"I simply refuse to get into that old jalopy of yours. It fair brings on me asthma it does"
"It's not an old jalopy, it just smells a bit when you stop at lights"
"Well, it seems to me that the whole car is filling with horrid exhaust fumes and I shall stand for it not a moment longer. Get it sorted"
And so he did.
It turned out to be a leaking injector seal. Gases blow by the injector allowing accrid fumes to be sucked into the car. Easily remedied though by simply whipping out the offending diesel injector, giving it a bit of a dust off, wipe off the seating surface and chucking it all back together again with a new washer, spacer and seal.
This lesson was brought to you courtesy of the British Institute of Gross Oversimplification