1995 Toyota 4Runner

Smells problem
1995 Toyota 4Runner 6 cyl Two Wheel Drive Automatic 155000 miles

The day after I bought my 4runner (within 20 miles) I put a flowmaster on. If it sits for more than a day and I start it up a puff of black smoke comes out pipe, which I have always associated it is oil in the pipe. While during regular driving it has a strong oil/gas smell, to where if the windows are done my clothes smell like it, there is no smoke coming from the tailpipe other than at start-up after a day or more of sitting. There are no oil leaks underneath vehicle and also had rear axle seal replaced within 75 miles ago.
Any suggestions on what to try to fix?
April 12, 2010.

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