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

I would check the catalytic convertors. They might be cracked hence the noise you describe also check your Mass Air Flow Sensor. I had same problem a while ago.

The diagnostic capabilities of a proper scan tool make simple work of most types of these problems.

The first thing would be to evaluate a number of actual values. The first I would look at would be smoothness. This is a bargraph representation of the crankshaft acceleration rate at the calculated moment of firing. Older cars had to use sophisticated testing tools to gain "power balance" readings. These now can be had by a proper diagnostic computer that draws the info from the cars own diagnostics.

From the "power balance" info one should be able to ID a misfiring cylinder. A futher testing allows one to add or subtract fuel in 7% increments (up to near 30%) while watching the same bar graphs. If a cylinder is low fueled an increase in fuel will cause the misfire representation to change (for the better in this case). If a cylinder is weakly fueled it will also be more affected by a 7% (or more) reducing in fuel.

The same testing can be done with timing increases and decreases.

Ignition can also be tested by eliminating the "A" coils or the "B" individually. If only one coil is working the cylinder will die while the working coil is subdued.

Adaptation values both over time and of the moment are also readily available and of great importance in diagnostics.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Well, took it to MB of Cincinnati and found that it had a bad catalytic converter. Thankfully it was replaced under warranty but the 'other' catalytic converter was not replaced since it hadn't yet failed. Should I be concerned about the other one failing soon? I only have 2500 miles until the warranty is expired.

It'd be nice if they could just replace it because you're almost out of warranty, but of course you can't fault them for not doing so either. I wouldn't lose too much sleep over it, if it happens it happens. BTW the cats are covered I believe by the Emission System Warranty which is alot longer than the 4yr/50,000 warranty, just so you know that. It may depend on what the exact type of failure is, but if the engine isn't running correctly because of it, you can kinda assume it's a type of failure that'd be covered. Check your warranty book to see what the length is of the Emission Warranty

Gilly

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Yup, you are correct. The warranty covers the emissions on the vehicle 8years/80K miles. I presently have 77.5K miles on the car. It really would have been nice to have both replaced but at least they took care of the immediate problem. Parts guy at the MB counter told me the wholesale is $899/per cat and the retail is somewhere around $1200...holy cow! Anyways he told me they'd sell it to me wholesale price it the other one fails. I guess time will tell....
Thanks! John