How did you tie the SL55's down?
I ran my SL55's without the front tubes for a few months without issue - apart from rubbing on the hood liner during WOT's.
No CEL.
I'm thinking if they intakes were loose - they may have caused an error from something else..? Coil packs still all good?
Note - I have an SLK32 - same engine - maybe slightly different clearances.