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Intermittent bad exhaust smell in cab?

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Hey everyone so my friend has a 2018 GMC Yukon that only has 30000km on it and since August she has been getting a intermittent very bad exhaust smell in the cab. She has taking it to severely dealerships and they can not find the problem. It only happens once in awhile while driving it. I recently took it for a drive with my personal gas monitor that reads CO and it it started to read a very large amount of CO when I was test driving it and I could smell it very easily. I the took it back to one of the dealers and took a tech for a drive with the monitor and of course it didn’t get any smells on the cab. I tried to change the setting with the heater controls and recirculating air control and it didn’t help. She now has called me again saying it was very bad on the way to work again. The dealerships said they have gone through everything possible? I mentioned maybe a manifold leak but the said it would be noisy if that was the case. I am wondering what could have been missed that I can look at? And what is the best step going forward? 

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My car had an exhaust leak that I could only smell when stopped.  When driving the air coming through the rad would push the exhaust down the firewall and out so no smell.  Exhaust when stopped would come up the firewall and seep into the intake vent for the heater.  No smell on recirculate but this is a 2007 so recirculate shuts off outside air completely.  Was somewhere by the manifold close to the engine as I could feel hot exhaust if I put my hand down there during a cool day.  Not any nosier though.  This was a 2.4 L 4 cylinder.  Sold the car.  In the old days we would put a potato up the exhaust and if the vehicle stalled then exhaust was sealed.  It is funnier if you just put the potato up there and leave and start the car and rev it up.  Just like a rocket blowing it out.  ?



Edited by jagabom (Esquire)
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Possibly the flex connection after the exhaust banks Y?  Not sure on a Yukon, but typically that would be under the firewall, cab area and would make sense if it's being smelt in the cabin. The rotten egg smell was typical of catylitic

converters of the mid-70's before they were located closer to the exhaust manifolds where they heat up and do their job quicker and more efficiently now.

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