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My wife's 2015 GLA with 65kmiles has started putting out dark black exhaust when she punches it. This happens mostly when the engine is cold and seems to go away as the engine warms up. The check engine light does not come on.
We bought the GLA 6 months ago. As soon as we got it the engine light came on with multiple cylinder misfires. The dealer replaced the number one and number four fuel injectors under warranty but said the number two and three injectors were okay. The engine has always been noisy since we bought it. Seems like more noise than the typical direct injection noise. Not sure if anyone here has had a noisy engine or black smoke but my wife is after me to get it fixed. Thanks for any help you can give !
 

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The black exhaust is a sign of a rich gas mixture. Most engines will run a little rich until they get up to operating temperature. Maybe yours is a more then it should be.
 

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It's now proper practice to drive away fairly soon after even a dead cold startup, but accelerating gently and avoiding "punching it" until the engine coolant is warmed up (in the case of my Audi's launch control the oil is also supposed to be up to temp which takes quite a bit longer).

If this cold/hard use is a common occurrence for your GLA then perhaps self-inflicted fouling of sparkplugs which in turn leads to a lowered combustion capability and causes unburned black smoke to pass? A 2015 model shouldn't need new injectors so also perhaps a poor vendor lot of factory parts?

Presuming of course you have a gasoline/petrol engine, as a diesel engine is much more prone to black smoke as a somewhat more natural occurrence.

btw: Blue smoke is indicative of burning oil from a "blown" (degraded) head gasket, worn piston rings, etc., so at least you don't have that.
 

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True on the start and drive gently approach. Pollution controls require fast heating of the cats. To do this the fuel gets enriched when cold so that there is enough left over to fire when it reaches the cats and force quick heating. You don't want this going on any longer than necessary to avoid damage to your plugs and, in the case of DFI motors, washing the cylinder walls with potential bore scoring. Thus the quick drive off after starting (remote start is no longer a good idea!!!). However, it is a really bad idea to put stress on a motor before parts have fully warmed up and expanded into operating clearances. Watch the oil temp gauge ... This is more important than water temp. Fortunately the GLA warms the oil quickly (my 997 requires at least ten miles before the oil temp starts to rise).
 
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