help,dont know what to do 420 sel
 

I bought a 86 420 sel around 2500 miles ago, after reading this great forum, I decided to replace the spark plugs. I posted a topic on what plugs to use. The previous owner had bosch platinum 4 in the car. I was directed by most of you to get rid of them right away and run the bosch super (wr9dc). After doing so the car ran bad i.e rough idle and missing. I put the platinum's back in and it seems to be running great again. My local parts house told me that the valves had set themselves to only run with theese plugs because of the extra firing of the platinum 4's, but this is bad because I have aluminum heads and that the platinum 4's will ruin my valves because theese plugs run real hot. Please any help would be great. this is my first MB and I wish to keep it.

I have never heard of the valve seat explanation.

But, if the gap is too wide on your spark plugs, the engine would miss. Recheck the gap.

I'd also make sure one of your spark plug wires hasn't shorted out, as well as checking out for a good connection between the wires and the boots.

Hogwash, pure hogwash.

Those engines will run on any plug you put in them. If they won't something is wrong. The only thing those platinum plugs might help is oil fouling. If you are in that situation you should fix the problem.

Just in case we've not had enough yet
 

Alright, I got suckered into Bosch Platinum plugs, not +2 or +4 just regular. Bosch only recomends this one for the M110. So I go to the Bosch website tonight, and what I've read contridicts some of the against arguments. Heres the link:

http://www.boschusa.com/AutoParts/SparkPlugs/Platinum/

eg: requires less voltage, cleans faster, remains cooler and better access to air/fuel mixture

Moreover, Bosch give you a money back guarantee of satisfaction.

All I know is for the moment I have a smooth idle, lightning fast responce and stonking acceletation, oh yeah I forgot its a 280 euro :)

I'll keep you updated if it changes.

You might check your ignition wire ends its likely you have disturbed a failing wire or not properly pushed one or more into place.

Good Luck,

sounds like either the gap was too wide on the supers or the base co is way off and the +4's are masking it. posibly the oil fouling issue steve mentioned as well. these engines will run on about anything you skrew in there but, seem to run best on the supers when properly tuned and with all fuel components working as designed. these are the last of the "tuneable" engines from benz and require a certian amount of knowledge to get right, when they are right they are wonderful mills.


Joe

Part of the problem is the new plugs you used. You used the wrong ones. I'm sure everyone on the board suggested using W9DC plugs NOT WR9DC. The "R" stands for resistor, your plug wires already have resistor ends so the resistor plugs would cause too much resistance. Get W9DC's and you should be fine.

hello steve

i'm not advocating any plug over another my question stems from yesterday when i tried to purchase the standard w8 or w9 plug for the 420 sel the person said the platinum plug replaced the others and it appeared that was the only plug they stocked. they did not have either w8 or 9 nor the champion plug listed in the OM.

so would you say that there shouldn't be a problem running the fine wire platinum or should i try to find the recommended plugs??

thanks much for the input
craig

ps: my car runs just fine. i'm just starting the engine after a cam replacement and doing some maintenance work on the rails and stuff.

Like I said they should run on about anything including the Plats. MB specifically says to not use the plats in those engines. I don't think it will hurt anything.

Well, good ....
 

catch Mike: "R" is bad news!

Regarding afmcorp's reply:
" . . . . when i tried to purchase the standard w8 or w9 plug for the 420 sel the person said the platinum plug. . . ."

I'll bet a nickel that the PERSON was a PEP Boys, AutoZone, Kragen, etc. clown! Don't buy parts from these places! Call Mercedes and get the right plug for the car! My 0.02 cents.

plugs
 

The 420 SEL does not have variable valve adjust, so they valves do not adjust to the plugs. I use W8dc, Mike is dead right the w9rdc has a resistor which will screw up your system as the wire connector has a resistor built in. I am sure the 420 will work on platniums but I personally noticed no difference on mine except the cost $32 vs $8. I would buy a set of w8dc's and make sure you gap them to spec.
m