1994 k1500 smog

Discussion in 'Chevy C/K Truck Forum' started by 65fj40, Apr 3, 2013.

  1. 65fj40

    65fj40 New Member

    Hey Newbie here,
    Let me start by saying i have searched this and other forums extensively and havnt found an answer to this yet but this thread came the closest
    However, It was never resolved, but i did try.
    My problem is i have a 94 chevy 1500 with a 350 and auto trans that when you go to smog it shows high on hydrocarbons but when you run the engine fast(2500) for about 5 min on the smog machine you can watch the hydrocarbons drop to acceptable levels and then they drop it back to idle and start the test and it will pass but if you let it sit there and idle the hydrocarbons start rising again. Now the smog guy told me it wasnt getting into closed loop and this was the cause of the hydrocarbons being high so i set out to find out why. I bought a ALDL USB cable and got a copy of Datamaster from the tunercat website. Got the software going and the first thing that i noticed was that it was in fact in closed loop about 5 minutes after i started the truck from dead cold and within 2 minutes of a hot start. The only thing that i noticed not being in spec was that as i understand it the voltage from the O2 sensor is supposed to fluctuate between .2 and .4 volts and mine fluctuated between .09 and .9 give or take. This truck gets great freeway mileage but about 4 mpg around town and i really want to get it fixed. I have replaced the O2 sensor, plugs and wires, Both coolant temp sensors, the distributor and the thermostat. The TPS appears to be functioning correctly according to datamaster. The only thing about this truck that is not stock is the previous owner installed a Gibson cat back exhaust that has a 3 inch pipe
    Any suggestions that could be offered on what is causing this truck to be so high on hydrocarbons would be much appreciated. THank you

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