Weak Vacuum Test Fail

Discussion in 'The BS Topic' started by TDPalmer, Mar 19, 2014.

  1. TDPalmer

    TDPalmer Veteran Member Lifetime Gold Member

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    Apr 25, 2011
    Cave City, Arkansas
    I bought a Cen-Tech pro code reader handheld from harbor freight. I hooked it up to my truck because it has been throwing a EVAP "gross leak" code periodically. I went to the live data monitor and saw where it has passed every EVAP test except for "Weak Vacuum Test 1" on the monitor it said it was pulling 1.2 (In H2O) and the requirement was 12.0 (In H20) it passed Weak Vacuum Test 2 and every other test that apparently the on-board monitor puts it though.

    My question is what could cause it to be failing the weak vacuum test, what all should have a vacuum on it?

    It was my understanding that the purge valve has a vacuum on it, I relocated the purge valve because it gets clogged with dirt in the factory position and I located it towards the front of the vehicle. I had to use 2 pieces of heater hose because one was not long enough. I used a brass barb and hose clamps to seal it together but I am wondering if that is where my problem is coming from.
     
  2. Mike N

    Mike N Moderator Staff Member Lifetime Gold Member

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    Jan 13, 2002
    Spencerport, NY.
    What kind of truck?
     
  3. TDPalmer

    TDPalmer Veteran Member Lifetime Gold Member

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    Apr 25, 2011
    Cave City, Arkansas
    04 Silverado Ext. Cab
     
  4. Joekool

    Joekool Veteran Member

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    Feb 4, 2002
    Hollister CA USA
    You have a leak in the EVAP system, start inspecting the EVAP system for any obvious failures. The fuel cap being in place and having a good seal is usually the first place I start. Without access to a smoke machine you might be fighting an uphill battle though.

    If your scanner is able, see if the vent and purge solenoids can be controlled. On some GM trucks the vent solenoid was mounted in a bad spot under the bed and could fill up with water and dirt and that could cause them to stick.
     
  5. Mike N

    Mike N Moderator Staff Member Lifetime Gold Member

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    Jan 13, 2002
    Spencerport, NY.
    Biggest failure item that we see here on that generation Silverado is the vent solenoid.
     
  6. Knuckle Dragger

    Knuckle Dragger Moderator Staff Member Lifetime Gold Member

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    Nov 2, 2002
    Waddell AZ
    We see that and the evap line at the top of the tank coming unpluged on the Tahoes.
     
  7. TDPalmer

    TDPalmer Veteran Member Lifetime Gold Member

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    Apr 25, 2011
    Cave City, Arkansas
    My scanner can test the solenoid and I have confirmed that it gets 12v etc and the scanner says its working but I could be wrong. I may have a look at my charcoal box and top of the fuel tank. I had a replacement gas cap but it seemed like it set it off more than the factory one did.
     
  8. BigBlock73

    BigBlock73 Veteran Member Lifetime Gold Member

    1,813
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    Jul 10, 2002
    Accord,New York
    I chased an "Evap" code on my 00 Silverado for a year and a half.
    Changed everything associated with the test.
    After searching the internet the whole time,I found a post from a mechanic who suggested it could be the "fuel take pressure sensor" it's not part of the test but used to do the test.
    I pulled the bed and replaced the sensor,because I was out of options.

    It fixed the code and haven't had a code in 2 years.
     
  9. Mike N

    Mike N Moderator Staff Member Lifetime Gold Member

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    Jan 13, 2002
    Spencerport, NY.
    You're knocking the evap line loose when you're changing 02 sensors.
     
  10. Widowmaker01

    Widowmaker01 Veteran Member

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    Apr 23, 2009
    North East, PA
    I threw an evap gross leak on my 03 Silverado a couple months ago. Turned out to be rusted fuel filler tube leaking vacuum.
     

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