2014 GL550 Spark plug removal and replace in 278 engine
Thread Starter
Junior Member
Joined: Nov 2018
Posts: 16
Likes: 1
From: Flowermound ' TX
Mercedes S 500…2014 GL 550, ..2011 GL 450
2014 GL550 Spark plug removal and replace in 278 engine
I just left an indy shop and wanted to have plugs replaced while they were doing warranty work, and was told if i had them, they would put them in during the teardown and back from the warranty work. low and behold , no where in writing anywhere, anything signed or posted, then tol no customer parts allowed from another asspciate. told them that i was already purchesed them and okd to do so. wated to charge close to $$1400 to do them, even though the old ones needed to come out anyway from the warranty work. A matter of swapping old for new when replaceing them in . Still wanted to charge the full amount as if starting at the beginning. Needless tosay lost my business as i felt mislead. so with the plugs, wanted to see if anyone had the proper steps to replace and about how long. Is it difficult as my S550 had the rear 4 plugs pretty c`l,osedmin and took forever to get out and in. Are these similar or open more room. Thanks so much. Is this a typical issue w Iny shops? never used to have an issue w bringing in my own Mb parts at a fraction af te price, even if they didnt want to warranty the part.
Senior Member
Joined: Jan 2015
Posts: 306
Likes: 78
gl450
First of all - $1400 for the plugs is silly. The dealer charges about $600-$700 for spark plugs on m278 GL - parts and labor.
As to doing it yourself - it is a pain because of the two spark plugs closest to the firewall. Very little room. Make you life a lot easier and get a proper wobbling socket for the plugs. I used a regular socket and a wobble extension and it was next to impossible to do the last two plugs.
Additionally, the spark plugs on m278 must be indexed, so make sure you have a good torque wrench. Also, depending on how many miles your GL has, you may consider replacing the coils while you are at it, as they are not crazy expensive. In any event, a tool for pulling the coil leads from the plugs helps as well (the kind that grabs the lead at the plug, rather than pulling on the lead)
As to doing it yourself - it is a pain because of the two spark plugs closest to the firewall. Very little room. Make you life a lot easier and get a proper wobbling socket for the plugs. I used a regular socket and a wobble extension and it was next to impossible to do the last two plugs.
Additionally, the spark plugs on m278 must be indexed, so make sure you have a good torque wrench. Also, depending on how many miles your GL has, you may consider replacing the coils while you are at it, as they are not crazy expensive. In any event, a tool for pulling the coil leads from the plugs helps as well (the kind that grabs the lead at the plug, rather than pulling on the lead)
Newbie
Joined: Apr 2013
Posts: 12
Likes: 0
S500
Boot removal
Thanks so much.. Did you use a special tool to remove the coil boots ? Heard they are tough to get off… hv a hard spot on top can use to pull off? Thanks
First of all - $1400 for the plugs is silly. The dealer charges about $600-$700 for spark plugs on m278 GL - parts and labor.
As to doing it yourself - it is a pain because of the two spark plugs closest to the firewall. Very little room. Make you life a lot easier and get a proper wobbling socket for the plugs. I used a regular socket and a wobble extension and it was next to impossible to do the last two plugs.
Additionally, the spark plugs on m278 must be indexed, so make sure you have a good torque wrench. Also, depending on how many miles your GL has, you may consider replacing the coils while you are at it, as they are not crazy expensive. In any event, a tool for pulling the coil leads from the plugs helps as well (the kind that grabs the lead at the plug, rather than pulling on the lead)
As to doing it yourself - it is a pain because of the two spark plugs closest to the firewall. Very little room. Make you life a lot easier and get a proper wobbling socket for the plugs. I used a regular socket and a wobble extension and it was next to impossible to do the last two plugs.
Additionally, the spark plugs on m278 must be indexed, so make sure you have a good torque wrench. Also, depending on how many miles your GL has, you may consider replacing the coils while you are at it, as they are not crazy expensive. In any event, a tool for pulling the coil leads from the plugs helps as well (the kind that grabs the lead at the plug, rather than pulling on the lead)
Member
Joined: Jun 2014
Posts: 92
Likes: 6
2014 GL450
Hi,
What does " the spark plugs on m278 must be indexed" mean in the quoted message? Does it only mean they have to be torqued properly, or does it mean something more than that?
