Long distance road test results are in.

After the disappointment of installing a new MAF meter I was looking forward to evaluating the performance of the car over a long motorway journey. Unfortunately, there were no pleasant surprises such as a miraculous improvement.

Overall there are so many confusing observations to make. To begin with I filled up at the beginning of the journey with the higher grade diesel as had been recommended by the BMW dealership. They advised that this was a good idea every fourth tank or so. (Presumably it contains less impurities and perhaps some cleaning additives). But as I've noticed before the performance problems can seem more acute after a fill up?!! However, I'm reluctant to get side tracked down fuel related routes.

So after a journey returning 50+mpg at 70mph I once again connected my ELM interface and collected data over an urban route with my smartphone. This was after the hesitation and faltering seemed to be getting particularly annoying.

I then disconnected the vacuum pipe to the EGR valve to see if it was in any way related to this device. It did reduce the symptoms but not completely and performance seemed flatter but was preferable to before. For urban driving the mpg was certainly in the 40's so nothing dire was happening to the fuel consumption. It seems that the EGR valve is more of an emissions related device and so I decided to keep it disconnected for the return leg up the motorway and collect data once again.

Having looked at the graphed data it is radically different to that collected previously under no load! There is very little in the way of a correlation between MAF, boost etc vs engine revs as there was before. Whether that is to be expected or not is unknown at this point.

One thing I did think that must have a correlation if boost pressure vs air flow rate and this is shown below with the EGR valve disconnected i.e. permanently closed. I'm intrigued that above a certain pressure the flow rate becomes erratic. An air leak maybe?

This is with the EGR connected during the urban driving. There are obviously a number of differences such as considerably lower flow rate for a given boost pressure which would be expected given that the EGR valve is letting in exhaust gas. But there is also no sign of the valve shutting and the flow increasing either.

Another exhaustive search of the internet revealed many more BMW 320D owners with similar problems and spending a fortune with specialist but no magic bullet!

Next step check for air leaks after the turbo.

New MAF now installed!

Apologies for the delay in posting. I'm moving house which is proving very time consuming!

After spending £130 + new air cleaner with MAF Shop in Berlin the new MAF is now installed and the result is ........... only a slight improvement! :-(

Possibly that might be an understatement because I've not had to chance to take the car a long trip yet. (That will be this weekend). Perhaps the fuel consumption does look better but the 'misfire' is still there and the MIL dashboard light is still lit.

My first reaction is annoyance with BMW who, to be honest, I'm sure are no different to any other manufacturer. Why don't they disclose the real meaning of the codes! And I mean the REAL meaning of the error code. Would it undermine their business to disclose what analysis the control unit is performing to generate the exception. It seems that they have been forced to comply with international regulations to provide a standard method of reporting but they have only reluctantly complied!

So the investigation continues. It's interesting that

• After resetting the light with Torque it illuminates almost immediately after the engine is started. There is some initial diagnostic taking place that is checking various parameters and finding them out of limits.
• The misfire/rough running occurs around the time the turbo is beginning to work and is generating positive inlet manifold pressure. 

This is now making me suspicious of the EGR operation despite the fact it has been cleaned and now seems to operate freely instead of being carboned up. What I have to find out next is how and when the valve operates. I know it's vacuum operated and I know where the electric solenoid valve is. But is it operating correctly? My suspicion is fuelled by the fact that the misfire seems to occur at 1800rpm which, as I understand it, is about the time the EGR valve is closing due to the inlet pressure being greater than the exhaust gas pressure.

All that said I'm still suspicious of the volumetric efficiency curve. This is calculated by measuring the theoretical air flow through the engine using inlet pressure, inlet temperature and rpm and comparing it against the measured air flow. The ratio of the two is the volumetric efficiency. 

I'm surprised that the maximum recorded value when the turbo is working is still less than 100%. (I'll try logging when the car is being driven rather than sitting on the drive). The second 'anomaly' is why the dip around 2500rpm?!

Vital difference between petrol and diesel engine operation

As I struggle to find simple ways of interpreting the data obtained from my car there has been one thing that has puzzled me. (In reality there are many more than one!) The question is, where is the throttle on my diesel? All the other engines I have experience of (all petrol) have a butterfly valve controlling the airflow into the engine. Where is it on my diesel? Well the answer provided by Denbury Diesels is that there isn't one!

The explanation is that petrol and diesel engines operate in two different ways. Whereas a petrol engine maintains a constant air to fuel ratio the diesel just alters the amount of fuel. Using a fire as a crude analogy the petrol engine controls the air vent to vary the heat output where the diesel is equivalent to getting more heat by adding another log. This basic information answers many questions raised by articles on volumetric efficiency etc that assumed the reader knew the engine was petrol.

So looking back at the volumetric efficiency graphs it could be expected that the efficiency is flat until the turbo kicks in. The really interesting work will be to see how the graphs looks when the new MAF is installed.

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!

Can volumetric efficiency help confirm my suspected faulty MAF meter?

I hadn't heard of volumetric efficiency until I found the excellent articles on the MOTOR magazine web site. These contain a wealth of practical information on engine operation and faults and have been the inspiration for my on going work.

Basically, volumetric efficiency concerns how efficiently air flows through the engine and is the ratio of measured flow to theoretical flow. On a normally aspirated engine this is not going to be 100% because of several factors. The more obvious of these are the resistance to air flow offered by the air filter, valves and ducting coupled with the back pressure etc from the exhaust system.

The torque app calculates volumetric efficiency and so, just like the MAF data, I plotted this against engine revolutions.

This was the data before cleaning the MAF meter. (Note: at approximately 2500 rpm the turbo begins to provide boost. At approximately 1800rpm, I think, the EGR valve should be closed and therefore the MAF meter is reading all of the air flow into the engine.)



And this after cleaning


It clearly demonstrates the effect of cleaning the MAF! Why is the improvement so much more noticeable on this data than the simple MAF figures? I'm not quite sure but air flow temperature and manifold pressure are used in the calculations and these might now be measuring more accurately as well.

One thing that does surprise me is that a 100% wasn't reached. My surprise is because the engine has a turbo charger and so the air is being forced through the engine which will tend to negate the air flow resistance mentioned earlier. So could this mean the MAF meter is reading low? That would be consistent with information I have read on the most common characteristic of a faulty MAF meter. Perhaps some readings taken whilst driving may be useful. (The above and earlier graphs are based on data obtained with the car stationery).

To buy a new MAF or not to buy?

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.

Can the ECU provide more information to diagnose the problem

With a genuine replacement MAF meter costing upwards of £170 (dealer price £312.02!!) is there any further diagnostic work that can help determine the cause of the error code. And as well as the error code is deteriorating performance and fuel economy. In attempt to diagnose the problem with more confidence the Android Torque app in conjunction with the ELM 327 interface was used to obtain some basic engine operating data.

This is the air flow rate entering the engine plotted against engine revs.


Next the MAF was cleaned and the air flow rate measured again.

Interestingly the correlation looks cleaner i.e. not so many spurious plots and for reference this is the cleaner used.