I have been happily looking at
SL350 7G-Tronic cars around the 2009-2012 era 28.5 mpg 315 bhp, 6.2 seconds 0-60 , when i spot the
SL350 AMG SPORT with 39.2 mpg, and 306 bhp, 5.9 seconds
There doesn't seem to be much difference between the 2 in looks but the difference in mpg is appealing to me, can anybody tell me if there are any other differences between the 2 and/or explain why there is such a difference in the mpg between them?

 

Don't know where you are getting this info from but there was only one SL350 engine in the R230 which was in production between 2008 and 2011. This engine was type 272.968, 3498cc, 311bhp, 266lbft. After 2011 the R231 was produced.

 

There is an optional sport appearance package (called the Wheel Package) that is sometimes called the "AMG package". It does not change any performance features - it's just appearance items, except for one thing:it adds a "manual" shift program to the transmission. Or, at least it does on USA models - I am not sure of that's a global option or not. I can't imagine the manual mode making any difference in MPG, or in performance.

 

There isn't a 39mpg SL anything, must be a mis-print. Probably 29.2 instead of 28.5, could be a different wheel/tire combo or possibly a different rating from a different source, generally the manufacturer doesn't offer that accurate of a MPG, they would both be listed as 29mpg.

 

Since the OP is discussing an SL350, which was not available in the USA, I assume his MPG is in UK terms (to the OP: Please update your forum user's profile with your location!). This means that UK 28.5mpg would be more like US 24mpg, and that makes sense for a V6 version, which does not have ABC (the ABC pump is a huge drain on MPG).

 

Thanks to all who responded, it seems that i was thrown by misinformation on the Autotrader adverts where some have given the R231 fuel consumtion figures for R230 vehicles. Thank you for clearing it up for me.

 

 

This not only affect the SL350, but the SL550 as well. The M272 and M273 engines were built with the possibility of the timing chain idler sprocket being improperly hardened. This is commonly refereed to as "balance shaft failure" because the sprocket is part of the V6's balance shaft. On the V8, it's just an idler sprocket, but the same supplier had the dame problems with that part, too. It wears prematurely, which can be anywhere between 20K miles and 220K miles. My 2007 CLK550's sprocket failed under 30K.

Here are details:

http://benzbits.com/BalanceShaftTSB.pdf

Note that Scenario #1 is the bad failure. The engine has to be pulled for the repair. Technically, the V8 can be repaired in place, but most shops pull it as you have to lift it anyhow to remove the oil pan, and at that point, it;s only an extra hour of labor for removal, and it makes the job much easier. The other two scenarios are cheap and easy repairs - $100 in parts and a fairly easy DIY job. Although this is a service bulletin, there was never any warranty extension. Some owners with expired warranties had it repaired under "goodwill". On mine, the car was under the 50K miles warranty, but was just over 4 years old, so MBUSA did agree to cover it. There was a class-action lawsuit that at first was dismissed by the judge, but was later negotiated with a settlement. Unfortunately, the settlement is useless now. It only covered out-of-warranty failures that were already paid for or occurred prior to the class action suit.

The TSB lists the engine serial numbers. Basically, any M272 or M273 engine built through 2006 is in the range for this potential failure. This means that all cars with those engine through MY2006 are included, and about half those for MY2007. By MY2008, the problem was resolved. If you are considering a MY2007, check the engine serial number by plugging the VIN into this website:

https://www.datamb.com/

Note that there are two schools of though on this issue. One is that only a small number of these will fail. Maybe it was an improperly heated furnace used bu the supplier that did not harden the steel enough, or maybe the operator was drunk or not well trained, so the parts from that run are bad. The other school of thought is that they are all bad to some degree, it's just a matter of how early they will fail. of course being a moving engine part, this sprocket will wear and eventually becomes a "maintenance item". Is that 300K miles? 550K? Whatever it is, I don't think anyone would expect it to last less than 200K miles.

The good news is that this failure does not appear even as a blip on the radar of the various consumer review websites and publications. Regardless, I have always recommended to used car buyers avoid engines in the range. It may be just fine, but why take a $5000 gamble? Especially when many of these cars (like W209s and W203s) are not even worth that much.