Thanks!
What does " the spark plugs on m278 must be indexed" mean in the quoted message? Does it only mean they have to be torqued properly, or does it mean something more than that?
Thanks!
First of all - $1400 for the plugs is silly. The dealer charges about $600-$700 for spark plugs on m278 GL - parts and labor.
As to doing it yourself - it is a pain because of the two spark plugs closest to the firewall. Very little room. Make you life a lot easier and get a proper wobbling socket for the plugs. I used a regular socket and a wobble extension and it was next to impossible to do the last two plugs.
Additionally, the spark plugs on m278 must be indexed, so make sure you have a good torque wrench. Also, depending on how many miles your GL has, you may consider replacing the coils while you are at it, as they are not crazy expensive. In any event, a tool for pulling the coil leads from the plugs helps as well (the kind that grabs the lead at the plug, rather than pulling on the lead)
As to doing it yourself - it is a pain because of the two spark plugs closest to the firewall. Very little room. Make you life a lot easier and get a proper wobbling socket for the plugs. I used a regular socket and a wobble extension and it was next to impossible to do the last two plugs.
Additionally, the spark plugs on m278 must be indexed, so make sure you have a good torque wrench. Also, depending on how many miles your GL has, you may consider replacing the coils while you are at it, as they are not crazy expensive. In any event, a tool for pulling the coil leads from the plugs helps as well (the kind that grabs the lead at the plug, rather than pulling on the lead)
Member
Joined: May 2008
Posts: 178
Likes: 17
broom
Indexing means making sure the open side of the spark plug tip faces a specific way, in this case somewhere within a 90 degree radius towards the center of the motor.
There is some debate about whether this is really necessary however.
Some say absolutely, some say just torque them down to factory spec (23 nm)
P.S. changing plugs yourself is not what I would categorize as a fun job... as one previous poster noted the plugs near the back of the motor, particularly on the passenger side are not easy...imo not only do you need a wobble socket, but a variety of extensions and swivels is useful depending on which plug you are doing. I found that a set of long needle nose pliers with the curved end specifically for hoses is helpful to get the coils off...depending on age and condition they might be a little stubborn, but I popped all mine off fairly easy with those pliers.
I thought this would be easier than the 157 on my W212 E63S, and it was actually more difficult as I had to disassemble the rear firewall plastic inserts and factory braces to access the rear plugs better. Plus the height of the truck makes working on the back plugs more challenging...just my $0.02
Last edited by boosted305; Dec 25, 2025 at 11:40 AM.
Super Member
Joined: Sep 2019
Posts: 622
Likes: 212
From: My house
SL55AMG, GL450, SLK, S550, Maserati Coupe
I have replaced spark plugs on many vehicles but the M278 I would not do. Only the Bosch plug specified for this engine, will do, any other will be totally incorrect as they were designed so that when torqued exactly right, the open gap on the plug is facing the right way inside the cylinder. There's some who believe that installing them wrong, could ruin the engine. Weird that they designed them this way. I wonder what a MBZ dealer would really charge to replace these plugs, I would call a dealer with the car's VIN and ask for a price quote. Because: I replaced all the plugs, plus wires, on my M113K engine in my SL55 AMG, and afterwards I called two dealers asking what they would charge. First one said, $3950.00 plus tax (!!!!!) and the second dealer quoted about $500 less. I bought correct parts online shopping and the cost was about $400, so how was this job a $3500 labor?! There is now ay I'd expect either of these two dealers to quote anything near $600 for the M278.
Trending Topics
Member
Joined: May 2008
Posts: 178
Likes: 17
broom
I have replaced spark plugs on many vehicles but the M278 I would not do. Only the Bosch plug specified for this engine, will do, any other will be totally incorrect as they were designed so that when torqued exactly right, the open gap on the plug is facing the right way inside the cylinder. There's some who believe that installing them wrong, could ruin the engine. Weird that they designed them this way. I wonder what a MBZ dealer would really charge to replace these plugs, I would call a dealer with the car's VIN and ask for a price quote. Because: I replaced all the plugs, plus wires, on my M113K engine in my SL55 AMG, and afterwards I called two dealers asking what they would charge. First one said, $3950.00 plus tax (!!!!!) and the second dealer quoted about $500 less. I bought correct parts online shopping and the cost was about $400, so how was this job a $3500 labor?! There is now ay I'd expect either of these two dealers to quote anything near $600 for the M278.
