Smog Case Study

Discussion in 'The Garage' started by longbedder, Jan 10, 2007.

  1. longbedder

    longbedder 1/2 ton status Premium Member

    Oct 26, 2005
    Likes Received:
    Ridgecrest, CA
    Maybe a long read, but...

    There are about a billion and one posts, searches, and web pages that talk about stuff related to passing smog in California. Very few actually offer any useful information. I thought I would post up my situation, as it may help someone in the future (as opposed to: well, my friend's brother once dumped XXX brand snake oil in, drove 23.1 miles in reverse, and showed up nekkid to the test and passed).

    Truck: 86 3/4-ton burb
    It came with Vortec heads and a non-EGR manifold. It also has no cats, as they aren't required for the 8,600 GVWR in 1986 (it's got H-pipe exhaust into twin Flowmasters, that's it). The Hedman headers are smog legal as they have inlets for the AIR pump. The AIR pump is functional and hooked up properly.

    What I did first:
    The truck would have failed visual with centerbolt valve covers and no EGR. There is an aftermarket EGR intake out there for Vortecs, but it's pricey, and you have to route the EGR gases over the valve cover from the exhaust manifold. I had a set of older style (but hi-po) heads in the garage, so I swapped the heads and put an EGR intake on there. I found an aluminum Q-jet EGR intake of an '81 Z-28 for $15 bucks at a junkyard with a good EGR valve on it.
    I didn't touch the carb - it's a Q-jet with size 73 main jets (.073" orifice). The main metering rods are .030" at the tip and .045" at the upper end. The cam is kinda lumpy (270 degrees duration roller cam), and this can kill emissions at idle. After the head swap I did nothing to the carb, and was running 10W-30 oil and a 180* thermostat. My locality does not require a dyno test, and does not test for NO2.

    Idle (FAIL) - Gross Polluter territory, but the tech ran it as pre-test
    -Idle Speed - 961 RPM, Timing 12*
    -%CO2 - 8.06
    -%O2 - 8.0
    -HC - 250 (allowed), 269 (measured)
    -CO - 1.50 (allowed, 2.10 (measured)

    2500 RPM (PASS)
    -%CO2 - 11.3
    -%O2 - 4.3
    -HC - 200 (allowed), 51 (measured)
    -CO - 1.60 (allowed, 0.74 (measured)

    This tells me I was burning rich at idle (High HC), and not combusting completely (high O2). The truck had sat (off) in the check station parking lot for about 1.5 hours before being tested.

    What I did next:
    -adjusted the carb float - it wasn't off much, but I set it dea-on for that model carb
    -screwed in the limiter screw for the main jets all the way - this allows the main metering rods to fall farther into the jets (thus obstructing more fuel)
    -changed the thermostat from 180* to 195*
    -retarded the timing 2 degrees. I couldn't back off much with my cam or it would die on me.
    -changed the oil and filter, using 10W-40 (normally 10W-30)
    -dumped 48oz of Iso-Heet (just Isopropyl alcohol) into 10 gallons of fuel that was in my tank. Alky tends to promote more complete combustion.
    -retained the same idle speed - this can be tricky, but is necessary in order to both stay off the primary carb circuit and not to fail the test for too high an idle
    -screwed both idle jets in (clockwise) SLOWLY, 1/8-turn at a time, and found the "edge" where the engine started to miss/idle slower. I then brought it back to juuuuust barely on the rich side of that edge. You gotta move slow here, as the effects sometimes take 8-10 seconds to settle in. You'll go back and forth with this and the idle speed setting a little bit, but you want it right around 1000 RPM when you're done, and sitting right on the "edge"
    -left the burb running in the parking lot of the station until the tech came and got it

    Idle (PASS)
    -Idle Speed - 982 RPM, Timing 10* Up 21 RPM, Retarded 2*
    -%CO2 - 9.10
    -%O2 - 7.1
    -HC - 250 (allowed), 141 (measured) Down 48%
    -CO - 1.50 (allowed, 1.18 (measured) Down 44%

    2500 RPM (PASS)
    -%CO2 - 11.6
    -%O2 - 4.3
    -HC - 200 (allowed), 34 (measured) Down 35%
    -CO - 1.60 (allowed, 0.18 (measured) Down 76%

    I'd be interested to see the numbers without the isopropyl, but I didn't want to go back another time to the test. CO did drop off dramatically at 2500RPM, but that could be solely because the engine was running hotter. Everything else I did was mostly affecting the idle performance.

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