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O2 Sensor and ECM

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by GRINCH, Dec 28, 2001.

  1. GRINCH

    GRINCH 1/2 ton status

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    As most of you know I have been batteling with Edelbrock to get a new chip for my MPFI cause it has been running so rich. Well they finally came trough and remapped the fuel curves in my chip. It is running a little leaner now but still not where I want it. My BLM and Intergrators are now around 117-125.

    I think some of the reason is my O2 sensor on my thorley tri-Y's is only picking up from the last 2 tubes rather than all 4. It does seem really lazy on idle and low rpm runs. I get very few cross counts. Would I be better off welding in a new bung further down in the collector where all 4 tubes meet? Or is 2 cylinders enough?



    Thanks

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  2. 502Burban

    502Burban 1/2 ton status

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    Just my opinion, but I would probably want to be measuring all four cylinders. But, I tend to be overly concerned about little details like that.[​IMG]

    Having said that, I don't think that moving the sensor will change your current situation much. I think that it would just be better to measure all four. Sounds like you probably just need to keep on Edelbrock to get your system mapped correctly.

    You may want to PM BlazzinOR if you don't get any further responses on this. He seems to know his stuff with the FI, especially the GM TPI-based systems which I know little about. DYeager also seems pretty knowledgeable. It sounds like the Edelbrock system is based on the TPI system since you're talking about BLM and integrator numbers.

    I've only had experience with the Accel and SpeedPro systems, which are programmable with a laptop, myself.[​IMG]



    Robb
    <a target="_blank" href=http://community.webshots.com/user/robbrj>'73 GMC Suburban - 502DFI</a>
     
  3. 95 Silverado

    95 Silverado 1/2 ton status

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    do they give any recommendation on placement of the o2 sensor, the farther down the exhaust pipe you mount the sensor, the farther from the engine, the cooler the exhaust will be, since these sensors work by measuring the heat of the exhaust, will this affect it. headers release the heat alot faster than cast manifolds, this could affect it also.

    '95 Chevy Silverado 1500
    5.7 V8-NV4500- 3.73 rears
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  4. GRINCH

    GRINCH 1/2 ton status

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    Robb,

    I feel the same as you. I would think I would get a more accurate reading from all 4 like the stock since I'm still using the stock ecm with edelbrocks chip.
    I will be taking some more scans this weekend. But peliminary scans show still rich. I don't want loose the performance I gained by having edelbrock take any more out of the fuel map.

    Sure wish I could learn to burn my own.

    Thanks

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  5. GRINCH

    GRINCH 1/2 ton status

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    I called thorley friday and talked to Randy there, He said that they don't determine the placement of the O2 Sensor. He said the Fed Gov't tells them where to put it for each application.

    I was under the impression that the sensor measures the amount of unburned fuel in the exhaust!!

    I know placing the sensor further down in the system like the stock would be a little cooler but what if I bought a 3 wire heated sensor? Is there different ratings on the sensors?
    Since I am using the vortec heads could it be I need a sensor from 1996 and up?
    Thanks

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  6. 95 Silverado

    95 Silverado 1/2 ton status

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    this is my understanding of how an o2 sensor works, when it is heated by the exhaust it produces a voltage that corresponds to the temperature of the exhaust, the computer is programmed to relate this voltage to temperature and adjust the fuel mixture to achieve the desired voltage that is programmed as the right mixture. lean exhaust is hotter, thats why too lean melts pistons, rich exhaust is cooler. if you put a very sensitive voltmeter (millivolts) on the lead from an o2 sensor and heat it, you will see the voltage change with the temperature of the sensor, that's why when the vehicle first starts running it doesn't use the o2 sensor input (open loop) after it runs long enough to get heated and in range the computer switches to closed loop and uses the o2 input for mixture.
    if I'm wrong on this let me know. that's why I would think location would play a part in getting the mixture right.

    '95 Chevy Silverado 1500
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  7. HarryH3

    HarryH3 1 ton status Author

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    Oxygen sensors do not measure temperature. They measure oxygen. Thus the name. Thermometers, thermocouples and pyrometers measure temperature. [​IMG] An oxygen sensor is an oxygen powered battery. Once it reaches about 600 degrees F, it begins to electrochemically react with oxygen in the exhaust stream. More oxygen produces more voltage. By reading the voltage that's coming from the oxygen sensor, the computer knows the content of oxygen in the stream. The amount of voltage that's created by the oxygen sensor is measured in millivolts.

    Exhaust temperatures vary all over the place, depending mostly on engine load. Pulling a heavy trailer up a long grade will make for much hotter exhaust temps ( by several hundred degrees!) than running at the same RPM on a flat road, so just measuring the temperature is not a good indicator of how complete the combustion process is.

    When the oxygen sensor is below 600 degrees, it will stop putting out any voltage and the computer will go into "open loop" mode. Our trucks run in open loop unless 2 parameters are met. 1) The coolant temp must be over 160 degrees F and 2) the oxygen sensor must be hot enough to function. The shop manual mentions that during long periods of engine idling, the oxygen sensor may cool off enough to put the computer into open loop mode. This can also be a problem with some headers. They don't contain heat nearly as well as a cast iron manifold, and can keep the computer in open loop mode much more often. Many vehicles now use heated oxygen sensors, that use an electric heating element to keep the sensor temp over the magical 600 degrees all the time. Retrofitting a heated sensor may provide more reliable readings if your engine is going into open loop mode too often.



    <font color=black>HarryH3 - '75 K5</font color=black>
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  8. 95 Silverado

    95 Silverado 1/2 ton status

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    thanks for the clarification, so, as long as it is close enough to get to the 600 deg. point, the location isn't that critical.

    '95 Chevy Silverado 1500
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  9. 502Burban

    502Burban 1/2 ton status

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    Well said, Harry.[​IMG] I knew there was another name I should've had on the list of people that know their GM FI systems.[​IMG]

    I agree about the heated sensor. Dennis, if your truck didn't have one from the factory, that would explain why Thorley had to put the O2 sensor so close to the head. Switching the the heated 3-wire sensor would allow you to place it farther downstream to measure all four cylinders. Again, not sure it would make a difference, but I would probably do it just to be sure.

    Robb
    <a target="_blank" href=http://community.webshots.com/user/robbrj>'73 GMC Suburban - 502DFI</a>
     
  10. GRINCH

    GRINCH 1/2 ton status

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    Well said..

    Once my truck goes into closed loop it stays there. I run a program on my laptop called turbo-link (It was designed for the t-tpye buicks) but they also have settings for the tbi motor.

    I can see real time readings of all ecm sensors. The voltage still changes during idle and city driving but not as much.
    Do you know what the cross counts should be for a well functioning motor? I was told it should be 15 or more. I ussally only see 3-6.
    I'm going to try a heated sensor. My question is should I get one for my 90 or should it be a 96?

    Since I stay in closed loop will it even make a difference.

    I can post some of the data in a csv file or excel if you can help.




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  11. HarryH3

    HarryH3 1 ton status Author

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    If it's staying in closed loop all the time, then a heated sensor shouldn't make any difference. On newer vehicles, they use the heated sensor to get into closed loop mode before the engine fully warms up. Just makes it a tad easier for them to pass all of the EPA tests.

    <font color=black>HarryH3 - '75 K5</font color=black>
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