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Thread: What are some signs of a clogged catalytic converter?

  1. #1
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    What are some signs of a clogged catalytic converter?

    Hey folks, first post here. I have a 1992 Silverado 2WD that just turned 200k miles, and I am going to drive it through the winter up here after I've only really driven it about 1000 miles a year for the past few years. I've spent the last few weeks getting it in decent shape, just odds and ends and a tune up. One thing I can't seem to figure out is the truck seems to "chug" almost like pinging, under load. Runs fine at idle, except for a slight intermittent stumble that I thought was spark related but it still does it once the truck is warmed up (it runs smooth as heck for a few minutes on startup and then once warm starts to stumble a bit, almost like it wants to stall).

    But the real concern is the chugging (pinging?), I have put a tank of 93 octane in and it still does it, and the plugs and wires and cap/rotor are all new, so I'm wondering if a clogged cat can cause this. I am thinking of removing the cat and putting a stright pipe w/muffler on it instead and I'm not sure if this will hurt drivability any. I'm not concerned with gas mileage, as long as it doesn't go down way too much. I'm pretty sure the truck still has the original o2 sensor also, so maybe that is in need of replacing too.

    I still need to pull the codes tonight to see if I can diagnose the slight stumble (or the "pinging"), but I just wanted to see if anyone had any info/advice on this. Thanks in advance.

    Not that anyone cares, but here is a picture of the truck from last summer, it's in pretty good shape for a '92 with 200K.



    Scott
    Last edited by scott92law; 11-11-2008 at 08:40 AM. Reason: added picture of truck

  2. #2
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    Welcome aboard nice unit! to check your cat restriction an exhaust shop can check it, they remove the front o2 sensor and read the pressure when running and revving the engine, should be near zero psi i think anything over half to one psi is too much restriction? but removing it would actually gain mileage, be noisier, and most likley be against the law in your state, i would maybe check for vacuum leaks, but see what codes you get first! >stump
    Gone.

  3. #3
    Senior Member 87c10slammer's Avatar
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    honestly if your just driving it during the winter then i wouldnt worry about cutting the cat out... no cop is gonna get out of his car just to check and see if you cat is there...lol.. but like stump said get it checked out if it fails just drop the pipe and take a long screwdriver or small pipe and knock all the crud out.. it may rattle a little at first but after a few days it will go away.. then if you get pulled over and they check its still there but its hollowed out... but they wont know that..

  4. #4
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    Great advice. Excessive temps expescially right at the cat will also be a good indication.

    Welcome to the site;

    Now your actual symptoms. While I am a little confused about the actual symptom, and do not know if this is a miss or a "ping", I will address the ping. Since you mentioned that you have only driven it a few miles per year in the last few years, I am led to believe that the engine only ran for a little bit. SHort trips are real hard on engines. Carbon tends to build up like crazy.
    It is entirely possible that you have some carbon built up inside of the cylinders (valvatrain really) and this carbon is holding temps when the engine gets warm.
    There should only be one ignition source in the cylinder, and if the carbon deposits can retain enough heat it will cause pre-ignition, and can actually start the combutions process. This is heard as a "ping" and typically only happens at higher temps or higher demands (under load).
    Clean the carbon with appropriate cleaning processes, and the problems should be gone.
    A decent method is to remove with water. Yes, water.
    Run the engine, and allow it to reach operating temps, and remove the airl cleaner element. This will expose the top of the throttle body, and here is where you add the water.
    Fill a decent sized bottle or container of plain old tap water. I like bottles because the small openeing helps prevent adding too much water.
    Open the throttle blade by hand until the engine reaches apporx 2500 to 3000 rpm.
    Once the engine speed is here, add water to the throttle body.
    Now, you can actually use a vacuum line for this, and add water just like any other engine cleaner, but addidng to the tb is just fine.
    You do not want to add so much that the engine is permitted to stall, but you do want to add enough to bring down the engine rpm.

    The theory:

    Water in the warm engine, introduced to the combustion process will change state and become steam, this steam will break up the carbon deposits from the valves and eliminate the build up, thus eliminating the ability for the carbon to retain heat within the cylinder.
    Does it work??? Sure it does, but you have to get a decen feel for how much water to add, and how many bottle you should run through the engine.
    Well this can be tricky, and each engine is different. Water inside of a running engine will not harm it, as long as it is not so much that it causes the engine to stall. So dont use a garden hose, or something that can hydro-lock the engine.
    Run a couple of bottles, maybe three or four.
    You will notice that the tailpipe will drian dirty black water. This water is the carbon that has broken free.
    Now, this carbon can build up inside of the cat converter also, and once in a while will add to the clogging problem in an already poor condition converter, but should not prevent you from trying this method.

