Coming up on that 100,000 mile mark 2004 SL500. Mechanic quoted me $500 to change my plugs. How hard is it, does a mechanic really need to do this? Look forward to responses.

Here is a video link for a 2005 S500. Same procedure for R230, only configuration of engine compartment may vary slightly.


It took me an hour to replace mine in my former R129 SL500SA.

Clearance in the engine bay is fairly limited. You'll need something to pull the plug wires off - the left and right rear plug wires were a real PITA on my '08 SL55. You'll also need a plug socket, extensions, a U joint, and a good torque wrench (torque is 20 ft/lb IIRC). The process itself is fairly straightforward, and removing the coil packs makes it much easier (be sure to label the plug wires by cylinder/location as you remove them).

If I had a tool to pull the two really hard plug wires (or more grip strength) I could have done the job in two or three hours - that's all 16 plugs on an '08 SL55. If your SL500 has two plugs per cylinder, the plugs alone will cost a couple hundred bucks - skinned knuckles and frustration can easily raise the value of the service to what your mechanic is asking.

Easy job. Took J.A. just an hour; took me about 1-1/2 hours.

Not unless you are paying over the MSRP for them. Sixteen plugs at autohausaz.com comes to around $83, and the shipping is free.

$83 sounds awfully cheap. Autohaus AZ currently lists OE Bosch plugs for an SL550 at $15 each - that's $120 for eight. With twice as many plugs an SL55 would cost over $160 for NGK plugs and $312 for OE Bosch plugs.

Pretty good, huh? That's for an updated iridium plug. With platinum plugs the cost is just $44.

I chose the iridium.

I just did mine and the hardest part was pulling off the wires I used a set of long needle nose pliers that had a bit of a curve to them. And yes definitely mark your wires mine got mixed up and it took me forever to figure out what went where. Once you get the first 2 out it starts getting easy save your $500

After changing out the plugs, did you notice any improvement in performance? Did the removed plugs look corroded or were they still in pretty good shape?

Mine had 110000 miles when I replaced them and to be honest they didn't look too bad, the rear ones seemed to have a bit of burnt oil around the threads so I had a leak down test done and everything was within spec. The car did seem to idle a little bit better after the new plugs not much tho, I then reset all the adaptions with star and it smoothed out the idle more. I think the oil is a result of the pcv system pulling oil from valve covers. I will be adding a oil separator soon. Also I used the colder ngk's

I only have 20,000 miles on my 2007 SL600. Lots of plugs to change. I am planning to get it done next year, so far the car still runs like a beast...the plugs must still be good.

My '03 SL500 has only 23,000 miles and it runs just fine. However, at age 13 I'm wondering if I should change the plugs? What advice can y'all provide.

Even if the car runs well, for the amount of time and cost involved, I would change them. I have 52K on my 2009 SL550 and I will be changing mine this summer. Like oil, even thought the mileage is low, the oil breaks down and should be changed based on how long it has been in your crankcase. 13 years is along time for corrosion to take it's toll on your plugs.

Nice car!

Agreed......although the car runs well it just feels right that a tune-up is in order. Will take it to my indy mechanic soon and post the cost here.

You'll be glad you did.

the service life is 10 years / 100,000 miles, whichever comes first

How do you determine the colder plug vs. the normal

A Spark Plug Wire Removal Tool aids considerably in getting the rubber wire boots off as over time the rubber has a tendency to fuse itself to the sparkplug. $10 for the tool is lot better than having to replace a damaged wire.

I replace my plugs in my 2007 SL550 and it was actually pretty easy !. Just remember 1 plug @ a time. My SL has ignition coils so you don't need a boot removal tool. You'll need a torx to remove ignition coils and a get a torque wrench to torque plugs. I also got NGK Iridium plugs , spend the money you saved on plugs !! Total time was about 1.5 hr. Good Luck !!!

I agree with Happypr, just replace one plug at a time and you're all set. Inspecting spark plugs tells a lot about the engine's condition.

Plugs on the M723 are a much easier job than the M113. For one, the M273 has 8 plugs where the M113 has 16. And then on the M271, they are right there on top, but on the M113, they are on the side of the head and in a tight space - real knuckle busters!

You know, I heard that just the proper blood to gas/oil mixture makes a car go faster

Yeah, changing those 16 plugs on the M113 can be a real chore. With all the extra cleaning, I can see t taking 5 hours.