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Need help with 1994 Chevy Sierra 1500C Bi-Fuel Impco System

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  • Need help with 1994 Chevy Sierra 1500C Bi-Fuel Impco System

    I haven't been able to get my truck to work on CNG as it is a OEM system by Chevy with Impco parts....I've had it in Utah for the last 2 years and even those mechanics haven't been able to figure it out....they say they need the schematic diagrams for this model...does anyone have them? Or if you are familiar with this truck and its IMPCO system please let me know...this was originally a truck from the City of San Luis Obispo....thanks!

  • #2
    Re: Need help with 1994 Chevy Sierra 1500C Bi-Fuel Impco System

    Im new here as well and am trying to learn all i can about cng. I have a 1995 GMC suburban that has an ANGI system and CNG information in this part of NY is not easy to come by, several unmaned clean energy stations though . Having an older truck like myself, did you find any information on your system? Mostly cars here and newer ones at that.


    • #3
      Re: Need help with 1994 Chevy Sierra 1500C Bi-Fuel Impco System

      ANGI goes back a ways. I checked the WEB and they are only doing fueling stations and associated stuff, no vehicle fuel systems that I could find.

      As I remember, the system has a high pressure regulator that feeds a low pressure regulator simular in purpose to a Impco model E. From there the low pressure NG goes through a vacuum controlled fuel valve (cone shaped inside) that controlls fuel to the venturi type mixer in the intake air stream. The system is a feed back type system that uses the oxygen sensor to control the mixture through a computer box, vacuum solenoid, and the fuel control valve in the low pressure line. The ANGI box sends simulated oxygen sensor signal (square wave) to the OEM computer O2 input line to keep the check engine light off.

      I used to have the ANGI diagnostic connector at the college where I taught, I don't know if it is still there. If the vehicle will run on CNG can "T" into the vacuum line to the control valve and goose the engine a few times and see if there is any change. I forget where the vacuum source is, but you can follow the vacuum line back to its source and see if there is a vacuum solenoid valve that controlls vacuum the fuel control valve. (If the regular inches of mercury vacuum gauge doesen't work try one calibrated in inches of water) There should be a pulse wide modulated signal that can be measured with a dwell meter or the mili-second scale on a good automotive digital volt ohm meter. The on-time signal should change when you goose the engine. If the signal doesn't change then the box is probably bad. These systems are usually set to rich (biased rich) and use the control valve to pull the system lean according to what the oxygen sensor sees.

      That is the best I can do with what information I have. You could try ANGI international through the "contact us" on the web site and see if anyone has will admit to knowing about the automotive fuel system. We took one off of a '92 Ford with a 302 that had burnt a valve and the owner didn't any more to do with natural gas, I may have some pictures of the components.

      Hope this helps