Help with a surging/rough running issue - PeachParts Mercedes-Benz Forum

Andy_M · #1 03-19-2008, 07:37 PM

I could use some opinions on this one before I go dump a ton of money.

I have a '90 300E 3.0L. 135k miles that we bought about 8 mos ago. I had the Head rebuilt 6 mos ago because I found a dead cylinder - burnt valve. New plugs, wires, cap & rotor, and replaced a couple of injectors at that time also. Car has been running great.

About a week ago, I started noticing a couple things about the way it was running: 1) gas milage seemed to be down a bit, as did power. Watching the economy gage, I could tell that I was having to use more throttle than previously to maintain a given speed. 2) I started noticing a surging while maintaining a steady throttle setting. It wasn't terrible, but it was noticable.

Last night it got noticably worse. Acceleration is very poor and is rough. If it was a carb motor, you'd be pumping the accelerator to get it to accelerate. The surging is much more noticable. I thought - fuel filter.

I bought a new filter today. Jacked the car up, put her on stands and climbed underneath. After removing the cover over the fuel filter/pumps area, I caould see that one fuel pump looks new. I would say easily less than a year old. The other looks ancient - might well be original. The fuel filter looks moderately new...not REAL recent, but not ancient either. The hard lines that connect the pumps and filter look fairly new as well. Looks like maybe the PO had this issue before. Now I'm thinking the older pump is probably on it's way out.

Not sure what the life expectancy of these pups is, but 18 years and 135K doesn't sound too shabby to me, so it makes some sence to me that the old pump becomes the primary suspect at this point.

Someone also suggested the O2 sensor, but if there is one under there I sure couldn't find it. I see what looks like a front pre-cat, a flat oval shaped cat and a muffler at the back. Nowhere downstream of the pre-cat can I find a sensor. Didn't look forward yet.

Sure would appreciate any comments from those more experienced than I.