How the fault is affecting performance

First thing to say is that cleaning the MAF meter improved performance for a couple of days but then deteriorated! Back to the drawing board. And for information it might be of interest to describe how the fault is affecting performance.

Way back when the fault first started the first component to be suspected and checked was the EGR valve. This, as I have learnt,  lets exhaust gas back into the intake which reduces the amount of oxygen available for combustion and helps, amongst other things, reduce 'diesel knock'. One of the symptoms at this time was excessive smoke from the exhaust under acceleration and logic might suggest that this could be too much recycled exhaust causing incomplete combustion. However, this was the condition of the EGR.

About as bad as it gets I would think and looks like no recycling was occurring! However, 'diesel knock' was and still is more pronounced than it should be.

After much cleaning the EGR was put back and an immediate improvement noticed in fuel consumption and smoke emission. On a long run and being very economy conscious fuel consumption improved from 50 to 60 mpg! However, fuel consumption variability continues to be a characteristic of the fault despite cleaning the EGR.

Sometimes the car performs as though there is nothing wrong and then like a switch being thrown the deterioration kicks in. Fuel consumption increases and a small misfire and hesitation can be noticed around 1000 - 1500 rpm under low to moderate acceleration or load. But floor the peddle and the performance is still there.

The BMW dealer suggested two possible causes. One was the MAF and the other carbon build up in the cylinder head. Having taken off the inlet manifold to inspect the latter the condition was as follows. This is the manifold port. Not good, so the manifold and head was carefully cleaned  as carefully as possible.

After this, unfortunately, no improvement in performance was detected  but it's worth also noting that two vacuum pipes that actuate the EGR and swirl flaps were in poor condition and were replaced.

Also what I think is the MAP sensor at the back of the manifold was found to be almost clogged.





This is the MAP sensor.

Overall, the engine in a modern car is so crowded and these items are not easy to inspect without disassembly.

As an aside modern diesels (perhaps petrol as well) seem to have variable inlet flow paths. On the BMW the path is controlled by swirl flaps and these can be seen in the rectangular ports of the inlet manifold below. Once again they are controlled by the ECU and the purpose of these is to modify performance. There is much written on  internet forums about removing them as there also is with regards the EGR valve. However, it must surely cause havoc with the ability of the ECU to manage the engine efficiently.

In fact testing the effect of disabling the EGR resulted in excessive exhaust smoke (but perhaps this was caused by the suspected MAF fault).









Finally, having run out of further ideas the decision has been made to buy a new MAF meter!