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Bad ecu? Bad ground? Bad luck?

Discussion in '1st Gen Tundras (2000-2006)' started by Germanviking, Jul 31, 2025.

  1. Jul 31, 2025 at 7:56 PM
    #1
    Germanviking

    Germanviking [OP] New Member

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    Long time reader new time poster. 2000 tundra 6/2000 what it says. 4.7, 4x4, auto, stock truck, has seen a used engine and used transmission installed years ago, not by me.

    Truck will start, and it will barely stay running while badly stumbling over itself. Sounds like it’s missing on just about every cylibder. Also like it’s not getting enough air cause fuel Oder is strong. Has a check engine light but when I hook up a scanner the scanner doesn’t recognize it’s plugged in. I have tried 3 different scanners.
    When I opened the computer up one of the heat sinks gets so hot you cannot touch it. When I unplug one of the wires to the ecu ( far left wire looking down with plugs facing away from body) the heat sink no longer gets hot, but truck won’t start. I have traced all the wires from that plug and can’t find an issue. ( yes pulled the dash and most of harness tape)

    Anyone know what my problem could be? I want to say computer but I think wires do to the truck running fine and when the computer was pushed back into mounting area is when problems started (computer was originally out for non related issues)
     
  2. Jul 31, 2025 at 9:23 PM
    #2
    Josue914

    Josue914 New Member

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    What was the ECU out for? and is there a chance it could have been dropped or contaminated with any liquid?
     
  3. Aug 1, 2025 at 7:09 AM
    #3
    shifty`

    shifty` Earth acid cleanses me, cleanses me clean

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    First off, you didn't do something silly, like disconnect the ECU while the battery cables were connected, right? (If you did, we need to know ... you shouldn't do that)

    Obviously, go back to your battery, unhook the negative terminal only, then pull your ECU again, unseat each connector, one by one, verify none of the pins are bent, and there's no corrosion visible anywhere, then reinsert/fasten that connector, and repeat with each connector.

    But more than that, verify EACH WIRE going onto EACH HARNESS is not loose, give a small tug, make sure one of the pins hasn't become unseated and is making a bad connection. Bad connections will cause heat, and it would NOT be the first time I've seen a loose ECU wire cause crap like this. But also note, ECU will run warm. There's a lot going on in that case.

    Also note: When you disconnect your ECU from power for more than 10 minutes it automatically goes into relearn mode. So, say you have a lot of buildup in your throttle body, and the ECU has, over the last several months, learned how to operate with all that gunk preventing your throttle body from closing fully, then you pull power from the ECU for more than 10 minutes, and it starts running on default values while it re-learns how to function with all that shit buildup it isn't expecting, it's going to run like ass. People who do stupid shit to these trucks, like install "hot air intakes" with oiled filters like K&N and Spectre, or just run oiled filters period, in addition to killing the natural cold-air intake properties of the stock high-flow filter, you get a LOT more buildup from oil droplets etc. when they pass thru the throttle body.

    That's actually one huge reason to never use oiled filters: Unless you're doing the extra maintenance of regularly cleaning your throttle body, you may initially get a better feeling of performance, but the amount of buildup that results isn't worth it. But anyway, I digress.

    Beyond that ...

    Before you anything else, you need to understand why this is happening, and if reseating the ECU cables properly doesn't solve it.. Until you can scan codes, you're flying blind, and it's 100% not normal to fail to get power to the ECU.

    After you check the fuse I'm about to point out, in the box next to the battery, if it looks OK, read this from @BubbaW and let's get to the bottom of whether the OBDII is correctly getting power, so we aren't flying blind: https://www.tundras.com/threads/obd-dead.131867/

    OBD power should be a 7.5A in the following location, I believe this is true for '00-'02:

    upload_2025-8-1_9-57-9.png

    Rich fuel could be a number of things:
    • You've let an upstream O2 sensor go bad, and the ECU no longer knows how to set the air/fuel ratio (a lot of stupid people think O2 sensors are commie/hippie bullshit, upstreams definitely serve a purpose!)
    • Your MAF is telling the ECU there's more air volume coming into the engine than reality (scanner will let you see fuel trims)
    • You've got a vacuum hose loose/broken/borked, and it's causing collateral issue
    • Your ECU is bad, or you've had the sorta-notorious water leakage into the in-dash wiring and/or ECU via bad/cracked windshield, cowl clip, or other well-known water entry issue
    There's a few more, but I'd be plugging at the air if I went on because ... again, we're flying blind without OBDII power, so let's figure out that easily-resolved issue first, because it is probably related somehow, and even if it isn't, getting OBD working again will give us at least a window to peek at the problem.
     
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  4. Aug 2, 2025 at 2:23 PM
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    Germanviking

    Germanviking [OP] New Member

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    1. I’m not even gonna post what I wrote. The level of “did you drop it? Is the battery have a good connection? Did you check your fuses? Gotta figure out why you aren’t getting power to know why your scanner won’t read your computer”

    thanks but no thanks. I’ll post what the issue was when I figure it out.
     
  5. Aug 2, 2025 at 3:32 PM
    #5
    hagrid

    hagrid The most diverse of Diversity Hires!

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    paynuss stretchers
    Bad karma.
     
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  6. Aug 2, 2025 at 6:01 PM
    #6
    NewImprovedRon

    NewImprovedRon New Old Guy

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    Well, that went south quickly...
     
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  7. Aug 2, 2025 at 6:14 PM
    #7
    Sirfive

    Sirfive Socially feral

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    Fuel filter.
     
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  8. Aug 2, 2025 at 6:26 PM
    #8
    Jack McCarthy

    Jack McCarthy Happy hour; beer goggles, not a fake profile.

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    Sounds like a short in the wiring. Best to use the wiring diagram from the Field Service Manual and trace out what might be causing it.

    If it was power being delivered to the ECU, you may need to either repair or replace it.

    All obvious stuff to look at first before you figure out your misfires.
     
  9. Aug 2, 2025 at 7:18 PM
    #9
    shifty`

    shifty` Earth acid cleanses me, cleanses me clean

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    “HELP!”

    “Here’s a breakdown of things…”

    “NAW FUCK AT SHIT, AIN’T NA BODY GOT NA TIME FI DAT!”

    Some days I sit here and wonder what the hell this world is we live in. And people wonder why I want to move to the middle of nowhere with a town population in the 4-digit range instead of staying in tbe 8-digit range.
     

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