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strong exhaust smell

7K views 5 replies 2 participants last post by  knt 
#1 ·
Last Friday my 2008 MCS started smelling strongly of engine exhaust. I usually don't smell it if all the windows are closed, but if the windows or sunroof are open I often get a whiff as I'm taking off from a stop, and by the time I get home from work my wife can smell it on my clothes. The odd thing is the smell reminds me of an old car from before cats, or a motorcycle - not a modern car exhaust smell.

I bought the car with 99k miles in March of '19, and it's had a P0420 catalyst efficiency code every time I've run codes and comes back shortly after being reset. I had decided not to mess with it because I've already spent enough doing walnut blast, thermostat housing and crossover pipe, and vacuum pump replacement, and oddly, the code can be present but not illuminate the CEL when there aren't any other codes active. But now, with the smell, I'm wondering if the cat has continued to get worse and finally crossed a threshold.

I reset the codes yesterday morning and have driven about 150 miles in 3 start/stop cycles since then, and the code hasn't returned yet. I also looked at live data and saw that the catalyst temp ran around 712°C while I cruised down the highway at 83mph. Readings from O2 sensor 2 don't look wildly wrong, and the short term and long term fuel trim values all look reasonable, though I don't really know what to watch for.

Please help...with as much as I've had to do with this car in only 4 months of ownership I'm seriously considering selling it and buying something boring for my commute, which is a sad thought given how excited I was about buying it. Losing the ability to drive with the windows down, or possibly spending several hundred more on cats without knowing for sure that's the problem may be the last straw.
 
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#3 ·
I hadn't really considered that, perhaps the P0420 code is a distraction from the real issue. Nor did I realize there were clamps...figured it would be flanged connections, don't have any significant rust and I haven't heard anything that sounds like an exhaust leak...but I'll get a look at those. Wish it was more like my Jeep in the ground clearance department.

Thanks
 
#5 ·
You were on the right track. I popped the hood when I got home and was greeted with the same odor I'm complaining about. Also found that I get it even with the windows closed off the AC isn't on recirc. So it's leaking somewhere near the first cat below the turbo but not big enough to hear. Should be fun to find it shoving my hand down in there to feel around all those hot components.

I am still curious about catalyst diagnosis. Upstream O2 sensor is reading around 1.5V and downstream sensor is about 0.74V at cruise. So by the O2 readings the catalyst is clearly doing something, I just don't know how much more difference there should be between the two sensors. And why the heck does the code not light the CEL??? (I know the CEL works because it's lit up before for other codes)
 
#6 ·
From what I read the CEL only lights up after a certain threshold has been exceeded.

I'm sorry, but I'm not mechanically skilled to help you diagnose the O2 sensors. Hopefully someone more mechanically skilled can help you soon.

Hope you'll get it sorted out soon as inhaling all that fumes can't be good.
 
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