The dealer is such an incredible rip-off.
The head unit in my GL550 died and, just for *****s and giggles, I went to the dealer to find out how much a new one would be.
The unit was $4k and "installation" was $1k...I told the guy, this is why Benz's depreciate so hard, because the dealer makes it punitive to own one out of warranty. (I ended up getting my own unit repaired for $800...still not cheap, but an incredible bargain comparatively)
Regarding the indexing, even factory plugs (which are just Bosch with a MB logo on them) don't always line up the right way when torquing to factory specifications...I had to end updoing the method that YouTuber "Exotic Car DIY" used: sanding the crush washer down in order to get the plugs to index in the right position...
MB World Stories
The Best of Mercedes & AMG
Manual Mercedes? 6 Times Sindelfingen Let Drivers Have All The Fun
Verdad Gallardo
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Verdad Gallardo
8 Classic Boxy Mercedes Designs That Have Aged Like Fine Wine
Verdad Gallardo
Flawlessly Restored Mercedes 190E Evo II Heads to Auction
Verdad Gallardo
Electric Mercedes C-Class Unveiled: 11 Things You Need to Know
Verdad Gallardo
Mercedes EQS Gets A Major Update: Everything You Need to Know
Verdad Gallardo
5 Underrated Mercedes-Benz Models That Don't Get the Love They Deserve
Verdad Gallardo
Mercedes 300D Has Pushed Well Past 1 Million Miles and It Ain't Stopping
Verdad Gallardo
10 Most Reliable Mercedes-Benz Models You Can Buy Used
Verdad Gallardo
Junior Member
Joined: Jan 2004
Posts: 24
Likes: 3
From: California
1983 300TD, 1998 SL500 (sold), 2013 GL450
Absolutely do not trust the factory Bosch plugs and torque spec- when I did mine the factory torque spec aligned each plug in a different orientation. Ended up hand sanding each copper washer to get the alignment within +/- 45 degrees of the 12 o’clock position to the injector. What a PITA but I’m convinced that if the dealership or an Indy did the job it would have been torqued to spec without regard to orientation. Now whether or not the orientation actually makes a difference, who knows…
Member
Joined: Jun 2014
Posts: 92
Likes: 6
2014 GL450
Now I know what "indexed" means under this context. I really doubt any dealer or any garage will make sure that the spark plugs are indexed correctly. Properly torque them is the best we can expect, I think.
Are Germans really good engineers? I used to think so but now I am having a different opinion. Their cars are not that reliable and their design making their cars so difficult to repair, I really don't know if I would call that great engineering.
Are Germans really good engineers? I used to think so but now I am having a different opinion. Their cars are not that reliable and their design making their cars so difficult to repair, I really don't know if I would call that great engineering.
Member
Joined: Jun 2014
Posts: 92
Likes: 6
2014 GL450
Once the spark plugs are installed, how would know that if it is indexed correctly or not? I have bought the Bosch spark plugs, and I checked them, I don't see how you can tell if they are indexed correctly once installed. Maybe I should mark the other end (i.e. the end that the ignition coil should plug onto) to show the direction?
Thanks!
Thanks!
Member
Joined: May 2008
Posts: 178
Likes: 17
broom
Once the spark plugs are installed, how would know that if it is indexed correctly or not? I have bought the Bosch spark plugs, and I checked them, I don't see how you can tell if they are indexed correctly once installed. Maybe I should mark the other end (i.e. the end that the ignition coil should plug onto) to show the direction?
Thanks!
Thanks!
MBWorld Fanatic!
Joined: Jan 2016
Posts: 2,749
Likes: 881
From: In the Shadow of the Tetons
2013 ML350 Bluetec
A witness mark on the end opposite the electrode would be an easy and permanent way to know the spark plug's orientation. A slight notch on the end of the plug ought to do it.
Junior Member
Joined: Jan 2004
Posts: 24
Likes: 3
From: California
1983 300TD, 1998 SL500 (sold), 2013 GL450
I marked all sockets and extensions with a sharpie and lined that up with the open end of the spark plug, once everything is torqued to spec you can then see which way the open face is pointing.
