I have a 520i euro spec, australian delivered E34 BMW. It has done 114,000km so far in its 14 years.
It's a great car which i intend to drive for a while
It seems to have a lag in the acceleration.
If i floor it, it takes around half a second then we experience some massive G forces, then at 3500rpm it holds for a bit, then goes full tilt.
I'm running it on 98 octane fuel (Shell optimax) and for a while, when i wasnt driving it... mum was putting 91 octane (Standard Unleaded) through it.
i plan to stick some kind of performance chip in it (it has a 405 ECU) to get a little more power from it and hopefully it will be a little less sluggish in the Economy mode and a bit more powerful in the sport mode.
Im curious as to why the car may be lagging a bit. It seems like it struggles a bit, and the engine does not idle properly... The whole car shakes, as if the engine was loose.. But i dont think that is the case. I think when the idle drops below around 800rpm, the rattling/shaking is at its worse. I am curious as to why the idle drops to this level and then jumps back up. Its very weird!
It has recently been serviced (113,500km) but this made no real difference to its running.
Im not sure what may be the problem, i'm just 17 and dont have a huge mechanical knowledge, especially with issues that seem to be fairly non-standard.
Maybe i should take it to the BMW dealership just across the road and get charged an arm and both legs to have them go over it, or just try and acertain what the problem may be and have my usual mechanic fix it - he is very good also, but BMW may be able to learn things he cannot.
Any help would be nice