I just took my 300E in for a big service (big $$$'s). For kicks, I asked them to diagnose my rough idle. It was a 'gentle' rough idle that only gave the car a little nudge every few seconds when idling at a stop light... not a huge deal.
Well after being scoped and fondled, the idle is a constant rocking/heavy idle. It's worse than it was.
The diagnosis: maybe swap out the injectors, fuel distributor, maybe even motor mount. Great ideas. The same shop performed ALL this work for a previous owner 10k miles ago. Now they think a compression ring in #2 is failing and they hope it works itself out. Works itself out???
Ok, they can't find the problem. But make it worse???
My plan is to drop the car off on Jan 2, take a loaner, and let them think about it for awhile.
I would appreciate other thoughts.

What's your comfort level with this shop??

Hey,
My 86 300e does the same thing...
Please please please let us know what you find out. I'm interested in fixing mine also. I've just learned to live with it, but I don't think it should be that way.

I'd check the plugs and plug wires/wire ends first. Inexpensive, and they could be the culprit...

anthony

Thanks Anthony. That makes good sense. I had actually been thinking that my 30k service could have improved performance due to new plugs.
I have low comfort in the shop. I took it there because I thought having a history at that shop could help with 30k checkup/diagnosis (previous owner used them for about 10 years)... and I'm shopping for a service place. It's maybe a 6 bay outfit that's basically all MBZ. I looked it over when I dropped by for parts a couple of times.
So it looks okay... results to date are weak.

If you change the plugs, make sure they're the copper core, not the platinum ones. The type to use should be listed on the back page of your owner's manual (or somewhere in that area).

g'luck,
anthony

What you are describing is a cylinder misfire. Misfires can be caused by many things. However, the first step would be to change the rotor, distributor cap, wires, and plugs. Unless you are certain that they were changed recently.
As far as mounts, if they were broken you would have constant vibration in the car during idling. If this is the case, you can check them visually.

Vinamg

Thanks for the thoughts. With strong documentation and 30k service performed 50 miles ago, I know all elements you mentioned to be less than 12k old - plugs, of course, brand new.
I had the vehicle broken down and diagnosed again today.
On the scope I could see #2 not contributing, bottoming out at 30%. But it had good compression. So the tech decided we had a fuel problem rather than an engine problem. He was hoping for an injector problem (though injector abt 11k miles old). He swapped #1, #2 and found #2 still crapping out. So by default, that left fuel distributor. That too is 11k miles old, $1k Bosch rebuilt.
Bad news: 12 month warranty at 21 months
Good news: 12k mile warranty at 11k miles
Really Bad news: Whichever comes first
Great news (resolving original issue of the post): When the parts were reassembled today, the resulting idle was much calmer than when I brought it in this morning. Mission accomplished, I guess.
I'll have to see how well the shop stands behind its work (ie. what kind of deal we can work for the fuel dist... if I get that, the car should purr).

Bob,
Glad to hear you've diagonosed the problem. I was able to clean up my idle simply with a new set of plugs. I have to admit, I was over due. My brain gets clogged easily...
Happy New Year

Kyle,
That's excellent news. The cheapest, simplest route prevails. I know basically jack-nothing under the hood. But after working through issues with the tech, I have a greater appreciation of the various potential causes for hard idle.
My 123 got a rare night in the garage last night. So I got to test the 124 at 20 degrees. It cranked quick and smooth - the standing idle is tolerable.
Happy 15th birthday for your car (sometime this year).

Hi Folks:

Since it sounds like you may have resolved your problem, this isn't really necessary, but I thought I'd add this to the thread anyway for general reaction.

You mentioned that your injectors were replaced previously. I was told by an MB tech with whom I have a great deal of comfort that MB has changed the injectors that were installed in the early 300Es (I have an 86). I guess the diffusion pattern turned out to be worse when fed-mandated fuel additives began being mixed into fuels used here in the US. My memory is a little hazy, since this is a conversation that happened a few months ago, but I believe he said that the result is that there is some small amount of unburned fuel left between cycles which ignites at the wrong time or something. My 300E also has a gentle bumping idle from time to time, nothing really annoying - it has the original injectors. My tech suggested that I try the following experiment: Load up a tank of 87 octane instead of premium and see how it starts and idles and also check my gas mileage for that tank. Regular, he says, burns hotter than premium and cold starting and idle would improve. Sure enough. I also got better mileage with that tank. I'm not all that comfortable with consistently burning regular (engine heat issues - I recall that there is another thread somewhere in this forum about that), but I thought the experiment very interesting.

As a discrete procedure, just how expensive is replacing the injectors in terms of parts and labor? I do some DIY, but I'm not really comfortable with fuel systems.

Very interesting test. I'll try that next tank full. 3/99 prior owner paid $211.26 for 6 injectors. The labor was buried in 4.5 hours that also included swapping fuel distributor, replacing chain tensioner and reset timing. I don't know how much time for injectors alone.

Plug wire ends
 

How do you determine if a plug wire end needs to be replaced? Is there a way to test or visually inspect?

Bullet:

Thanks for the info. Please let me know your experience with the lower-octane test. It may be a superfluous test in your case because I would assume that in 3/99, the replacement injectors were the MB redesign/replacement my tech told me about. That should've eliminated one possible cause for rough idle in your car. At least, if what my MB tech told me was accurate it should have. Part of the reason I posted was to get reaction from MB folks with way more knowledge than I currently possess.

dtanesq,
You got better mileage with 87 octaine?!?! I get an average of 50 miles more per tank with my 86 300e when I use 92 octain fuel than when I use the 87. That's how I justify the extra money. My car also runs a bit rougher on 87 than with 92. Interesting that you had the opposite results.