Hello, I’m in the middle of changing my plugs and ran into this. I got NGK LFR6AIX-11 plugs online specified for my 2006 SL55. They seem to be longer than the OEM NGK R’s in the car. I’m the second owner and have no idea if there were even the correct plugs in the car. Can any one please tell me if I have the correct replacements. It’s long trek back to my 24th floor home to do proper search on my desktop. Thanks
I think you are looking for WO 133-1631747 (SKU number) - NGK Iridium - Theses plugs also carry a ILFR6A part number - It appears that the ones you have are correct.
Bosch is FR6MPP332 if this helps
It states on those plugs that they are pre-gapped, but it also states - must adjust gap - 1.1mm = .044 in.
LFR6AIX-11. I’m wondering if the IX-11 means the are not correct plug? They are T least 1/2 “ longer! I’ve never had to replace plugs in any of my bikes or cars that had such a big difference in length. I’m afraid of plug contact with a piston. 😜. Hopefully some will chime in before I pack everything up and try this again another day. Thanks for your input though. It’s much appreciated.
I just got off the phone with my mechanic and he said there may be a cataloging problem with NGKs! The Lfr6a’s listed on a lot of vendor sites.
The plugs that came out of the car are NGK IFR6Q-G. He said these are listed on his MB parts list for 2007 and up SL55’s. Mine is a 2006 with production date of 12-2005! He told me not to install the current plugs I have or it can cause seriois damage!
Thanks guys. I’m going to the dealer tomorrow and talk to the parts guru there and see what he says. You are absolutely correct about why these longer plugs may cause damage. I really don’t think heads were modified so much as a longer plug would make contact with pistons or ?????? Thank you again.
In researching the NGK IFR6Q-6 that you took out of your car, that is for a Mercedes V-12 motor.
In researching the NGK LFR6AIX-11 that you purchased for replacements, The IX stands for "fine wire Iridium center electrode" and the 11 stand for the plug gap (1.1mm).
Also, even though these plugs may come pre-gapped from the spark plug manufacture, ask the dealer for the gap spec for your car. The plug purchased may be different than what is recommended by MB. (NGK recommends .032 on the LFR6AIX-11 for your car).
In researching the NGK IFR6Q-6 that you took out of your car, that is for a Mercedes V-12 motor.
In researching the NGK LFR6AIX-11 that you purchased for replacements, The IX stands for "fine wire Iridium center electrode" and the 11 stand for the plug gap (1.1mm).
Also, even though these plugs may come pre-gapped from the spark plug manufacture, ask the dealer for the gap spec for your car. The plug purchased may be different than what is recommended by MB. (NGK recommends .032 on the LFR6AIX-11 for your car).
Thank you so much for doing all this research and solving the mystery. All the service records of the car from day one did not indicate a spark plug change. The car was dealer serviced 100% it's whole life! So I have to assume the plugs I pulled out where installed at the factory! I'm heading over there now and bringing the new NGK plug to verify gap and thread length is correct.
Now I have this other concern! I have some mods from Renntech, ECU upgrade, carbon intake, pulley and resonator delete. The tune was with the old "incorrect" plugs. Will the new correct NGK's drastically effect my mods now? I'll have to contact Renntech on Monday I guess. Also my car had the P30 package which increases the top speed to 186 MPH - I doubt different plugs were needed for this factory option right? Thanks again for you help.
I confirmed after some much needed sleep you are correct on these plugs being for an SL65!
Went to see my parts guru at the location dealer today and confirmed what NOR CAL SL55 has kindly investigated for me. He brought out the OEM NGK plug for me and was identical in specs and size to the NGK's I purchased! The plugs in the car were incorrect plugs for an SL65 V12 motor. Thanks again Nor cal SL55! I can't wait to see what if any improvements the correct plugs will provide!
One last word here. NGK makes a spark plug, specific for the MB SL55 with the preset gap at .031. It is ILFR6A.
While the LFR6AIX-11 plug you purchased will work with gap adjustment, NGK recommends to not modify the plug gap by more than .008 in either direction.
One last word here. NGK makes a spark plug, specific for the MB SL55 with the preset gap at .031. It is ILFR6A.
While the LFR6AIX-11 plug you purchased will work with gap adjustment, NGK recommends to not modify the plug gap by more than .008 in either direction.
Yes, gaps have been corrected to spec. I think they were at .040 out of the box. Parts guy re-gapped then for me at the counter. What service! I didn’t even buy them from him. 👍
Now I just have to wait for my twisted back muscles to heal. I’m 6 feet tall and may jack the car up before attempting the job again.
fcp euro has the one step colder plugs, you can go with that
Yes, I know! I have had the car just over a year and put 10K miles on her. As stated, Renntech mods were also done with the incorrect plugs! The car ran smooth from day one and never misfired or stumbled. Significant power gains were also felt and verified on a dyno. I'm calling Renntech tomorrow to see if there are any concerns or possible "surprise gains" with correct plugs installed. I guess this may prove in some way how plug reach, type or even gap can have little or no ill effects in this engine??? My non car friend asked me "Isn't a spark all the engine needs to work? How does an engine know the type of plug you stick in it?"
I can see his point but, again in my particular issue, having the wrong plugs and gap didn't have any ill effects (except maybe for a 10 mpg average in spirited & daily driving) I'm sure my case is unique and other factors may be involved.
Let us know of the results of the plug change and what you find out from Renntech. I bet you will find it more responsive.
Will do. I found more horrific things when I finished the job last night. There were 4 correct plugs in the engine. I don't remember which cylinders they were in because I didn't notice until I already installed the new plugs! So I was running 4 correct NGK MBZ plugs gapped @ .034 and 4 incorrect ones @.030! How did this thing even idle?
Well, gentlemen.....she is alive! Holy crap! I feel like I gain 20 HP! She idles just as smoothly as before even with the wrong plugs but, throttle response and WOT are much more improved. Renntech is still baffled why my car ran so well for over a year with the grossly incorrect plugs and gaps! They credit the Mercedes ECU for probably "fixing" things. They have offered a reflash of my ECU to see if there can be even more improvements. I guess a lesson learned for me at least. Incorrect plugs with incorrect gaps will still allow the M113 to run well especially if you have nothing to compare to. No misfires, stumbling, erratic idle etc. I'm waiting to see if my mileage improves too. I'll update again soon, in the mean time, I'm going to enjoy my "new" car for a the weekend. Thanks again for all the vital suggestions and insights! You guys are awesome.
Good to hear! You have a great looking Car - Enjoy!
Thank you again for all your help! I owe you one for this. BTW, I’m now averaging about 14 mpg compared to 9 mpg! With the new found power, I suspect I’ll dip back down to single digits on “spirited” drives. Mileage is something I don’t care about in this type of car but, having correct plugs really do make a difference guys. Check your plugs if you haven’t since purchasing your SL55.
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I'm heading over there now and bringing the new NGK plug to verify gap and thread length is correct.
My non car friend asked me "Isn't a spark all the engine needs to work? How does an engine know the type of plug you stick in it?" 
