Intermittent exhaust smell in cabin?

'90 Maxima 256k…recently I’ve noticed exhaust coming in through the vents when I have the ac on and have been idling or driving in slow traffic. I know I have a small exhaust manifold leak, because I hear the clicking. The problem is this exhaust smell happens very intermittently. Usually on hot humid days when I have the ac on. I have tried to duplicate the issue to no avail. Sometimes I can sit in a parking lot at idle with the ac on for 30 minutes and I don’t smell any exhaust (not even when I smell under the hood). How can this be? If it truly was an exhaust leak than it would leak all the time and I would smell it. Could my egr valve be leaking? Does anyone know when the egr valve is activated? Thanks.


Wouldn’t it make sense to fix the exhaust manifold leak first before worrying about why you “only” sometimes smell exhaust in the cab?

That’s how I look at it.

The leak is in the exhaust manifold studs which is extremely difficult and time consuming repair…basically I am trying to find out what could cause an intermittent exhaust smell and also how often the EGR valve works on a car.

@mattT@db4690 gave you some good advice. If you “hear” an exhaust leak then you have an exhaust leak. Many people have died from carbon monoxide poisoning without having ever smelled it. The EGR valve works from the time you start the engine until the time you shut it off. The EPA would have it no other way.