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Thread: 7747 Won't Enter Closed Loop At Idle

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  1. #1
    Electronic Ignition!
    Join Date
    Aug 2020
    Posts
    17

    7747 Won't Enter Closed Loop At Idle

    I have read hundreds of posts on this forum and it has been really helpful, but I think I have a new problem...

    Vehicle: '85 Toyota FJ60, 4-speed manual
    Engine: 89 C1500 5.7L TBI, all stock except that AIR system has been removed
    ECM: 7747 from the 89 C1500
    BCC: ASDW, no modifications

    Problem: Failed smog due to high CO at idle (0% CO at 2500rpm). I think my problem is that the ECM is not staying in Closed Loop at idle

    What I have done:
    1. Verified sensors and actuators (MAP, TPS, IAC, EGR, CTS & ground) are nominal
    2. When I raise idle (either with pedal or adjustment screw) to >850rpm the ECM will enter closed loop. When I reduce rpm it drops back into open loop. Target idle in the bin is 600-650 depending on CTS. I think the reason it enters CL at higher rpm is due to the "RPM above which open loop idle is forced off" (825rpm).
    3. Verified O2 sensor (singe wire installed in OEM manifold) is moving between 0.2-0.8mV in closed loop
    4. Confirmed Park/Neutral switch input (B10 to ECM) is OPEN = Drive
    5. Checked for vacuum leaks (not 100% sure there isn't a leak somewhere). MAP is seeing 20-24" at idle
    6. Rebuilt throttle body
    7. Verified fuel pressure (14psig)
    8. Tried biasing the TPS to 1.5V at idle to try to force closed loop (juicing the "Maximum TPS for Open Loop Idle" parameter") (didn't work, does this ECM set 0% throttle at key on? I saw conflicting info)
    9. Lots of other stuff that didn't make a difference
    I read in one posting that "older TBI trucks didn't really enter CL at idle" but it's hard to figure out what that really means.
    I am at the point where part of me wants to just drive the idle speed to 900rpm to force closed loop and try to pass smog, but... the engineer in me needs to figure out wtf is going on.
    Thanks for any help, again this forum has been a huge help to me!
    Adam
    Last edited by SBAdam; 02-22-2021 at 05:04 AM. Reason: messed up the post title somehow

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