Lumpy running

I have a problem that has just turned up on my 2007 Mercedes 320 CDI auto in the past week and is only affecting the car when it’s cold or has been left standing overnight. When you start the car from cold, it fires first turn of the key and ticks over smoothly, but if you increase the revs a little it misses badly, as if it’s running on only three of four cylinders. If you then drive away, it’s very lumpy, stuttering throughout the rev range and sometimes jerking. As soon as the engine starts to warm up to operating temperature, it drives fine, with no misfiring or roughrunning. If left for a few hours and started up again, it runs smoothly.

The problem only occurs when the vehicle has been left for 24 hours or more when it’s cold. No engine warning lights come on when it is running roughly. I have also noticed my MPG has dropped a lot from what I had been getting. There also seems to be a lot more exhaust smoke and a lot of soot has built up around the exhaust pipe and bumper surround.

The car came with a year’s warranty from the garage for any problems like this, so I will get on to them and get them to fix it. Steve Mccreery From the symptoms you describe, it sounds as though you are suffering from sticky injectors. This would explain both the lumpy running and the high fuel consumption coupled with the black soot and smell of fuel from the exhaust. As this is only occurring when the engine is cold, you may be able to clean out the system by using a fuel system additive. If this does not work, you should look at replacing the injectors. As you have a 12-month warranty on the vehicle, I would advise getting the garage to carry out the work.