I have a 1997 Chevy Silverado, 2wd, 5.7 V8 with 129000 mi.
I am getting an exhaust smell through the vent system. These are the conditions get it:
Heat temp only, with or without AC on, when I call for heat to the floor or defrost, not through the vents on the dash.
I can smell it outside the car too when standing near the engine compartment.
When that happens, you are getting some exhaust gas escaping under the hood. You could either have an exhaust gasket leak, an exhaust pipe leak, or a bad connection.
Another source of this smell could be that you have blowby due to engine wear and the PCV valve can’t handle all those gasses, and they escape from the sysem and enter your heater system.
I would have this checked out by a good echanic. If you’re lucky it’s just a PCV valve and tubing, or an exhaust gasket.
You can easily check for exhaust leaks.
When the engine is at the condition where you smell the exhaust gas, with the engine idling, have someone take something like a brick and block the tail pipe. Now inspect the exhaust system from the engine on back. If there’s a leak it should be easy to see.
Tester