Black and blue smoke are more common in diesel engines ... fuel additives can change the color of your exhaust smoke, too. So, figuring out gray smoke involves checking out many different things ...

An expert from The AA who helps stranded drivers at the roadside has explained what to do if you see different coloured smoke coming from your car's exhaust ... while black smoke signals excessive ...

But the smoke test means something else for cars, especially those powered by diesel ... in exhaust passing through the tube will scatter light in a predictable way, and the numbers work out ...