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Thread: Symptoms of clogged cats?

  1. #26
    Join Date
    Oct 2007
    Location
    Boca Raton, Florida
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    1,558
    My Cars
    89'750iL,98'740iL,00'M5
    Im having similar symptoms, car initially has great pick up, but after 2,500k it sounds like the engine is struggling to breathe. Horrible smell from the exhaust, 14mpg no matter what, and codes 420 430 and a few others I cant remember.

    Would removing the pre-cat sensors make enough of a difference to tell while driving if the cats are clogged?

    BMW 7er Website www.7er.com
    1989 BMW 735i Schwarz (sadly, sold) // 1989 BMW 750iL Cirrusblau Metallic // 1998 BMW 740iL Oxfordgrün Metallic // 2000 M5 Carbon Schwarz ///

  2. #27
    Join Date
    Dec 2011
    Location
    Lancaster CA
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    5,051
    My Cars
    e34,e38,e39,e46,e70,e90
    Quote Originally Posted by SergeBMW View Post
    Im having similar symptoms, car initially has great pick up, but after 2,500k it sounds like the engine is struggling to breathe. Horrible smell from the exhaust, 14mpg no matter what, and codes 420 430 and a few others I cant remember.

    Would removing the pre-cat sensors make enough of a difference to tell while driving if the cats are clogged?
    Yes, if you notice an increase in power with those sensors removed and the engine runs without any vibration or missing, your cats are gone. You will get codes for the O2 sensors, but you can reset them afterwards.

    Clogged cats can cause high heat in the exhaust valves and can cause the valves to burn up or crack and fail. The excess heat can also cause a head gasket failure. Don't drive the car around too much unless you're trying to destroy the engine.

    02 e39 540i Sport (Son), 01 DINAN 7 (Me), 12 e70 X5 x35i (Mrs), 95 e34 525i (Daughter 2), 01 e46 325Ci vert (Daughter 1)

  3. #28
    Join Date
    Oct 2007
    Location
    Boca Raton, Florida
    Posts
    1,558
    My Cars
    89'750iL,98'740iL,00'M5
    Thanks for confirming. I'll give this a try and see if this is actually my issue. I'm hoping it is because it would explain a lot.

    Sent from my ONEPLUS A3000 using Tapatalk

    BMW 7er Website www.7er.com
    1989 BMW 735i Schwarz (sadly, sold) // 1989 BMW 750iL Cirrusblau Metallic // 1998 BMW 740iL Oxfordgrün Metallic // 2000 M5 Carbon Schwarz ///

  4. #29
    Join Date
    Dec 2013
    Location
    Chicagoland area
    Posts
    4,372
    My Cars
    EXOTICS
    An exhaust backpressure test can confirm an obstructed exhaust system.

  5. #30
    Join Date
    Oct 2013
    Location
    Portland, OR
    Posts
    1,293
    My Cars
    328i,740i,e36 m3 x2,e46
    My recent experience with bad cats....

    Car was wayyyy down on power, felt choked up, misfiring intermittently, at high rpm it would often shudder and hit a wall in acceleration. Mpg went as low as 12mpg. Car threw no codes besides intermittent misfire codes

    First thing I did to diagnose was pulled the primary o2 sensors, noticed no change whatsoever with them removed, besides the horrible noise lol. Also hit the cars with a dead blow hammer and there was not any noticeable rattling.

    Next thing I tried was drove the car 25 miles and then used a laser temp gun to check temp of exhaust pipes before and after all 4 cats. I did this a few times and my results led me to suspect one warm up cat had failed and the other was failing.

    The thing that made me pull my exhaust off was live data in INPA. Looked at my multipicative and additive values and low and behold one bank was negative and the other positive. So I pulled manifolds and my whole exhaust and found one warm up cat to be completely blown out, the downstream cat was severely blocked as a result. Couldn't see an LED flashlight through it at all. The other side was starting to do the exact same thing, hadn't progressed as much though.

    Car runs WAY stronger now after fixing my issue.

    Pics are of one bank, didn't take pics of the bad side.


    Sent from my SM-N920V using Tapatalk
    Last edited by MtekJosh; 10-22-2018 at 04:12 AM.

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