Very High Idle HC, won't pass smog ('95 K2500)

Discussion in 'Chevy C/K Truck Forum' started by FB73tii, Jul 17, 2012.

  1. FB73tii

    FB73tii New Member

    Having trouble passing smog--very high HC at idle (1139 ppm, standard is 175 ppm). History and actions to date below. I do have the 3-volume GM 1995 C/K Service Manual and have read through a lot of emissions-related posts on this site but have not identified the problem(s) yet.

    The truck:
    1995 Chevy K2500
    350 V8 TBI, Automatic Transmission
    Stock engine, no mods
    Used for street and towing racecar, no offroad use
    124,000 miles

    Two years (10,000 miles) ago:
    HC high (about 220 ppm, standard is 175ppm)
    --trouble code 32 = EGR code
    --Idle fuel pressure = 13.5 psi
    --SeaFoam, oil change, fuel filter change, new 195°F thermostat and new spark plugs did not help
    --New GM EGR valve solved problem

    Changes since then:
    Very slight exhaust manifold leak as left rear manifold bolt fell out
    Added free-flow cat-back exhaust

    HC at idle = 1139 ppm (standard is 175ppm) = FAIL
    CO% = 0.71% (standard is 1.00%) = PASS
    Dilution check (CO + CO2%) = PASS
    Gas Cap Test = PASS
    Catalytic Converter Test = PASS

    Actions and observations so far:
    --no trouble codes
    --idle smooth, runs well, not down on power
    --Idle fuel pressure = 13.5 psi
    --spark plugs look fine (all light brown)
    --new plug wires and coil wire
    --new cap and rotor
    --vacuum hoses seem good (did not use vacuum pump though)
    --EGR valve appears to function (hand compression slows idle)
    --new EGR solenoid
    --Base timing confirmed to be 0° at idle

    Things not tried yet:
    --compression test
    --leakdown test
    --Coolant temperature sensor test
    --O2 sensor test (how to do?)
    --coil output test (how to do?)
    --vacuum pump test of EGR system
    --replace TBI pressure regulator
    --ECM tests (how to do?)

    Given the excessive richness I assume this must be a distinct and well known failure. Any suggestions on how to diagnose and remedy appreciated! Thanks--Fred
  2. K15 Blazer Guy

    K15 Blazer Guy Member 2 Years 100 Posts

    sounds like a vac. leak or, more likely, it is running in closed loop because the engine thinks its alwasy cold.

    so i guess check these first:



    Coolant temperature sensor test

    vacuum pump test of EGR system

    --O2 sensor test (how to do?) check all your spark plugs and see if they are burning well. if theyre dark and crappy, the engine is dumping fuel because it has no o2 sensor input.

    all that can be done for free. if you dont pass smog, you dont pay. so dont let them charge you for every single test.
  3. FB73tii

    FB73tii New Member

    Thanks for the quick response. Plugs look fine so hopefully not the O2 sensor. Will do the items you suggest. --Fred
  4. FB73tii

    FB73tii New Member

    Follow-up: Well, it turned out to be something simple. I thought the plugs were good but putting in a new set of plugs and throwing a different cap and plug wires (also new) dropped idle HC and passed smog! Thanks for people's help here. --Fred

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