DIY / Electronics / Maintenance / Performance

Change Mercedes Spark Plugs: Tuning? Go for Newer, Colder Spark Plugs

Spark plugs actually have a temperature range that they are designed to operate in.  A colder spark plug doesn't fire colder, but it does help evacuate heat from the cylinder head faster.  Pressures with emissions and finding the right balance between a spark plug that will burn fuel completely even during short trips and one that can handle long hard driving is difficult.

Merecedes over time has revised spark plugs to one step colder on M278 and M157 according to Jason at OE Tuning.  Who recommends the current Mercedes spark plugs with oem gapping for the M278 and M157 even when tuned. NGK and Weistec do offer spark plugs which are colder I believe and some tuners may recommend changing the gap on either setup.

When tuners started with these engines they almost all suggested one step colder plugs.  Over time revisions occurred, and even official power output numbers revised.  This is an important step in your pre-tune checklist.

Changing Mercedes spark plugs are a DIY job but harder than normal.  With a DIY scale of 5-6.  It takes time, you have to go slowly and may not finish in one go.   After changing sparks you'll notice the ECU recalibrating and after this recalibration you may want to consider flashing to a tune if you have had previous trouble.

How to Change Mercedes Spark Plugs

  1. Get the instructions from Mercedes Star
    1. Ask, Search on FB, MBworld as needed
  2. Note the torque and special tools suggested
    1. Buy generic versions of the special tools
  3. Undo electrics with tool
  4. Unscrew two bolts per coil
    1. Unclip any electrical from fuel rod
    2. Unscrew wire harness between cylinder banks
  5. Remove coil with special tool
  6. Remove Spark Plug with special tool
  7. Add antiseize to threads optionally
  8. Add dielectric boot grease to end of boot and spark plug boot area
  9. Put spark plug in spark plug socket, tigheten to specifications, in the case of m278 17 pound feet, approximately tight plus a quarter turn.
  10. Insert boot back on
  11. Plug in coil
  12. Repeat

Fuel injectors have also been revised overtime as I understand for the M278 application.  Some I believe some upgrade these as well.  I'll do more research on the fuel injectors.

5 Comments

  1. I have read conflicting reports about putting die-electric grease on the spark plugs which changes the resistance and allows possible over-torquing when installing. A user on MBWorld suggested that the torque spec. by Mercedes is taking into account no grease added. Do you have any further information?

    • I understand the antiseize on the threads is always going to be a bit of a debate. With this application I kind of understand it, because the plugs index by torque. And in theory they should be changed often (or more often than the average Toyota). Die-electric on the boots I would 100% do. Anti-seize on the thread. If you’re in a wet environment, like the UK. I would do it, but maybe not go to town with the torque, use more feeling in terms of the tight +1/4 turn to where you get resistance to index. You can kind of feel when they are seated. If you’re not I believe I would go with STAR/MB instructions which likely don’t specify any anti-seize on the threads. NGK says to reduce torque by 30% https://www.ngk.com/learning-center/article/522/plug-torque-settings . I think this is sort of a fielders choice, with on one hand using MB directions, and procedure, on the other hand you don’t want to have the sparks seize. They both have risks and rewards, pros and cons. Thanks for the question Chris it is very good.

  2. I think the site might be broken, I see it says “2 comments” which I’m assuming you responded to mine, but mine was never visible and I don’t see anything at the moment in this section except for the “2 comments” header and then immediately after the “leave a reply” box…

  3. What’s the name of the coil bolt at 4)? What’s the best tool to use, 7mm 12-point socket ? Thanks.

Leave a Reply