So onwards with the BMW and giving half a chance to redeem itself
What's up with it that makes it terrible? Well in short the engine
BMW designed these engines to work with this and that with various gadgets to be more economical both in emissions and fuel and althering the torque curve for that extra grunt in the RPM range. Their designs and work were brilliant, and it must have cost loads to design all this because what was left of the budget went into the materials used to create it.. and there my friends is their downfall
Nearly everything plastic or rubber on these cars are prone to failure and being an engine, that's quite a lot of stuff to go wrong. The plastic becomes super brittle and the rubber hoses turn to play-doh and soon split. Heat fatigue and vibrations wrecks them.. great place for this material then in an engine bay
Now I'm going to pick at their design, the fuel pressure regulator's vacuum take off is linked to the CCV system (CrankCase Ventilation). Well it works don't get me wrong as its linked to manifold vacuum, but jeez making the CCV unit from plastic that's prone to cracking under heat stress and vibration.. in an engine bay.. time bomb waiting to happen. So you lose your crankccase separating ability and lose the vacuum to the fuel pressure regulator and of course a vacuum leak in the manifold. Brilliant.
Doesn't stop there either, they have a Secondary Air Pump system (Referred to as SAP) that the forums say pump fresh air to the exhaust to help warm the catalytic converters up faster. I call bully on that. I've inspected that system and in no form does it connect to the exhaust whatsoever. Theres only a vacuum line from the manifold to the EGR type looking valve. But i'll show that further as I've already removed it. I'm not going to say that's it they're all wrong, but from what I've seen (not tested) it doesnt apply
There's the DISA valve which is designed to change intake runner length to suit the RPM range for peak torque across the board. Except the flap that controls that dries, crack and falls into the engine, then the little pin it rotates on also falls off into the engine causing complete engine failure
The best piece by far is the inlet boot. The piece of rubber joining the MAF from the airbox to the throttle body.. that dries and cracks, causing yet more airflow leaks and fuelling issues. The idle control valve is separate from the throttle body, which is also prone to failure, that in turn complicates the inlet boots design further and that's where the crack most often, allowing yet more unmetered air in
I'll stop there, and I'll pass by each piece as I go through this. I'm sure the picture is pretty clear from now, everything on these is prone to failure, expensive to replace for the same results a few years on and everything one way or another causes some kind of lean running condition