I had a fairly strong smell of gas that was hard to pin point until I stuck my head under the hood and sniffed around a abit- turned out to be a leaking EHA valve on the fuel distributer- they often start to leak around 180-190k.

Of course it could be something completely different if you're smelling it strongest at the exhaust pipe.

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1993 320TE M104
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past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
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"music and women I cannot but give way to, whatever my business" -Pepys