Mercedes-Benz Forum banner

Can a cat throw misfire codes?

2500 Views 17 Replies 5 Participants Last post by  euro_flaw
97 E420 142000. It happened all of a sudden, stopped at 7-11, started the car, running very rough. Restarted, no help. Felt like misfiring 2 cylinders on the same bank. I had to fly cross country for a month to help my parents the next day, so I parked it. While I was gone my gurl drove it "sparingly" presumably. When I get back the problem is still the same, codes say cyl 1 misfire, cyl 3 misfire, and random cyl misfire. Plugs are about 2 years old so I start with that. Every plug looks like it's firing even, no carbon, but well used. The only issue I could see was #3 was loose enough I didn't need a ratchet to loosen it and there was approx 1/2" of oil in the plug well, but the plug itself was not showing any signs of fouling or oil in chamber. They were all Bosch super +, no signs of anything serious.
When I fire it up it runs much better, just a slight, random miss that smooths out with rpm's, more like a small vacuum leak. Leaving the top cover off so it's free breathing, by the time I get to the end of my block the "check engine electronics" reappears, and it stumbles at the stop sign. Thinking of the 1/2" of oil in #3 plug well, I decide to "clean it out". Huge difference in power. It never had this kind of power before, I'm thinking that might be that. But when I get back it still has that slight stumble at idle that goes away quickly with rpm, and when I look underneath the front passenger catalytic converter is about to go supernova it's so hot. The other 3 are hot, but not glowing so bright they look look like they might melt. The codes say cyl 1, 3, and random misfire only, no cat codes.
So here I am, new day. I'm next going to swap 1&3 coils to 5&7 to see if the misfire follows, and run it with the the top cover on, then off, to see if that has any effect on power. I'm thinking (praying actually) that the misfires are sending unburnt gasses to the cats, and the passenger cat by some twist of physics is receiving more gas than the other making it hotter, and that by taking off the top cover I get so much unrestricted air it irrefutably proves alll I need is a cold air intake to run laps as fast as nascar.
Any thoughts? Can it be a bad cat? Can a plugged cat throw misfire codes?
See less See more
1 - 1 of 18 Posts
In theory you may be right, and there is nothing wrong with continuing to pursue your diagnostic work.

However, a plugged cat can cause misfires, and your symptoms match a plugged cat as well, with the stumble at idle that disappears with increased revs (that's a classic symptom).

Still, I'd continue your investigative work, that's just some time and effort. If the misfires don't move and the symptoms remain, then you'll have probably reached the point that you need to check backpressure on that bank or bite the bullet and go cat shopping.

Good luck.
1 - 1 of 18 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.