BMW Repair: running rich, air flow meter, bmw technician

Question
hello josh,please help! e30 1989 318i m40 engine 2door motortronic.the problem is she is running rich or at least smelling rich when warm after i have pulled up to a stop,the heater motor draws in a rich mixture smell for  a short time.when cold it does not seem to happen.the problem started  about 12 mnths ago when all of a sudden the car started spluttering and belching black smoke to the extent the car was nearly undriveable,the local bmw specialist reckons he did loads of tests (fuel pressure,temp sensor etc) and the only thing he could suggest after he failed to get co value down was to fit a replacement air flow meter.i got a used one which worked ok (after getting 2 duff ones),the co value was set and all seemed ok,but soon after i got the smell of a rich mixture.re checked the c o value again when engine hot but was within tolerances.fuel consumption was ok so left it at that.recentley (say Last 2 months) the rich smell has got worse with fuel consumption getting worse.i began asking around people at work who have had dealings with this model before (i worked for audi at the time) and had a few ideas what it could be so i have done a) fitted a new (although second hand but guaranteed working) blue two pin ecu temp sensor that fits into the head b) renewed all intake gaskets including throttle body to head c) reset the c o value.  everything seemed fine for a couple of days but sure enough on the slow stop start journey to work i could smell the tell tale signs of the rich mixture coming back.aaarrrgghhh!-petrol consumption seems to be ok though,any ideas? another guy told me that due to my normal m40 hydraulic tappetts being a little loud he mentioned valve overlap is not as it should be and it will always 'smell rich' ,he had the same car as me and could not get the hydrocarbons down at mot time and the only way to cure it was to install new lifters wich cured it ok.this was passed onto him from a bmw technician,incidentley i have replaced the intake pipe because it had a slight split.any ideas? surley hydrocarbons (unburnt fuel) would not smell this rich? i can understand it showing up on the c/o meter but not that much as to smell it out of the exhaust?  many thanxs in advance 07941 952653.

Answer
Steve,
 Unfortunately I dont have much experience with the 4 cylinder E30's.  I mostly work on the 6-cylinder cars.

 Have you checked the catalytic converter.  By some of the words you use It seems as if you are not in the US, so you may not have a catalytic converter.  Some times they get clogged from running rich, and fail to burn extra fuel in the exhaust.  
 You can check the injector duty cycle to make sure they are not being overworked, and also check the injectors to make sure they are not clogged.  Sometimes the pintle in the injector gets stuck open, and continuously sprays fuel into the cylinder(s).

 Have you checked the O2 sensor?  This reads the exhaust for oxygen content and adjusts fuel (duty cycle) accordingly.  If I remember correctly, they only have a life of 50k miles, although many last twice that long.  If your engine has been running rich, chances are that you have ruined the O2 sensor.  

 Thats about all I can recommend at this time, especially since I dont have much knowledge of this engine.

 Good luck,
 Josh