Hi All,

So i recently changed all my plugs on 2013 W204 with MB original plugs from the dealer. The original did not have the resistor built in but the new one did. I replaced all 6 and torqued to 23 Nm. I replaced boots on #4, 5 & 6 because they were damaged during removal. Think I did everything correctly. Car is running fine, no rough idle like i was noticing before the replacement. No smell of raw gas or anything either. But I am getting 17 mpg mixed which seems fairly low. I think I was getting 24 mpg mix before. It is winter here and maybe it might be related to the change in fuel for winter, but I dont think I have ever noticed that big of a difference before.

I checked everything through OBDII and I am not getting any codes or issues. What should be my next step and figuring out what is causing the lower fuel mileage?

Only thing I can think of is it replace the boots on #1 - 3 and check the torque on the plugs again but I am dreading having to do the whole process again.

Ken

 

I doubt that plug boots could be the cause without obvious performance decline.

Take note of how long it is taking your car to warm up to 80 C. If the thermostat is beginning to fail or has failed partially open the longer warm up can cause fuel mileage decrease, especially if most of your driving is short trips.

My C250 is doing that now and I see a 3 MPG decrease.

If the thermostat fails completely open, it is obvious because the coolant temperature never reaches 80 C, then a DTC will be set and the CEL will be on.

 

Confirm the secondary air pump hose is attached properly...removal of top engine cladding to do plugs necessitates the one hose being disconnected.

 

Thanks I will check, I went through everything throughly when putting everything back but its possible I could have missed something. I am hoping its that. Another thought is that one of the boot plug springs that touches the spark plug came out while I was changing the plugs. I put the spring back in but maybe its not making good contact. I am also going to have to back track and remove everything to check if I torqued correctly. I am pretty sure I did 23nm but i guess i dont have much to go on. Thanks for the help!

 

Just found out my main battery is bad. Tested it with a battery tester which showed its capacity is at 43%. Wondering if that would cause poor fuel economy? I am averaging around 24 on highway. I dont remember what it showed before but in my trip meter, it shows 2.4 mpg when i first start my car and it goes up from there but in local street driving it goes up to around 14 - 16 mpg after around 8 miles of driving.

 

I doubt it.
If that is a M276 did you check the spark plug indexing ?

 

Well I torqued the plugs to 23nm during install. I used the Mercedes plugs with resistors. Original plugs did not have resistors.

Here is the part number 004-159-81-03. On the plug itself it says ZR6SII3320. On the old plugs the R was missing but the rest of the number was the exact same.

I am assuming the plug is correct. Will be checking the torque again tomorrow.

Is it ok to loosen and retighten these plugs? Or are they meant to be only tightened once?

 

thoughts on what others are seeing during winter driving on the northeast. Does this look right?

I went to my local Greenwich,CT dealership and the service person was very helpful. He showed me the plug I should be using which confirmed the spark plug I bought is correct. He also checked with a mechanic who told him if I don’t have check engine light I should wait till summer because in winter gas mileage is lower.



Mixed highway and screen driving

 

Can’t comment directly on winter mileage because we’re having one of our 80 degree Decembers. The winter gasoline blends are different to help with starting in cold weather. Unless the temperatures are a lot less than the 46 F in your photo, the difference will be minimal. When below freezing in the mornings is anticipated, the ethanol in the blend is reduced for a different oxygenation agent that attracts less water. When very cold temps are expected, butane is added to increase volatility. I kept detailed mileage records for company cars for years and don’t remember any significant change in winter. But the advice you got at the dealer is technically correct.

 

Thanks thats helpful. Whats the average I should be expecting? I am almost starting to forget what my normal was before the spark plugs replacement.

 

Well you said 24 mpg in your 1st post. That’s about what I get in my C250 in short trip suburb driving. With a 4matic C300 I guessed 21 mpg would be about right because you’re a bit heavier. I’m not in the northeast, but we get the reformulated gasoline (Houston & surrounding counties) to reduce NOx. That costs 3 mpg on the highway. When I fill up in areas that don’t have it, the difference is measurable.

 

I torqued the plugs this morning by loosing them and retighten with the same torque wrench. Put everything back together and I believe I am seeing a difference now for the better. Not sure what changed because the torque seemed correct but who knows. Maybe I missed something.

will report back after more driving.

 

happy to report my gas mileage is significantly better

 

28 MPG seems about right - - - in 50/50 City/Highway driving with fairly new spark plugs.

C250 coupe
39k
(wife's car)

 

Thanks for confirming. This was mostly highway driving with a mile or two of inner roads with lights and stops. I believe the car is rated at 19 / 28 mpg so its spot on.

 

You should really measure your mileage from RESET instead of START. Your two examples of 18 mpg and then 28 mpg were under totally different driving parameters. That said, when I replaced my tires from the original Continental tires with Michelin (both Mercedes OE designated tires), I saw a significant loss in mileage, as well. The Michelins were MOE for my 2004 SL500, but not the C350. So, I'm not sure if the Michelins were softer rubber or what, but there was a definite difference. I had gone with the Michelins because they were on sale at the time, but next time I will go back to the MOE Continentals.

 

you are right. The measurements were under different driving conditions. I was trying to see what kind of mileage I get on local or highway after the new plugs. Just the fact that I am getting 28 mpg on the highway confirms the engine is performing optimally otherwise I would not get that kind of mileage in any scenario.