Post tune-up erratic idle - 300E
 

Hey guys -

I need your advice/help. I recently did some minor tune up work on my engine following a long summer of driving. I replaced the distributor cap (with original Mercedes part), the rotor (with Bosch) and the spark plugs (with German-made Bosch H9DC). I started the car up after replacing the parts and it now idles a bit more roughly...weird. The roughness in the idle is more prominent when the car is in "Park" and occurs once the car has warmed to operating temp. When the car is idling and in gear, the roughness is much less. It sounds like the engine is "stumbling" when in park.

Any ideas why this would happen? I installed the parts very carefully and didn't notice anything out of place. I was careful removing the spark plug leads as well.

This is driving me nuts as it ran better before I tuned it up!! Argh!

TIA,

Did you gap your new spark plugs prior to replacing them?

Yes, I carefully gapped them all to 0.8 mm. I read an earlier post below about setting the gap to 1.0 mm which may help. Maybe I'll try that.

Any other suggestions???

By a chance, did you purchase a tune-up kit from EBAY?
I've been wanting to buy one of those because their pretty cheap and my car needs a tune-up too, but was tell that EBAY tune-up kits are alot of times outdated chappy parts.
Like with those H9DC Bosch plugs, from what I heard, those are not to good for our cars. Platinums are better.
Maybe someone could help us both out on this.

Hey Jimmy,

No, I got most of the parts from my mechanic. I've always been wary of the e-bay tune up kits.

The rough idle has been fixed. I tried a few things - some fuel injector cleaner and a new air filter, as well as the good ole' "Italian Tune-up" and it worked wonders. Basically, I drove the car until the engine was warm, found a long, steady hill and hit the throttle. I ran the engine hard and it belched out some black clouds a couple of times, then ran clean. Ever since, the idle has been very good. It's hard to tell what worked, but it was probably a mix of all the things I tried...

As far as copper vs. platinum, I've been warned off the platinum plugs for my car by many qualified people. The M103 engine was not designed for platinum plugs - the copper ones are best.

Do A search on platinum plugs and you will find that they are not recommended fo earlier cars. I found this out a little late and I'm going to take $40 worth of brand new platinum+4's out and replace them with copper. It may be possible that the roughness is coming from a damaged lead or boot, seems to happen rather often. Once again, the search feature may give a lot of good info on that.

I agree with CJ87911. Stick with the OEM copper plugs. Also check that your wires are pliable. You can check the ohm resistance of each plug wire. If there is major variances between each of the six wires, they are deteriorating. If they are 14 years old replace them regardless.

Thanks guys for clueing me in on the plugs. I have alot to learn before I get my hands and wrenches into my 300e. That's why I'm on here.

I've always used platinum plugs in my v6 Mazda-powered Probes and they always performed very good. So naturally I thought platinum.
So looks like tomorrow I will be making a trip to AutoZone to return the platinums I bought last week. Glad I didn't install them.
Again, thanks for the info.
And I will search and research before doing anything.

brian, how do you test the ohm resistance for the
plug wires?

use a multimeter set to a certain KOhm setting(i believe MB wires using the resistor caps are at 5KOhm, i'm not sure), and stick one test lead into one end of the wire, same for the other end.

thanks

Supposedly your OEM 1987 plug wires' resistance are matched to OEM Bosch spark plugs. The Platinum plugs have a different resistance. Bosch has a chart that says these plugs will fit and have similar heat range but the resistance isn't the same. I am not sure what the danger is, but it is known that the insulator on Platinums can crack. However, I do use Platinums in my 1984 Rabbit race car with absolutely no problems.

italian tune-up
 

Zeus,

I'm glad you found the problem. This happened to me a month ago. Changed my oil then the rough idle. Changed plugs, no improvement. Thought it was the wires so I tried to rebuild the tips to not avail and ended buying a new set. The rough idle persisted... Performed Italian tune-up and viola!!!! It appears the #6 plug well or plug had gotten wet with oil. I'm still stumped on why it did that. At least I know now that the oil burned off and all is well.

FYI... the wire tips have a 1Kohm resistance while the plugs (H9DC) don't or have a small amount. The platinum plugs have their own resistance and that's why it's not recommended as per many of the posts. As for Japanese cars, the resistance is in the plugs and not on the wire and that's why their wires are so readily avail I suppose.

Oh yeah, use dielectric grease on the plugs so you can remove the boot easily. Never pull the wire by the wire. I think there's a special plier for removing the boot from the plug... but it's like $50.

have fun,

Check out Pep Boys if they are in your area. I bought the wire puller for $, I think, $24. Its like a pliers with plastic covers on the ends to protect the boots on the plug wires. You can also move them into different angle positions to reach those difficult plugs.

The best part of an Italian tune-up...it is FUN!!! :)

We need a Pep Boys in Canada I think. I should open one and give Canadian Tire a run for their money...