Although there does appear to be some noticeable improvement does this indicate the MAF is the key cause of the problems? Could it just be a contributing factor and perhaps another component like the EGR valve?
Without a doubt a modern internal combustion engine is a complex system. Before starting this work I had very little knowledge of turbo's, exhaust gas recycling (EGR), mass air flow (MAF) meters etc. but with the help of my smartphone, Torque Android app and device to connect to the car management interface my knowledge is increasing rapidly. I never thought a computer would become as important for maintaining my car as my spanners, allen keys etc!
On my diesel engined BMW I now know that the air passes first through the filter and then the flow rate is measured by the MAF. Next the air enters the turbo where the pressure is boosted and sent through the intercooler before it enters the EGR valve. The EGR valve then mixes the clean air with exhaust gases (for combustion reasons to reduce diesel knock and emmissions) before it enters the inlet manifold and two possible routes. One is a longer route and controlled by a butterfly valve in each inlet manifold port. The purpose of this route, as I understand it, is to get better driveability e.g. smoothing pulling away/ torque at low revs.
From the above description it can be appreciated that there is great margin for error and great necessity for monitoring and control which, of course, is the job of the Engine Control Unit/Engine Management Computer/Power Train module or whatever you want to call it. And as I've found there are a multitude of sensors measuring things such as pressure and temperature and actuators and controls to make things happen.
On my car the actuators tend to be vacuum powered. For example, the EGR valve is opened and closed by an electrical valve which controls a vacuum which acts on a diaphragm and opens a hole to allow the exhaust gas into the stream of clean air. But the decisions the ECU has to make are complex.
For example, the ECU has to know that the exhaust gas pressure is higher than the inlet pressure. So it has to close when the turbo is beginning to develop pressure (which implies the EGR valve only operates at low revs). Then there is the question of how much exhaust gas to admit. This is deduced by measuring the air flow by the MAF and knowing how fast the engine is turning. Overall, there are many things that can begin to go wrong as they get old, dirty and worn.
On my diesel, one of the first things I looked at was the EGR because comments I had read in other forums and blogs seemed to point to this being a potential cause of some of the symptoms. I took it off, simply cleaned it, and got 10mpg better fuel consumption on a long run. That was an amazing return for simple cleaning. However, unfortunately the necessity for cleaning, I'm reasonably confident to state, has been caused by the problem I suspect is the MAF meter. The coking up of the EGR valve really does look like too much unburnt fuel and I hope this is not affecting things like the turbo or exhaust system. But I want to be confident it is the MAF before spending a lot of money on a replacement.
One of my biggest problems is that I don't know what a correctly operating engine should look like. So are there any other parameters I can measure to help? Well I've come across something called volumetric efficiency. This might give me further indications. It's a measure of how much 'gas' is actually passing through the engine compared to the theoretical amount. More about this in the next post I think.
