On returning home fr. a 2000 mi trip my wagon started losing power (slightly) & when I stopped at the off ramp there was blue smoke coming from the exhaust (it went away). Car has 181,000 mi, burns no oil, gets 26 mpg on trip. M-B dealer replaced orig. radiator cap and ran fine to home (300 mi). Temp. gauge was normal, no "check engine light" came on. I'm concerned it might be something else with that mileage. The loss of power feeling wasn't as dramatic as when the Air Mass Sensor went a few yrs. ago. That was more of a surging. Any ideas what happened & why a new radiator cap would cure it? Thanks,