Perhaps you could put up a camera to show cars backing up and the length time they r there.

At the end of that letter give a date when they need to get back to you. Letters are hard copy – a ‘legal’ document – proof that you told then – then send it off certified mail keeping copy of letter and the receipt.

Next call the DBI department of building inspection- although it may not be their jurisdiction they can come out and can refer the problem to EPA or public health – or you can call those groups yourself.

Remember please that renting is business and friendly is not a friend = I don’t know what state you are in but there must be state laws governing renters and landlords+ see if you have rent control or renters rights groups/

Exhaust is compound being burnt – heated up the liquid changes into airborne chemical vapour/not good for health – anything burning is bad for health when breathed in.

In california WARRANTY OF HABITABILITY
Under a 1974 California Supreme Court decision, Green v. Superior Court, all leases and rental agreements are deemed to include an implied warranty of habitability. This means that regardless of any conflicting lease agreement, the landlord is required to keep your unit in a habitable condition at all times.

“Habitable” means that the apartment conforms to the standards set forth in California Civil Code Section 1941.1, as listed above. Even if you knew that a unit was below code when you moved in, you still have the right to demand that it be brought up to habitable standards. Habitability could include a fume free unit

Peaceful quiet enjoyment of ones rental unit is another subject that should be considered. Paying rent to live in a fume filled unit

How much worse can the quality of indoor air be than that of the air outdoors?
Ten times as bad – Enclosed space without proper ventilation can lead to a buildup of hazardous pollutants.

How do combustion gases cause harm?
by preventing oxygen from flowing through your body
Such gases are associated with a host of symptoms, such as dizziness or fatigue. At high levels, inhalation can lead to unconsciousness or even death.

Anti-Idling Primer: Every Minuter Counts
http://www.thehcf.org/antiidlingprimer.html

The Harmful Effects of Vehicle Exhaust
http://www.ehhi.org/reports/exhaust/summary.shtml

Ann Arbor Restarts Talk on Vehicle Idling 2012
http://annarborchronicle.com/2012/01/19/ann-arbor-restarts-talk-on-vehicle-idling/