bosch platinum+4 plugs 420 sel - PeachParts Mercedes-Benz Forum

Louis · #1 10-09-2002, 07:55 PM

I recently bought a 1986 420 sel, it has a rough idle and is very sluggish from a dead stop, once up to normal speed it runs great. I noticed it has platinum4 spark plugs, are theese good or is there better plugs out there. Any help would be most useful as this is our first MB. Thanks Louis

MikeTangas · #2 10-09-2002, 09:24 PM

Dump the plats and get some regular old copper core Bosch plugs, the non resistor type. I think you need W9DC plugs. Working from memory on this one, but should be close.

blackmercedes · #3 10-10-2002, 09:25 AM

Mike is right! Most MB's abhor platnium plugs, and you're not only wasting money installing them, but probably going to end up with running problems that you'll have a devil of a time finding, as you won't blame brand new plugs...

stephenson · #4 10-10-2002, 06:39 PM

C'mon Guys,

Given the correct length, temp range, etc why would a MB know what kind of material the plug is coated with? I get confused enough about things ...

blackmercedes · #5 10-10-2002, 06:46 PM

Well, this topic has been discussed at length, and sorry to say, but it seems for whatever reason, most MB's, especially ones of your car's vintage, don't so well with platnium plugs.

The right plug is listed in your owner's manual, and experience shows that it is usually the best plug for the job.

old blue · #6 10-10-2002, 07:19 PM

I can vouch for this firsthand,

My 85 500SEL had the same platinum plugs in them from the PO.
(Probably got the typical upsell from the local auto parts store)

I replaced them with the Bosch Super's first thing, and the old girl was so appreciative.

Get those things outa there ASAP!!

old blue · #7 10-10-2002, 07:26 PM

The Bosch Super's should help with the rough idle.

As to the sluggishness, I have three easy recommendations.

1. Be sure to use premium grade fuel
2. Check your fuel filter and replace (easy DIY) if in doubt on age.
3. Run a bottle of Techron Fuel system cleaner with a full tank of gas on your next long (hwy) trip.

That is if your 420SEL drives like it is thirsty, and not getting enough fuel delivered.

Good luck, Michael

MikeTangas · #8 10-10-2002, 07:27 PM

Goerge, as I understand it the biggest problem with the plats is the size (diameter) of the electrode. The plat electrode is barely a pin prick while the Cu core plugs have a huge electrode. Picture the difference between a Big Chief school pencil and that fine .5mm mechanical on your desk. That pin prick of an electrode gets fouled, or otherwise throws an inconsistant spark while the Cu core throws a large well defined spark.

stephenson · #9 10-11-2002, 10:21 AM

OK, gotcha. If there is a physical difference in size, I can see why the combustion physics might get mucked up ... thanks for making it make sense.

I can also see the danger in getting a thread going on this ... sort of like the "do you think synthetic oil is OK?" maelstrom of inputs!

Talking about plugs makes me wanna go out and find that '99 S320 (long) I have been lusting for .... and, then I remember the discussions on $10K worth of electrical parts ...

#10 10-11-2002, 01:36 PM

Dittos, all the way around!!

Myself, just sold my well-maintained 88 420 SEL. Last year, put the plus-4 plats in them. A big mistake!!

They ruin the car's idle and low end torque, etc.

Stick with the copper plugs. No question about it!!