I recently bought a used Mazda 6 2006 diesel car. After a couple of days, I noticed it was producing lots of white smoke especially after the engine is warm. This usually occurs when I stop at traffic lights etc. I noticed that the smoke stops for about 5 days when I drive it on the motorway for about 10 miles. Then it will start smoking again.

I took it to a local garage who checked it and told me that the problem is not very obvious. He suggested that I take it to a Mazda specialist (dealer garage) as they will probably have more experience in dealing with the issue. Their diagnosis was inconclusive. Typical causes of white smoke like coolant leaking and serious engine problems (e.g. rings etc) were ruled out. They however said they suspected the EGR valve - but they were not sure as the signs were not apparent. I had to get the car from them because I booked for a day diagnosis.

I communicated this development to the trader who sold the car to me and he poured 2 cans of BG244 fuel additive to a full tank of about 60 litres and said we should try that out. Its been about 2 weeks now and the problem keeps reoccurring. Like I said, the white smoke stops when the car is driven for about 10 miles on the motorway.

Please can you advise on what I should do? Is there any additive I could try that could stop/suppress this problem?

Thank you.