    This method is pretty close to free, and is rather effective. Once finished, and the tailpipe runs reasonable clear water, you can stop adding water, and leave the engine running.
    Take it out for a spin, and check to see of your symptoms have gone.

    You do have quite a few miles on your engine, so be easy on it.

  5. #5
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    Wow, thanks for the quick replies. I should have been a little more clear in my post about the symptoms of what I am seeing, sorry about that. I suppose it could be a miss as well, but I'm not positive. There are two things going on that I am concerned with- the "chugging" coming from the exhaust under load (which might be pre detination?) that sounds almost like bad gas or something, and the tendency for the truck to stumble a little when I stop at lights or when it's idling. It will do this once it's warmed up it seems. I should also add that the exhaust is a bit modified- it's the converter with a single chamber race muffler and a dumped tip that is located under the rear of the cab and not at the back of the truck. This was done out of neccessity a couple years ago when my muffler rusted off and I happened to have a flowmaster race muffler lying around.

    I have heard of the carbon issue(s) but was not aware I could run water into it to clear it up. I will try that asap. The converter is not an inspection (or legal ) issue for me, although I realize it's a no no to remove one, but what about ones that are faulty- surely these can be replaced I assume...I have a supercharged Mustang that hasn't had the cats on in over 5 years and runs like a scalded dog. Really the main reason I asked about the cat was because it is something I can easily do in my driveway if that were in fact an issue.

    I have also read through many threads here and have seen some mention of the temp sensors on these trucks causing all sorts of issues if faulty- is this in reference to the sensor at the top of the motor by the thermostat or the temp 'switch' on the side of the motor? What is the difference between these two, and wouldn't a problem with either throw a code?

    Sorry for all the questions, I'm hoping I can address these issues the easy way first!

    Scott
    1992 2WD Silverado
    200k+ on the clock

  6. #6
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    These engines can really become a problem and drive someone crazy trying to diagnose. If you have a sensor failure, you should have a check engine light. You did not indicate that this was the case, so the guys did not mention anything like this.
    Now, if you do have a detonation problem, the ecm will detect it via the knock sensor, and modify fuel and spark curves to try to eliminate this.
    This could be a multi symptom problem, and isolating each little item will get you pointed in the right direction.

  7. #7
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    Yeah I figured the CEL would be on if I had a sensor problem, but no light yet. It's been awhile since I pulled codes from the truck so maybe that will shed some light even though the CEL is not on.

    Quite honestly I really only need to drive the truck until about April, when I will be looking to buy a newer one. I just hate to hear this one struggle, it's almost as if it's a family member at this point, owned it since '96. In the last several weeks I've put a new driveshaft on it, had the front brakes done and replaced all the fuel & brake lines as well as put a new fuel pump sender in. I also "fixed" a leak above the windshield that was making the headliner sag and mildew with some black RTV (best temporary fix I've done)

    I have folks around here telling me to just run it until it dies, but I have a problem doing that to any vehicle I've owned, and I won't junk it even if it just sits in my driveway for the rest of it's life!

    Scott
    1992 2WD Silverado
    200k+ on the clock

  8. #8
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    if using water, make sure it's hot water....

    cold water and hot aluminum is asking for trouble....
    and that's not an opinion. a factual mistake on my own part.

  9. #9
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    Anyone know the difference between a coolant temp sensor and a coolant temp switch? My haynes manual (I know, they suck) shows both, what does the switch (on the side of the motor) do? When I look up the part at Napa it appears to be the same thing??

    Scott
    1992 2WD Silverado
    200k+ on the clock

  10. #10
    Senior Member gmctrucks's Avatar
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    Welcome to the forum and howdy from TEXAS. Ebay has some free flowing Cats for good prices now when 75K30 talks it's like E.F.Hutton people listen he's one smart member here were lucky we have him im spreading love tonight also I would add a can of Seafoam to the gas tank or look at video below it will help you with Seafoaming your truck.
    Be good! GMC
    YouTube - instructions for seafoam (break booster)

    YouTube - E.F. Hutton 1983 Commercial Look at the whole video. LOL This is for you 75K30 you bad boy but in a good way not bad ugly but bad good.

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