1uz running better with AFM unplugged

Discussion in 'Diagnostic & Repair Tech' started by Jay351, Sep 18, 2017.

  1. Jay351

    Jay351 Member

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    Hey everyone,

    I have been doing some searching but I still feel the question I have has not been directly answered.

    Im running a 93 ls400 1uz in my 91 hilux. Im running the factory ecu. With everything plugged in the truck runs pretty good but at around 3800rpm at it seems to gain noticeable power all the way up to redline.

    When I unplug the AFM the truck has amazing power from way down low all the way to redline, it feels much smoother and seems to run better. Aside from it running rich.

    So far I have Adjusted the TPS, replaced the water temp sensor, cleaned the primary o2 sensors, tested the AFM, replaced the caps in the ECU.. nothing seems to help.

    Now I am currently running no sub 02 sensors and no EGR, so the ecu usually has three codes for those.

    My question is, what sensors are ignored once the AFM is unplugged? Something must be pulling alot of timing out because this is a very noticeable amount of power being lost/gained. Any suggestions from the veteran 1uz people out there?
     
  2. Jay351

    Jay351 Member

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    I forgot to mention. When the engine is cold and the AFM is plugged in, it runs very similar to once its warm and the AFM is removed.

    Im running an oem toyota thermostat with a giant aftermarket aluminum radiator and a 16" puller fan. Could it be running too warm?
     
  3. eicca

    eicca Member

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    Bump for more experienced people than me.

    My first guess is to try another AFM for good luck. With the AFM unplugged it defaults to preset fuel maps, which you probably know. It looks like there is an air temperature sensor in the '93 AFM, and my guess is that it's pulling timing because it's getting a falsely high reading.

    My next guess is that the ECU doesn't like having the EGR/secondary O2s missing, but I would expect the power loss to be on the high end, not the low.

    Too bad it's not OBDII or this would be a whole lot easier.
     
  4. Jay351

    Jay351 Member

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    Thanks for the reply!!

    So recently I began to modify my y pipe to add the secondary o2 sensors, although they are only two wire sensors. This should be done in the next week or two depending how cold it gets in my shop.

    I also tried running the truck with no hood to eliminate potential false high readings for temperate due to ambient standing air, no difference.

    I'm going to grab a spare afm from my friends parts car and will throw that in after I'm done with the exhaust.

    As far as the egr, I will throw in a resister to trick the computer but I doubt it would have this drastic of an effect as I'm experiencing.

    In addition I swapped in another throttle body and will try adjusting the tps on this to confirm it's not the issue.
     
  5. cribbj

    cribbj "Supra" Moderator Staff Member

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    Try a different coolant sensor as well.
     
  6. Jay351

    Jay351 Member

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    Should I bother going oem?
     
  7. eicca

    eicca Member

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    Coolant sensor. Forgot that one. Always go OEM if you can.
     
  8. Jay351

    Jay351 Member

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    Okay, so I got the truck running today and took it for a spin.

    I now have the sub o2 sensors installed (with anti foulers) and the egr valve wired back in to prevent any codes.

    For the first 1-2-3rd gear pull it was super strong, but as soon as it reached operating temperature it reduces power !!!!


    I am going to pull the throttle body and make 100% sure the tps is set. I will get my friend to bring over a spare air flow meter. I'm hesitant to replace the coolant temp sensor again, the old one tested within range, as did the new aftermarket sensor.

    Is there a way to narrow this down at all? I'd really rather not throw parts at it as I'm trying to save up for suspension and paint work.
     
  9. eicca

    eicca Member

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    I wonder if your ECU has bad capacitors. I remember reading something about early 1UZ ECUs having trouble. The reason I suspect this is you're not having trouble until it switches to closed loop mode and it's making calculations based on sensor input data. But usually if there's a sensor sending bad enough data to affect performance that bad, you'd have error codes related to something more crucial to engine management. This makes me think something in the ECU itself could be off.
     
  10. Jay351

    Jay351 Member

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    I had a friend replace the caps in my ecu a few months back. None of the oem ones looked bad at all, nothing changed when that was done.


    I can most likely get my hands on a spare ecu to test at least.

