I would like to replace my '89 420SEL's spark plugs. (I figured that step-by-step instructions for doing so would be located here, on ShopForum, but searches keep coming up empty.)

Would someone please help me (and others) by either providing a straightforward set of DIY instructions for this job OR by directing me to an existing thread containing the same?

Thanks as always!

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Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)

1.- Remove the boot from the sparkplug (do not pull by the wire, grasp the boot and pull)
2.- Blow some pressurized air around the sparkplug (to remove debris, not necessary, but a good idea)
3.- With the socket and ratchet wrench, loosen and remove the sparkplug (counterclockwise)
4.- Install is the reverse of above, do not overtighten the sparkplugs. A good idea is to put some anti seize paste on the sparkplugs threads (if they do not have any)

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1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

Thanks for the speedy reply, pesuazo!

Questions:
1) Which plugs are the best replacements? (I know copper-core is preferred, but confused as to the correct Bosch part number to order.)
2) What is the correct gap spec for the replacement plugs?
3) Is there a proper torque spec for tightening the plugs?
4) You recommend use of antiseize on the threads--are there any heat issues that need to be addressed here?

Sorry so detail-oriented, but I've never done this before and want to get it right the first time...

Regards.

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Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)

Also,

It is a good idea to only do this on a cold engine.

Goldstone, I would only use Bosch w8dco plugs. They are copper and will work great. I definately would not recommend platinum. They seem to want to mess up a lot of times. The stock gap is .32 on those. I would recommend pulling one plug at a time and replacing it so that you don't accidentally get confused as to which plug wire is which. If you do this, you are probably looking at a 30 minute job max and that is checking the gap on the plugs you are installing before doing so and I would definately recommend checking the gap. Of course, do the pulling and installation should be done when the motor is cold. If you have not changed the distributor cap and rotor, it might be a good idea to put a fresh set and even inspect the wires. Hope this helps.

Thanks for the sage wisdom!

Checking my owner's manual, I note that the MB specification calls for a 0.80mm gap. SCOTTFISH, is there any significance to the Bosch plugs' "stock gap" of 0.32mm, or did you mention it only to indicate that adjustment would be necessary (to meet the 0.80mm spec)?

Also, I found some good info on the Service CD's (I always forget how useful these discs are... You just have to keep hunting through the myriad menus 'til you find what you're looking for.) Anyway, therein it states that the new plugs should be torqued to 25-30 NM.

Good generic advice about the distributor cap, rotor, and wires. However, in my case, I had this done about 18 months ago, so I'll wait awhile longer before biting the bullet on these parts.

Thanks again; I think I'm "good to go" now!

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Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)

I think he meant .032 inches, not mm.

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1982 300SD

Your also going to need a couple extensions for your ratchet to get at some of the plugs. And, when your beginning to set the new plugs in and get the threads started do not use your ratchet for this. Use only your spark plug socket and an extension and turn the extension with your fingers until you feel that the spark plug is threaded properly and turning smooothly. (another good reason to use a bit of anti sieze). Once your confident that you got the plug threaded properly then attach the ratchet and tightend to spec.

Surely he did! And since 0.32" = 0.81mm, it looks like we're all on the same page here.

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Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)

Goldstone, sorry for any confusion to you or anyone else. I stated earlier the the plug gap should be .32 and I meant .032 in. and in mm terms is .8 like the others have stated. I actually was thinking of my 300se by mistake when I quoted the gap. It turns out that the .032 is the gap for the 300se (inline six), but it sounds like you are on the right track of gathering information to help you in doing this service. Again, sorry for any confusion. I do have a question to anyone that can respond. I was advised to gap the plugs on my 300se to .035 by a mechanic and I have done that. But, now that I think of it and I researched the information, it turns out it should be .032. Can anyone tell me what differences, good or bad, it would make by widening the gap on the plugs? Thanks.

they say a slightly wider gap than called for produces a smoother idle. for my 126 the stock gap is .32 and my independent put new plugs with a gap of .36 or .38 I forget what exactly. Do a search on this you will find some good info.

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**A few Hondas etc... etc... Not Worth Mentioning**
83 Euro W126 380 sel Money Pit (Sold)
91 W124 300e 2.6. (Sold) 3.0 Motor is Much better!!
93 BMW E34 Touring Wagon (Sold) Sweet Car!!
91 300e (Just 200k with Ease.)Best car ever ???(Sold)
90 420sel (Black with Tan Interior) Best car ever!!!!!!!

Last time I tried to remove my spark plug boots, it required the use of a pair of pliers. I nearly tore one of the wires trying to get that damned thing off. Seemed superglued or something.

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http://comp.uark.edu/~dmgill/signature.jpg

Break the boots free by twisting before pulling them off. There's some kind of grease you can put over the spark plug insulator so the boots don't stick.

Sixto
94 S420
87 300SDL

S U C C E S S !!!!!

Following the good advice you each provided, I swapped my plugs tonight, and WOW---what a clear difference. First off, the engine turned over so quickly I couldn't believe it; I hardly touched the key! And the idle was as smooth as silk right from the get go.

The time was right for the job too: the old plugs were pretty badly corroded--especially (for reasons I don't quite understand) the two on the starboard (passenger) side nearest to the firewall.

Anyway, I just wanted to extend my thanks to those of you who offered tips and guidance, and of course to the rest of the Shopforum members for making this board such a great resource for all of us Benz owners.

Gotta say, it's pretty cool owning a Mercedes pushing 225,000 miles that still sounds, feels, and handles as well as she did 150,000 miles ago

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Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)