Hey, thanks for all the replies and information. His truck is a Ford F-350, shiny bright red and looks fairly new. We're in California and have not had a morning below 50 degrees since he's been living there. i don't recall seeing any kind of smoke coming from his exhaust, but perhaps if there is some it would be whitish. Definitely no black smoke. After i spoke with him, he has been moving it to the other driveway, a bit further from our house when he runs it in the morning. But those fumes are still making their way clear to the far back side of our house in the morning. Not good at estimating distance, but i would suspect it's at least 200 feet.
i'm hoping to be able to talk to him in a manner to get to the bottom of things. i like the idea of taking something over, maybe a dozen or so eggs? That might start things on a good note. i also like the idea of perhaps having him come over into our yard when his truck is running, so he can see how stinky it is. He does seem to be trying to comply, and i appreciate that. i'm just getting so frustrated (and sick) waking up with diesel fumes in my lungs every morning. Even tried wrapping my face in my sheets this morning but the smell permeated even that.