750IL 1991 air quality problem

I need advice for 750il 1991model. I fixed this one after 2 years of struggleing. No more "check engine " light. Now this one runs very well and I am enjoying the 12 cyliner 290HP power.
But it still need some more improvement. I cannot have fresh air while driving. Sometimes have raw gas smell in damp day. and I don't kow to descrive the smell of not clear air. I wonder some exhaust gas is creeping in or not through vent. If I use heater, ther is more a little sweet smell, if I use air conditioner, there is less. Do you think this is a problem from some exhasust manifolder leaking? If so, what to do and how? Thank you for your opinions.
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When did you last replace the microfilters? If they're blocked the system may be sucking air in from any nook and cranny it can.
--
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Email: snipped-for-privacy@unixnerd.demon.co.uk, John G.Burns B.Eng, Bonny Scotland
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How are you? Thank you, Mr. Burns. I suspected the microfilter but my car record by the manufacturer's record, doesn't have microfilter. It seems my car made just before the installation.( some cars in 1991 start have it. but mine made in April and the micro filter not installed/designed). 1) Do you think( if you ever opended the air duct route), that I can still install the microfilter I bought to try? I am just worrying that the microfilter will change the air flow pressure and will cause false waring because this car has very sensitive sensors)? 2) if engine oil burns in the valve stem. would the exhaust gas come to cabin? 3) if ehaust manifold gasket is not tight, would the gas come to cabin?
Also, I believe this car was designed 12 cyl 290HP at the time of techniology. I wish it could have 350HP. Thank you.
John G.Burns B.Eng, Bonny Scotland

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Even the 4.ol V8 has 285bhp, the V12 should have 326+ n'est pas

After fifteen years the filters may need changing / unclogging. If the AC hasn't been run you may have escaped refrigerant in the vent system, if it has then condensation might have lead to mould growing...
Many systems will smell musty if unused for a while.
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