Doctor Diesel

Limping home

Web01As an avid reader of your magazine, I wonder if you can help me with the following problem I am experiencing with my 2008 Mercedes-Benz E 320 CDI. The turbo has (only apparently! Doc.) stopped working, the car will accelerate very, very slowly, so therefore I am rather afraid to overtake. If I pull into a lay-by, switch off the engine, then start it up again, the turbo will work for approximately 20 miles, then it will stop working again. Is there a problem other than the turbo, e.g. some electronic sensor somewhere, or will I have to have a new turbo fitted? The car has only done 60,000 miles, as being retired, I only do 10,000 miles a year. I live in West Yorkshire. I would value your expert opinion.
Basil Davies

Well, here I am sat down at the PC at the unholy hour (for a man of my advanced age!) of 8.30am, with my thinking cap put on, all ready to try and tackle your problem. I have a feeling that this may turn out like one of those property search television programmes where the credits roll at the end and you don’t know whether the people ever bought one of the houses. By the time that we go to press in a few days, we may not have sorted your problem, so that readers know the outcome… but I hope that at least we will be making some positive progress.

Anyway, let’s look at it logically, as I sense that you possibly have not really done yourself, which rather surprises me, you being a Diesel Car reader and an avid reader of this column. In saying that the “turbo has stopped working”, you are jumping to too many conclusions. What you really mean is that you are diagnosing that the expected normal turbo boost has gone, and that the power output is seriously down. Fair enough. But it doesn’t mean that the turbocharger itself is necessarily the problem. Turbochargers usually “go”, as you might say, by starting to blow black smoke, as the bearings wear out, and gradually lose performance, as the boost pressure goes with leakage, and then nasty noises become obvious etc., etc., You haven’t mentioned any such gradual mechanical deterioration. That’s good news. Neither does your sudden loss and reappearance of boost really tie in with the turbocharger being worn out, which might possibly allow us a sigh of relief, although we’re not out of trouble yet, Basil! So what might be happening to cause a “transient” (good word that!) failure of turbo boost? Well here it gets quite complex, and I’ll list some of the possibilities: ECU fault – boost control instructions not being given correctly; a sensor failure – engine speed sensor, boost pressure sensor, engine temperature sensor etc.; a sensor misread that shuts the engine down into “limp home” mode, which usually clears with an ignition on/off sequence; an air flow problem, such as inlet air obstruction, a hose leak, either on low or high-pressure side, before or after the turbocharger, again causing a sensor to shut the engine into “limp home”; a fuel supply problem – low or high pressure fuel pump, or even possibly a blocked fuel filter; sticking vanes in the variable geometry feature of the turbocharger – but rather unlikely to suddenly self-cure, as yours does. But a proper diagnosis on a machine should spot which of the many possibilities is the case, and there’s a very good chance that solving it may not cost you much money.

If you’re using the Mercedes-Benz dealer for servicing, then you must go there for a diagnosis. Otherwise you need a diesel specialist with specific Mercedes-Benz diagnostic equipment, as many simpler multi-make diagnostic systems fail to pin down the precise problem area. I would put my money on a malfunctioning boost sensor, or another related sensor, or a hose leak, both of which are not unknown on such engines; but there are quite a few possibilities, none of which should be too difficult to pin down. So I think it’s relatively good news from me Basil, and I very much hope that I’m right, and that you’ll let me know as soon as you know, and I hope you’re going to do this without further delay! Many thanks for your letter, and please let me know how you get on.

Whilst I have your attention, might I just suggest that your engine needs some good regular exercise. I don’t think it sounds as if your engine is choked with the automobile equivalent of catarrh, but do give the engine a regular bit of proper exercise, maybe by holding it in a lower gear and pressing on up one of your many Yorkshire hills! It’s the equivalent of us older folk doing some regular walking, and helps keep it in good shape! Best regards,
Doc Diesel

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