    What sensors could possible pull power without throwing a cel? It's a night and day difference as soon as it reaches operating temp, or I unplug the afm.

    As bad as it sounds I just want to leave the afm unplugged and run it like that, although it seems to have idle issues when it's unplugged.
     
  11. eicca

    eicca Member

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    See if another ECU does anything. In the meantime, somebody with more experience ought to chime in. I can't think of anything but maybe the knock sensors, but I think those signals are processed regardless of operating temperature.
     
  12. gloverman

    gloverman Active Member

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    In a situation like this I like to know what the ecu is seeing from each sensor so some live data is a must. From there I add some fuel mixture information so connect a wideband unit. If it's lean how lean? What are the stock oxygen sensors doing? If you suspect it's pulling timing then check the timing instead of guessing. Some of the stock early ecus are mapped quite lean down low so something like old hard injector seals can push them into being too lean to run well once cold start enrichment is removed. I also check everything is a matched set. I've had ecus wired onto the incorrect injectors so too lean or too rich depending on the ecu.
     
  13. eicca

    eicca Member

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    How does one pull live data from a 93 ECU?
     
  14. gloverman

    gloverman Active Member

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    Wire it correctly with the underdash diagnostics plug and connect a suitable scan tool is the best and gives the best data. This is the semi circle Toyota connector.

    With most I wire i also modify the under bonnet connector (rectangle Toyota) to allow interface with a scan tool. It works ok but some info like temp is a voltage instead of degrees etc.
     
  15. Jay351

    Jay351 Member

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    Im a amateur when it comes to this in depth of troubleshooting. I really had no idea you could pull live data from an OBD1 setup. I dont own a timing light or a OBD1 scanner tool. The timing light I can most likely get , the scan tool I will have to ask around and see what my friends have available. I also do not have a wideband unit available unless I drop the $350 for one. If it makes any difference I left all the wiring from the ECU to the engine stock (except for the sub o2 sensors and EGR which I have rewired into place).

    This engine I have is giving me hints that it may have been run hot at some point as most connectors are quite brittle. So perhaps the injector seals could be hardened at this point.

    On a side note, I had pulled the throttle body off last night and re-adjusted it using the idle switch as reference as per a post I had found on here awhile back. The measurements I was getting from the TPS all fell within spec, but were extremely close to the low end of the ohmic values given in the FSM.
    Took it for another test drive and it was the same deal. I unplugged the AFM and took it for another spin. All power was available again, only this time it had a gnarly loping idle.... Im sure im just chasing my tail on that subject.

    I am still waiting for a friend to drop off a spare AFM and ECU since these are easy things I can throw in and test.

    It would seem that the only way to go at this point is to try to get some live data out of this pig, of course I did NOT wire in the under dash diagnostic port as I figured as long as I could pull codes from the engine bay port I would be fine... gah!

    Could you describe modifying the under bonnet connector to allow interface with a scan tool?

    thank you for your knowledge!
     
  16. gloverman

    gloverman Active Member

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    brittle connectors are very common on old looms. Sometimes I end up fitting all new ones. I replace the injector seals on every engine i fit and often when doing jobs like starter motors.

    Can you do a little text for me please. Unplug the IAC and check for power at one of the centre pins. key on. Start engine and keep running for about 10 seconds. Turn engine off and tell me if power to IAC turns off straight away or not. Thanks
     
  17. Jay351

    Jay351 Member

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    Do you by chance have a source for replacement connectors?
    Trying to source these seems to be a real pain, junkyards are pretty bare when it comes to these cars in my parts.

    Okay, probed a centre pin, ran the truck for 10 seconds and then turned it off. I had +12v at the connector for about 2 seconds after ignition was shut off. Im assuming this is how its supposed to function, perhaps to reset itself to a home position?
     
  18. gloverman

    gloverman Active Member

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    I have an awesome source of connectors. I have two big boxes of new ones as I use that many and can only buy them in bulk amounts. Ive been meaning to set up engine sets to sell but have been a bit busy.

    Yes there should be power at the IAC for a few seconds to allow the unit to reset.

    Have you checked your fuel flow? While I dont think its the issue its a quick easy item to check and confirm its not an issue.
     

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