 on the left bank and I have not been able to duplicate any codes that were in the vehicle to start with. I am wondering if I may have a vacuum leak somewhere or maybe an EGR issue. I have very little experience with mercedes cars so I am not to sure where to go with this