I sometimes see a fairly new diesel car near where I live belching out black smoke. Aren’t diesels supposed to be all cleaned up these days? What could be causing it? And what’s the difference between black smoke, blue smoke, and white smoke from a diesel exhaust? 

Bob Hill, Staines

Well Bob, your neighbourhood rogue diesel could be suffering from any of quite a few problems, but if I was a bookie, I would probably be offering the following odds: Evens – a badly maintained high mileage car with a blocked air filter and/or worn fuel injectors, causing very poor fuel ignition: 6 to 4 – a lady’s short run shopping car being taken out and given a bit of welly by her husband, which is clearing out the accumulated carbon build-up (it probably disappears after a mile or two): 7 to 4 – a tuned car that’s had the adjustable tuning box turned up too far by the owner: 10 to 1 – the owner had the diesel particulate filter (DPF) taken off, although any diesel engine in reasonable shape will not usually belch black smoke, even with the DPF removed. Well maintained diesels stopped belching black smoke years ago!

As for white smoke and blue smoke – well how many words have I got Ed? Basically, white smoke almost always means neat unburned fuel in the exhaust, unless it is actually steam, and not smoke, which would probably be down to water from a cracked cylinder head, or cylinder block, getting into the cylinders. I was once told on very good authority that most white smoke problems are usually down to incorrect fuel injection timing – meaning that the compression ignition just doesn’t take place, but this would most likely be only on one cylinder, as the engine would not otherwise start. I’m not sure this diagnosis does not relate to past times and older cars with mechanical injection and rotary injection pumps though. More likely white smoke is the result of a faulty pre-heater plug, resulting in one cylinder not firing, particularly when the engine is cold, creating white smoke. It could also be the result of poor cylinder compression, and failure of the fuel to detonate, or a badly worn injector that is failing to atomise the fuel well enough for it to ignite. It might also be any of a number of rather more obscure things. Blue smoke is pretty definitely from the engine burning engine oil, and lots of it! So it generally means worn piston rings, pistons, or cylinder bores, or it could possibly mean worn valve stem seals. Hope this helps Bob. I think you should have a word in the ear of the owner of that smoky car!

Doc Diesel