So I buy this '90 300E is super nice shape with 120k on it. My first MB. Guy tells me that the car sometimes idled rough for his wife, but he thinks it is just because she only drives it 1 1/2 mile to and from work...never gets to temp. Guys says it has just been tuned up, and the oil changed by his "Mercedes guy." A sticker on the window and the maintenence records bear this out.

I drive it for two days..never does it once. The on the third day it does it to me, but clears out as I bring the rpm's up and is fine the rest of the day. I determine that it is actually a miss that is causing the "rough idle." For the next 4 days it does this once per day, each under different conditions, and at different times. In the meantime, I have filled up with a good quality 92 octane fuel and added a can of injector cleaner. On the fifth day I fill the tank again and drop in one more bottle of injector cleaner. The rough idle does not repeat for the next full week. Car has quite a bit more power. I think I have it nailed down - cheap fuel and maybe dirty injectors.

Then Friday I take a trip to a football game. It is raining steady. On the way to the stadium, when I'm almost there, I stop for a coffee. The rough idle starts again as I wait at the drivethrough. This time it isn't clearing up when I raise the revs, and it doesn't clear up as I drive down the road. I'm almost at the game, so I complete my journey and park it at the stadium. To my suprise, when I come out of the game and start the car it continues...and does so all the way home. It has not done this before.

Next morning I start by pulling plug wires. Cylinders 2 and 3 are dead. I pull the plugs. Problem #1 found - the guy has Bosch Platinums in the car and cylinders 2,3,and 6 are all fouled. Plugs 2 and 3 are really bad, but the cyllinders seem to have good compression. In goes a new set of Champion plugs. The miss is still there. Recheck 2 and 3, 3 has spark jumping along the outside of the metal plug wire end and I get zapped as I remove it. I don't see anything on 2, but 2 zaps me as well. Realizing I won't be able to get a set of wires locally I try something and wrap the metal ends of 2 and 3 with electrical tape. The miss gets much better, but is still there. Back to the wires - 2 seems OK now and the idle drops as I remove it. 3 is dead as a zombie. No spark from the wire at all. I also see that, what appers to be a fairly new distributor cap, is cracked from overtightening.

So a new set of OEM wires, a distributor cap, cap seal, and a rotor are all on order from mercedesshop.com. Hopefully this will put an end to my mysterious intermittant rough idle problem.

Lessons learned:
1) Take nothing for granted. I would think a "Mercedes guy" would have known not to use HEI plugs in a non-HEI motor. New tank of fuel, FI cleaner, new plugs, cap, rotor, filters as soon as you buy the car. Establish a base right away.
2) Maintenance records don't tell you anything about how GOOD the mechanic was or wasn't....check it anyway.
3) These boards are fantastic information sources, but be careful reading the sources of other peoples problems that are "the same." as yours. They may just SOUND the same because of someone's perception or description of the issue. Use them as information, but each problem has to be troubleshooted (not sure that this is a word, but troubleshot doesn't sound right either....) on it's own merits....shortcuts don't usually pan out.

 

I have had the same issue and am going to change my wires, that is the last item to be replaced for my TE.

I would be very careful with that energy going through those wires, these ignition systems really pump out the juice.

 

I would suggest doing three additional things as a form of "preventative maintenance".

1) Check and replace any leaking or dried vacuum line.
2) Unplug electrical connectors in the engine compartment, visually check for corrosion and spray with something like WD40.
3) Locate the main ground/earth connection in the vicinity of the battery, remove the connections, clean/wire brush the area, spray with WD40 and reconnect.

The W124's are known for "Gremlins", most of which are the result of vacuum leaks and poor electrical connections and grounds !!!

 

You think dielectric grease would make sense for all the terminal connections?

 

Sounds like good advise. I will do that as well.