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      11-30-2021, 05:54 PM   #2
rjahl's Avatar

Drives: Z4 35is
Join Date: Jun 2011
Location: Tampa

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2012 Z4 35is  [0.00]
Originally Posted by pbedrosi View Post
Moving this from main forum....

So my son's E91 LCI w/N52 developed very rough idle. I had the BMW Scan Tools and noticed 02 post cat readings were fluctuating to 0 at times. So I replaced the post cat o2 sensors.

Soon after this the car started to run rougher and developed an exhaust leak right at the post-cat flange. This area was super rusty and I'm not surprised that I may have created the leak while replacing the 02 sensors.

When I mean rough, I mean it sounds like it is running on 4/5 cylinders. Before heading to the muffler shop I did order new spark plugs and coils...sort of maintenance and possible fix, but not installed yet. Previous owner addressed the vanos motor, new valve cover. Can just hit 80k.

The exhaust shop just called and confirmed the exhaust leak, but also found that cats were plugged up during flow test. Cats had 3% back pressure vs. 0%-1% target. Not sure how they run this (first for me) but they say the cats are "plugged up". So the exhaust leak turned from $200 weld job to $3,000 cat replacement. I'm just having them cut and weld in new flanges at the moment, but I do need to address the "plugged" cats.

Any suggestions?

I'm thinking either;
1) eBay headers and 200 Cell magna flow cats... still want 02's working, emissions, etc..
2) Used N52 exhaust w/cats from eBay/Forum classifieds

also....We did use seafoam cleaner in the tank months ago, could this have plugged up the cats??
I have suspicions about the back pressure readings they provided. I'd ask how those where measured.

Miss fires are ignition or fuel related. Have you read the engine codes using a BMW specific code reader?
Appreciate 0