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  1. #1

    Default 91 silverado 6cyl, failed smog!! help!!!

    ok, i'm sooo bummed, I love my truck, yea she's probably neglected, but she's always been there for's what I got....not bad for 266k miles!!!

    %CO2 %02 HC (PPM) CO (%) NO(PPM)
    test rpm mea mea max ave mea max ave mea max ave meas
    15MPH 1585 14.5 0.3 121 35 153 0.70 0.11 0.26 1006 332 841 fail
    25mph 2440 14.6 0.3 101 24 98 0.90 0.09 0.29 866 274 742 pass

    they said my hc(ppm) at 153 was too high......what's that mean??? I passed all the other stuff with the exception of Fuel Evaportive Controls Functional, if they are functional, how'd it fail??? i'm sooo confused, and they won't tell you a thing either!!

    I did a oil change about4 days ago, and changed the air filter...what's sad is I changed the cat. the last time I did smog....not sure it that was 1 or 2 years ago....anyways, any help would be appreciated!


  2. #2


    Try running a fuel injector cleaner and or premium gas...

    06 Red GMC 4X4 CrewCab 4.8L
    MagnaFlow Muffler
    Lubed by AMSOIL...

  3. #3

    Default new info...

    ok I lied, I smogged 2 years ago...I had to replace the cat, because I had the same problem. BUT I didn't have to do the fuel evap thing, it say's N/A. What is that fuel evap part?? it's something new. It wasn't applicable in 07....why is my cat going bad?? I was reading that i'm running, rich??? how do I fix this?? is my egr bad?? I know that lil hard plastic hose under my air filter is rigged, what is that anyways??? i'll see if I can find apic,....Thanks for any replies!

  4. #4


    X2 on the premium fuel. Run the regular out first,then fill it .

  5. #5

    Default premium fuel?

    what is premium fuel going to do for me?? I did run sea foam through engine, but only through gas tank. I forgot to mention this. I did this 2 tanks of gas ago....about 2-3 weeks ago...

  6. #6


    If there's a problem with the EVAP system your canister can become saturated and cause the engine run very rich. Any codes?

    2004 SILVERADO 2500HD LS
    Pro Comp Leveling Kit
    285-75-16 Cooper ST All Terrains
    Pro Comp Programmer
    Bilstein Shocks
    Linux Bed Liner
    Tinted Windows

    Other rides:

    1967 Landcruiser <350 Chevy>
    1970 Camaro
    1990 Geo Metro Convertible
    2007 FJ Cruiser

  7. #7


    The oxygen sensor will adjust rich/lean as will the coolant temperature sensor.
    92 GMC Sierra 2500

  8. #8

    Default ok...

    I keep reading about these codes, what is that??? obviously i'm a moron when it comes to this stuff....hahahaha..........ok so what got saturated??? the charcoal canister? how is that fixed? where's the o2 sensor? if the coolant sensor was bad, wouldn't the truck overheat? thanks for all the help, the truck gets 20mpg, usually in my experience when gas mileage drops, that's an indication that somethings wrong, but yes, I am a true, change the oil, put gas and go person....I do little things, but nothing major....I did do a tuneup on this 2 years ago.....but that was easy...another friend told me, I could fail the fuel evap thing by a visual inspection of a cracked/old hose, is this true??? how hard is it to change all the fuel evap hoses.......??? I guess I should buy a manual???

  9. #9


    Well you are asking questions and that is good. You can order a GM Factory Service Manual Set from your GM dealer and this will give you complete detailed troubleshooting and repair instructions for your specific vehicle.

    These are a set of 3 or 4 books for a specific year and model vehicle. So basically everything in the books is about *your* vehicle. The older GM service manual sets have a general service manual, electrical diagrams, fuel and emissions manual, and a unit repair manual (to the set).

    For your situation, you would read through the fuel and emissions manual and this would give you detailed instructions for testing various components. Then how to replace faulty parts. The books also explain how each gizmo operates - what makes it work - what its function is.

    Testing is mostly done with a multimeter. Or a fuel pressure gauge. And sometimes other test devices like an exhaust pressure gauge.

    These days vehicles have computers in them and electrical sensors which provide information to the computer. For example there is a sensor to monitor how hot the engine is (just for the computer). Based on the information provided by that sensor, the computer would adjust how much fuel is given to the engine.

    But that temperature sensor might fail and give faulty information. Well they built a bit of "smarts" into the computer and in some situations, the computer can tell you what is not working right. Or where in general a problem is. This is called a "diagnostic code".

    The factory service manual will tell you how to get your check engine light to blink a series of blinks to let you know what the diagnostic code is. Or for a 91, you can buy a OBD I code reader at the auto parts store which you would plug into a connector under the steering wheel. This would also tell you the code(s). (96 and older have a different connector and use what is called an ODB II code reader.)

    Anyway if you want to do your own repairs, it is best to get it "straight from the horse" and that would be a factory repair manual. This would be the most detailed and informative books you could buy on your specific vehicle.

    Then there are other books like a Haynes book called "Automotive Electrical Manual - All Common Repairs". This would explain electrical basics.

    Some of the things in these books might be over your head and you might prefer to take your vehicle to a mechanic. But as you read them, you will begin to understand things and be able to do more and more over time. But it helps to start with the right instructions!

  10. #10

    Default New information...

    ok, well had diag's done on the truck, here's the skinny.....

    it needs a valve my question is i'm no mechanic, this is way above my skilset. I live in Sacramento, is or are any of you weekend mechanics? or can you recommend me a good shop. The shop I took it to gave me an estimate of 1500. He said other places will go as low as 700, but I went to him because that's where Bureau of Auto Repair sent me.

    I can give the info later when I get the paper work from him. I asked him to write everything down, things I can remember is plugs 3 and 4 are fouled, with oil. They are shorting out so his scope said. He said my 02 sensor is bad as well. But it's probably because of the oil leaking....anyways...any help to point me in the right direction would be appreciate.

    I love my truck and I don't want to "reitire" it, nor clunk it....

    Thanks in advance for the help!

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