Hello,
Today my '99 ML320 (78,000 kiles) started missing when I first started it this morning. I could clearly feel that it was not firing all cylinders. After a moment i 'thought' it was running better but as soon as I got on the highway I clearly found that it had no "get-up and go". The engine just sounds different, indescribable but something is different sounding.

The CEL has not lit up but something is clearly wrong. Maybe I am jumping to conclusions since I have seen so many MAF failures posted on here but maybe that is it or maybe something else.

When I pulled it back in the garage I could smell gas which I've never noticed prior to this problem.

Any ideas? Can a code even be pulled from the computer if the CEL hasn't come on?

Thanks! John

If you have a Multimeter, you can check your plug wire resistance prior to paying for further diagnosis. If you don't own one, you can get one at Sears for pretty cheap. Simply pull each wire, one by one and stick one probe on one end and the other probe on the opposite end. It should read 2.2k ohms. I had 2 wires test out of spec. One was 2k ohms and the other was 8.5k ohms. I could have replaced the 8.5k one for $40 on fastlane, but I wound up buying the full set for $190.

I was able to get my hands around the metal connectors of the wires pretty easily after popping the top plastic engine cover off. It simply pops straight up and off. Be sure to give the ones connected to the coils a little twist prior to pulling them off.

Replacing all the wires took me about 15 minutes and that was including measuring the resistance of each.

The plugs are good to about 100k miles.

Glad to see that the forum is back up and running. Per Q's suggestion above I pulled all the plug wires and tested them with a multimeter. I found one wire was bad, all others seemed to be within range. I went ahead and replaced the entire set of wires with new. I still have the same problem even with new wires. I took it to Autozone to see if it could pull any codes, but nothing showed up since the CEL hasn't been on.
I dumped a bottle of injector cleaner into the tank to see if that helps. I believe I saw somewhere in the past that Mercedes didn't recommend using injector cleaner but didn't see any real harm in using it once to see if it improved. I am boggled at this point. Still running bad and no Check Engine Light.......
Plugs have 15K miles on them and the fuel filter and lines "should" have been replaced at 60K miles but I have since come to realize that the mechanic I was using wasn't as trustworthy as I had hoped. Just for the sake of it I pulled the cover under the back seat to see if I could lay eyes on the filter to see if it looked new (I have a pretty good idea what it looks like as I hand carried the filter and new lines to the mechanic) but all I see is a couple of fuel lines, and an electrical disconnect plug on top of a white plastic looking lid. Do I have to remove that plastic top to see the filter?? Who know's, hopefully it was already replaced (the silicon seal on the lid had already been broken loose) but it never hurts to double check just to rule out parts.

Our vehicle set a CEL immediately when it started running bad. Good luck. Might need to bite the bullet and have the dealer pull the codes and diagnose for you.

Autozone will only be able to pull up OBD2 codes. You may get more proprietary codes by using the dealer or an independent shop with a scanner that cam pull up more than OBD2 codes. The lack of the check engine light coming on would tell you you don't have any stored OBD codes.
What comes to mind is maybe the engine control module, it may need the updated control unit, do a search, you should find a thread or two about this. Does the problem go away after awhile?

Gilly

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Gilly, yes the problem is consistent. It is noticeable to me by the way the engine sounds because I know the normal purr of my engine but it doesn't feel like the engine is completely not firing on one cylinder I would think that could actually be "felt" when I drive it. I can hear a fluttering sound when it's idling and more of a sound like a manifold leak would sound (and lack of power) when on the highway. Kind of a wierd description but that's the best I can come up with.
Our regular indy mechanic is out on sick leave so I haven't been able to get it scheduled to be looked it. Dunno if I want to wait for him to get back or try a different indy.
I was not able to search for the engine control module thread. Every time I try a search it comes up with an error message in my browser...sounds like the website may need some tweaking since it was down.
-John