240D running poorly after routine maint.

A few weeks ago, I snapped a belt. No problem... replaced all belts and back on the road. Now, my 240D is running a little rough and sucking the fuel - down almost 15 mpg on last tank.
Last night, I peeked under the hood and I found a crack in an air-check valve that is in-line running to the intake. Could this be my problem with poor mpg and sluggish driving? All else seems fine, fuel lines are intact, no evidence of leaks, and nothing else out of place. Also had valve adj. less than 4K ago.
I am going to replace it anyway tomorrow, but sure would like insight as to why this crack would offset my mileage and power this much.
Thanks,


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William
76 240D - 550K miles
78 300D - 200K+ miles