I have been getting exhaust in my car cabin since last fall. It is worst when I first start up the engine. Once I get driving, it's not so bad. Switching off the heat or recirculating the air does nothing to change things. I used a CO detector in the cabin and the alarm didn't go off, but it still worries me that I might be breathing in carbon monoxide. And none of the shops have been able to diagnose it, let alone fix it. The first shop installed a new 'flex pipe'. The dealer thinks it's coming from a gasket near the flex pipe. The exhaust manifold is not cracked. A muffler shop said there was no leak. Is it possible that exhaust is somehow getting mixed into the air via a faulty valve or vent? Thanks!