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Matt, Engineer
Category: Mercedes
Satisfied Customers: 21627
Experience:  Mechanical Engineer with 23 years experience in the auto industry, 8 yrs in formula 1 engine testing
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I have a 2009 SL600. That is the twin turbo V12. I had both

Customer Question

I have a 2009 SL600. That is the twin turbo V12. I had both coil packs and all plugs replaced within the last 2 years. The car is also Tuned and slightly modified by Renntech. I have been possibly over-using Royal Purple's Max Boost Octane booster and it seems that it might have caused some issues. I noticed an orange color inside by exhaust tips and read that it was also likely that other internal components might also have build up.
I got check engine light on a cold start and the scan read that is was multiple Cylinder misfires. And the scan on my ScanGauge II read "Open loop" The car ran in limp mode until it warmed up and was able to clear the codes. as i was not able to clear the code properly while the car was still cold. The CEL would return immediately. Then once the car is warmed up and I am able to clear the codes it runs beautifully. Until the next cold start.
however once while driving I got a check engine light and the code read that it was the "
Submitted: 1 year ago.
Category: Mercedes
Expert:  Matt replied 1 year ago.


do you have the fault code that was generated?

does the car run / drive OK when the check engine light is out?

Customer: replied 1 year ago.
the cars drives perfectly after CEL is clearedI get the p300 codes for misfires.. always the dems to be the same cylinders too..
Expert:  Matt replied 1 year ago.


OK as you've already replaced coils and plugs then I'd suggest the next step is to measure fuel pressure on a running engine if its less than

If thats OK then check over the exhaust for leaks before the Oxygen sensor as

this has beenexperiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.

this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst

The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers

Customer: replied 1 year ago.
hey matt.. So the fuel pressure is good.. no leaks in the exhaust. Cylinder 7, 8, 9, 10, 11, and 12 all misfire. Only one exhaust is heating up and it is the passengerside. the drivers side exhaust does not. This only happens at cold start and when I let it warm up i can clear the code and run perfect until the next cold start.
Customer: replied 1 year ago.
I think the left cat is clogged from using too much octane boost and or the Ignition control module? I really feel it is related to the octane use.
Expert:  Matt replied 1 year ago.


Ok thanks for the extra info, I' agree that a clogged / broken cat is a possibility

If you remove the oxygen sensor in front of the cat you can possibly peer inside to view the condition of the cat honeycomb, if its anything other than uniform and flat then it will need replacing