I had to drop my Y pipe to replace my starter and now there is a slight leak under acceleration. I triple checked and everthing is tight. It looks like these don't use any type of gasket at the Y, just a metal to metal flange. Is there anything I can use to stop this annoying leak. Also, since air is getting in or visa versa, its tripping the "check engine" light - probably due to the O2 sensor not seeing what it wants. What can I do?

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89 RS V8,Borg-Warner 9-Bolt rear,Open element,Flowmaster.