Hello Doc. 

Last Friday evening, I was travelling home along a de-restricted dual carriageway and was held up by some slow moving vehicles in the outside lane at about 45mph. When the lane cleared, I accelerated firmly and, as usual, my Mercedes GL 420 CDI took off with its customary amount of vigour, but only for a few seconds! The engine management light illuminated on the instrument panel and the car then had much reduced power, with no apparent turbo boost. Oh dear! Next day it was the same thing. So I decided to use the Mercedes-Benz “Mobilo Life” service and a technician was duly dispatched to my home with a van full of equipment. 

Interrogating the ECU with a laptop revealed two separate fault codes, both relating to low common rail fuel pressure. The in-tank fuel pump sounded noisy which gave both me and the mechanic the reasonable impression that it was failing. Investigations ultimately revealed that the (low pressure) in-tank fuel pump was fine, and the main fuel filter itself was to blame, being almost completely blocked. Once a new filter was fitted the fault apparently remained – until the new filter was re-coded into the ECU, whereupon the car became fault free. 

Whilst the engine covers were off, I asked the dealership to replace both air filters and also re-code them to the ECU, as they were also apparently almost completely blocked. On modern Mercedes-Benz cars you just can’t simply replace consumables such as these filters without re-setting the ECU to recognise the fresh part – something that certainly would complicate home car maintenance. Any comments, Doc? “Lack of maintenance” you will undoubtedly say, and I would have to agree! But not according to the owner handbook, where the fuel filter is supposed to be replaced every 50,000 miles or four years, and the air filters at the same interval. 

I had a Mercedes-Benz service plan until recently and, under this plan, only the work outlined in the handbook for that scheduled service would be done, and anything else would be extras. These filters were due for replacement this August, at the next scheduled service, and four years and 32,000 miles since they were last replaced, so well inside the Mercedes recommendations. 

I was always led to believe that regular diesel fuel filter, air filter, and oil and oil filter replacements were all vital to ensure the longevity of both the fuel injection equipment, and indeed the engine. From now on, and as long as I own this car, I will get the fuel and air filters changed at two year/17,000 mile intervals, to be on the safe side! 

I have worked out that my “Gargantuan Gas Guzzler” is averaging about 21 to 22mpg so, over 32,000 miles the fuel filter will have filtered about 7,000 litres of diesel, and over the Mercedes-Benz recommended 50,000 mile recommended filter life (if it lasted that long) the filter would have to filter perhaps 11,000 litres of fuel! I assume that recommended maintenance intervals are more related to the perceived cost of maintenance than the actual needs (and life) of the vehicle? 

So, a warning to your good readers. Treat recommended maintenance intervals with a great degree of scepticism and change oil and oil filters at least annually, the fuel and air filters every two years (assuming 8,000 miles or so per annum) if you want your car to last and not break down on the road. I assume that you would agree, Doc? Incidentally, since getting the filters replaced the fuel consumption has improved to about 25mpg, indicating perhaps just how blocked the air filters were! Regards, Danny Gillis

I shall take advantage of your invitation to relate your experiences, for the benefit of our readers. As I think you concluded, it all makes sense and merely highlights that a few pounds extra spent on changing filters on a time, rather than mileage, basis are always well spent.

Your treatment by Mercedes-Benz (through Mobilo Life) sounds quite exemplary, and highlights the value of such arrangements. Had you been using an independent garage you might not have come out of it all as well as you did. As you suggest, sometimes recommended service intervals are cut to the minimum to offer fleet managers lower on-paper running costs – and we see this particularly reflected in oil service intervals.

I wonder if you have reflected upon where you normally buy your fuel, and whether their housekeeping might not be up to standard, resulting in excessive foreign material in the fuel? I’m not sure about all this re-coding business though – things like new injectors always need re-coding, but that is usually to re-match individual injectors that may differ minimally in dimensions, and therefore in terms of output for a given pressure. But I’ve not come across this before with things like filters, and I don’t quite understand why the fault was only cleared after a re-coding. I would have thought that other engine management systems would have taken care of that, but I’ll certainly dig into this a little bit further.

Why have you since seen an increase in fuel economy? If an air filter is partially blocked, then the MAF (mass air filter) would sense that and adjust the fuel injection accordingly, one would think – unless, as you suggest, the air filter was very badly blocked. One might think that bigger engines like yours naturally would have filters with greater capacity and therefore filtration area, but 50,000 miles does seem a long interval to me, particularly for the air filters.

Doctor D

P.S. I did then check out in some depth the Mercedes-Benz recoding procedures, for my education. Apparently the ECU will have adapted to the blocked filters as the miles (and muck!) accumulated. When the filters are changed, the required ECU reset puts the engine back into the factory settings and then re-adapts to any new parts, based on information generated by multiple engine sensors. As I later suggested to Danny might happen, after the reset the automatic transmission had to embark upon a re-learning experience, which was evident to Danny from the noticeably altered change-up points, which should settle down with time.

I also managed to source a procedure for Danny to do a Mercedes-Benz DIY ECU reset, which may come in handy for future DIY filter changes – at more frequent intervals!

Doc