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.

Andy_M · #2 03-20-2008, 10:12 AM

Nobody ?????

GDC · #3 03-20-2008, 12:40 PM

The O2 sensor on my '936 is in front of the cat, on top of the downtube in the front edge of the transmission tunnel. Pull up the carpet on the passenger side floor and you will see where the wires go into the trans tunnel

GDC

Ivanerrol · #4 03-21-2008, 03:23 AM

Change your fuel filter in the fuel distributor, Change your fuel filter near the fuel pumps. If you have let your fuel tank run low then it's possible the filters have picked up some residual junk that resides in the bottom of the tank. If you let your tank almost run dry it only takes once for this gunk to fill up your filters.

If you have no cat and no O2 sensor by lifting up your passenger side carpet you will see the wiring meant for the O2 sensor. see photo. If nothings plugged into these then there is no cat. To install an O2 sensor improves the engines response and economy however the cost to do this may not be justified as it may require alteration of the header pipes to the first muffler.

The famous resistive plugs can have an effect on performance as they get older, - make sure you have the non resistive plugs in. If you can't get Bosch non resistive then NGK BP6ES are the equivalent.

nissanzx1 · #5 03-21-2008, 10:25 AM

So, did you go ahead and change the fuel filter anyway?

-GH

Andy_M · #6 03-24-2008, 10:00 AM

Yes. Changed the fuel filter & took it out for a ride - no help. Checked fuel pump operation while I was there - it's OK. Checked for any vacuume leaks - can find none.

Used the car yesterday and it ran fine. Used it again a little later last evening and it was running a little rough again.

This morning comming to work it ran fine again.

Almost seems like when it is cold, it runs ok. The more I use it the more likely it is to start acting up, like as the engine temp or temp in the engine compartment rises the problem becomes more evident. This has me thinking that when the alternator went a couple of weeks ago, low voltage may have damaged the ignition coil.

JEdwards · #7 03-24-2008, 04:15 PM

How old is the coil on your car? I didn't see that mentioned. I was having a terrible time with my car and noticed the performance going south when it was wet mostly, and a new coil fixed pretty much all of it - power returned and everything.

Also, its been mentioned that perhaps your mixture is off, so that might need adjusting too.

When I spoke with a guy whose worked on these cars since their inception, he pointed out that sometimes rough idle is hard to track down because the CIS system isn't electronically controlled and that there were new cars that they weren't ever able to fix.

On a side note, be wary of replacing the fuel distributor - its very expensive to replace and most likely isn't your culprit. Its probably electrical.

edit: I see your already suspecting the coil... another thing I was told by the same mechanic is that if you are going to keep the car and its not a new coil, go put a new one on there. There is no way to test them other than replacement, apparently they can test out fine but can also be the problem.

rocky raccoon · #8 03-24-2008, 07:43 PM

You said that you eliminated vacuum leak. Too bad since the symptoms sound classic. Before you start chenging every high cost item in sight:

You say the car runs fine when cold then worsens as it get warmer. If so, my guess is a problem with the engine temperature sensor that feeds the fuel injection computer. That solved a similar problem on my car. Although we have different years and models, the FE system is similar.

Andy_M · #9 03-25-2008, 09:35 AM

Thanks Rock and JEdwards, those posts were helpful. The engine temp sensor isn't that expensive so I think I am going to pick one up as well.

I'll let you know how it shakes out. Might take a while now though...I ran out of heating oil at home this morning and it's going to cost me over $600 for 150 gals. today !!!

U-n-b-e-l-i-e-v-a-b-l-e