W124 idles extremely rough when operating temperature reached
 

I've had my 87 300E for about a year and a half and this is the first time it's stranded me. A few days ago, I stopped at my office on my way to my other job (my office is only 2 miles or so away from my house). When I got back to my car a few minutes later, it would crank, but refused to start. This has happened before, so I let it sit a few more minutes ... nothing. I bummed a ride to work and when I returned a few hours later, it started right up and I drove it home. When I was pulling into my street, I noticed that it was idling a little rough, but forgot about it.

The next day it started out fine, drove like it was new (I stopped at my office again). Then it wouldn't re-start. A few hours later, I drove it home.

I thought, well, this must be a hot-start problem. I got info on the OVP, checking the cap and rotors (which were replaced les than two years ago by the previous owner), etc.

I figured THIS time I'd drive straight to work. About half-way to my work (nearing my office), the car started sputtering. I was able to park it by my office, and it was REALLY running rough at this point.

It wouldn't restart. It was at this point that I realized that the problem started as soon as operating temp was reached.

A few hours later, (the car hadn't completely gone cold yet), I managed to get it cranked and proceeded to slowly head home. It DID NOT like idling, but I was surprised that as soon as it sputtered into 2nd and 3rd gear, it would smooth out and ran great (until I had to get it back to idling). I somehow made it back to my house...

I had installed new BOSCH Platinum Plus plugs two weeks ago. I'm wondering if that may be the culprit (it had run great until this week).

Might a bad cap and/or rotor/wires lead to a stumbling idle when the car is at operating temp?

Thanks for reading my dissertation...

Those "Platinum Plus" Plugs are completely the wrong kind for that engine. Find some good standard plugs, either NGK, or Bosch, and put those in. Preferably the ones you get at a MB dealer. NAPA or autozone should be able to hook you up with some proper ones too. Bosch "Super Plus" would work much better. Platinum plugs are not meant to be used in that engine. Get them out of there ASAP.

Yes, put in the correct plugs, but I would be very surprised if that is your problem. Try disconnecting the EHA and then seeing how the engine runs. It will never run as good, but if it doesn't make the same kind of drastic difference when engine is warmed up, then you will need to get a manual and go through the FI troubleshooting tree.

I agree, get those platinum spark plugs out ASAP and replace them with the new Bosch Copper Plus spark plugs ($20), and throw a new coolant temp sensor in as well ($35). The Bosch Platinum spark plugs are junk,
I have tried them in MBs and BMWs and they just don't work. My 300TE was running like crap last summer, to the point where the wife wouldn't drive it anymore, stalling at stop lights, hard warm start and so forth. No check engine light but just generally crummy performance (I could smell
unburnt fuel out of the exhaust)...did those couple things and the car has run like a champ ever since. When I bought the car it had Platinum plugs in it, I really don't like the Bosch Platinum spark plug in engines that don't specify them.....

Correct plugs and think about rotor arm.
 

Mine had exactly this problem after 2 years with the RIGHT (NON_RESISTOR - see item 2 below) plugs and 18 months on a 'new' rotor arm.

For the price of a rotor arm (I got my MB original for £30), I'd recommend trying that. I say this for seveal reasons:

1. On my 104 engine, the distributor is at the top of the engine, bolted close to the hot bits and it does get hot.

2. The rotor arm contains a 1K ohm resistor and this can break down under heat, giving exactly these symptoms. The suppression resistor is here and if you add more resistance at the plugs, misery is the only result.

3. My cheap Bremi rotor arm died after about 18 months.

4. Er, I can't remember the 4th reason.

>>> I've just remembered - I had a leaky EHA (no, the EHA, not just the grommets) and I replaced it with an old one I recovered from a 1987 4-Matic and all's been well ever since in that respect.

So, to recap, correct plugs and original quality distributor parts in that hot and hostile area are a good starting point before worrying about expensive things.

Bonne chance.

RayH

I replaced the old plugs...
 

Then I took off the distributor cap. As you can see, it had a bit of oxidized copper crud on the bottom and the center electrode is just about gone. I think I may have found the culprit. I'll let you know what happens when I replace it.

Yes, that looks in need of replacement ...
 

Mine was never that bad but I would certainly recommend changing cap and rotor arm.

Also, remove and clean the dust cover behind. I bought a new one and I think they are expensive - I shall find out today. The o-ring on the old one was very compressed so I feel better about the new one, although replacing it may have been overkill.

In that hot and hostile environment, I do recommend installing genuine MB-sourced parts as I've suffered with cheaper Bremi parts.

All the best.

RayH

Replaced distributor cap, runs great!
 

Thanks for helping me track down this gremlin!!!:) I can't believe the car ran at all with the dist. cap looking like that!

I recently had symptoms similar to yours. Turned out to be my rotor - and my rotor looked a lot better than the one in your photo! Mine looked good and tested good but it was not good.

Rotor replaced at last
 

I finally was able to replace the rotor today. When I replaced the cap a few days ago, two out of the three 3mm allen bolts that hold the rotor on were on so tight that they stripped. With the help of PB Blaster, an easy out and a trusty vise-grip, I was FINALLY able to safely remove them and replace the rotor. Thanks for all the advice! Running smooth!

I used a Bosch rotor on mine when I replaced it back in December....so far its been fine.....is Bosch the OEM/MB part supplier? The cap was Bosch too....the old one had some oxidation as well, but very minor compared to the pics in this thread!

Bosch is the way to go
 

I have no idea who made the awful cap and rotor on mine, but I made sure to buy Bosch (it was actually cheaper than what NAPA and Autozone wanted for Bremi's).

Bosch is/was OEM
 

TO my mind, there is no 100% guarantee that a Bosch part is as good as an MB-supplied part (that may also be Bosch). Different Quality Assurance standards may be applied when the batch goes to MB and, certainly, the MB-supplied part will have an MB reference number and the 'star' logo.

I imagine Bremi could win a contract to supply MB in which case they would have to apply MB QA standards.

My original rotor arm was Bosch/MB, as was the cap. What went wrong was that the resistor in the rotor arm (1 KOhms) broke down at high temperature and I had to wait for things to cool before it would start again.

And this resistor in the rotor arm is a key thing here as you do NOT want to add another resistance in series with the spark gap by installing resistor plugs. I have 12 high-priced plugs supplied by MB in France who swore these were right for my car. Only when I installed the correct non-resistor plugs did things start running right.

It might be interesting to short out the Bremi rotor arm and refit the resistor plugs. Any comments on that, anyone?

RayH

My dizzy cap was similar to yours in the photos. I was using resistor plugs as "specced" by my local parts store. I eventually realised that they should be non resistor, since then my dist cap is a lot cleaner (I check it and clean it every now and again) and the car runs a lot better.
The old cap can be cleaned and used as a spare, use needle files and fine emery or a mini die grinder to buff up the brass terminals. Yeah the clearances aren't the same but its a spare!

yeah, with Bosch I always look at the box and if it doesn't say, Made in Germany, I won't buy it. If you look at the packaging closely, Brazil or Spain or whatever may be the country of manufacturing. Nothing about or against those countries but I will not buy a Bosch part not manufactured in Germany. Really, if you go to the dealer and buy these parts they will ALWAYS be made in Germany. If you go to AutoZone or PepBoys or whatever, probably not...truth..it'll say Bosch on the box, but it's not same Bosch that you are looking for...costs more from the dealer for the real stuff..but whatever...