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Mambeau / Admin
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Discussion Starter #1
Posting new thread based on PM from Jedidiah Stephen Parker

My tacoma takes a long time to start and when it does it mis fires and when I drive it it stinks really bad. My computer throws a thermostat code and the Engine temperature jumps from 190 to -40 and the air intake goes from 85 to -40. I replaced the Engine Temp sensor and noticed one of the pigtail connectors looked burnt. It had a bit of black on it but the sensor fixed the problem first crank then when I drove it it acted the same way. I'm hoping Auto zone just sold me a piece of junk and I will try to get another sensor from oreily
 

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Mambeau / Admin
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Discussion Starter #2
What are the specifics on:

- The Tacoma?
- The code?

:doah: :rolleyes:
 

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Engine temperature jumps from 190 to -40 and the air intake goes from 85 to -40.
-40* is default for a system open circuit
with the default readings, your ecu is probably compensating by injecting a lot more fuel than it should which will cause your catalyst to overwork (the odor you are smelling)..and possibly meltdown internally $$$.


Like Enola said..need more specifics
 

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The tacoma says 200k on the odometer but it's broken so there is no telling. As for the code I can't remember the number but it said thermostat. I will try to hook it back up to the computer but what should I do what should I be looking for so we can get this figured out. I'm located in Ga so it's hot and never freezing.
 

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The tacoma says 200k on the odometer but it's broken so there is no telling. As for the code I can't remember the number but it said thermostat. I will try to hook it back up to the computer but what should I do what should I be looking for so we can get this figured out. I'm located in Ga so it's hot and never freezing.
How about letting us know specifics like
1. YEAR of Tacoma
2. Model.. Pre-Runner or 4x4 ?
3. 2dr, 4dr, Extra-cab?
4. Auto or Manual?
...You know...the kind of info we would need to help determine what your issue may be?


Exact DTC (diagnostic trouble code) would really be to your benefit...


5. Was there any work done previous to you experiencing the issue?
It sounds like a bad / loose harness connection somewhere
 

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I'm sorry I thought I included that.
Its a 1996 Toyota tacoma 2.7l 4x4 extended cab.
The code it was throwing was P0125 the air flow temp was going from 85 to -40 and the Engine Temp was going from 185 to -40 and the throttle position sensor was going to -40 but they would go every now and then. I don't know what was done to the truck before. A meth head owned it before me and it looked like it had been sitting for a while. The only thing ice done is the Engine Temp sensor and thermostat.
 

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Mambeau / Admin
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Discussion Starter #8
I'm not sure how you got a thermostat code. The only thermostat-related Toyota diagnostic code in 1996 was a P0128, and that code wasn't listed as one used on the Tacoma.

PO125 makes a lot more sense in relation to the symptoms you describe.

I agree with ZIP - the most likely cause is an electrical fault (short / broken connection) in the wiring - specifically in the harness leading to the E7 connector on your Engine Control Module (ECM). There are 3 connectors; the E7 is the middle one.

The E7 plug provides the ECM with the signals from:

- Engine Coolant Temp (ECT) sensor
- Heated O2 Sensor (Front; Bank 1 Sensor 1)
- Throttle Position Sensor (TPS)
- Mass Air Flow (MAF) Sensor
... and some other, less likely, suspects ...

The P0125 code means:

PO125 Insufficient Coolant Temp for Closed Loop Fuel Control

... and the probable causes are:

- Open or short in heated oxygen sensor circuit
- Heated oxygen sensor

Like ZIP mentioned, if you're getting readings (presumably from a scanner) that go as low as -40, it indicates a dead sensor / circuit. Notice that the other fluctuating readings you mentioned relate to the items connected to the ECM through the E7 connector.

All these things reading 'dead' (even if only occasionally) points to their shared connection to the ECM - in other words, a wiring fault, bad connection, or a bad ECM.

NOTE: There are two engine temperature sensors - the coolant temp sensor that drives the cluster temp gauge, and the ECT sensor that signals the ECM.

I'm assuming it was the ECT sensor you replaced.
 

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Im also thinking E7 connector at the ECU itself..located behind the glove box (see page 28 of "E8" PDF attached below) . Like Enola had said, the sensors you are mentioning all share that connector..as well as the VCC (voltage constant 5v) supplied to those sensors from the ECU.
It also could be a bad ECU.. but I would check that connector first, then monitor the supply voltage to the sensors since it is intermittent..before condemning the computer. There are some tips included in the system description in the Engine Control PDF I also attached
 

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Mambeau / Admin
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Discussion Starter #11
... Zip the pdf you sent me will not load. Is there any other way you could send it to me?
Are you referring to some email ZIP sent you, or are you saying you can't download the files attached in this thread by clicking on their links?

His PDF files (posted here) are downloading / opening in every browser I've thrown at them.
 

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I replaced the ecu in my truck and am still having the same issue. I ran a jumper wire from the tp sensor to the ecu to check and see if there was a broken wire and the reading was still dropping from 7 to -40. Replaced the air intake temp sensor and the reading was still dropping to -40. The only sensor I have left to change is the Throttle position sensor.
 

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Mambeau / Admin
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Discussion Starter #15
What have you done to troubleshoot the wiring harness leading to the E7 connector on the ECM?

Bypassing the single line for the TPS is the only thing you've mentioned that's relevant to the wiring connections. Have you tried bypassing the individual lines for the other sensors to see if you get the same results?

This bypass test will only help tell you if the problem lies with one or another individual line leading to / from the E7 connector. If the harness is shorting out or broken overall, the bypass testing on individual lines won't necessarily help.
 

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Mambeau / Admin
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Discussion Starter #17
I see what you mean, but ...

I don't think that bypass would necessarily have proven anything with respect to the TPS. There are 4 lines running between the TPS and the ECM.

For example, if you bypassed the brown / black striped wire at pin 4 on the TPS, you'd have been bypassing the ECT sensor (which should have set off a code).

If something's shorting out in the TPS or any of the wiring between the TPS and (a) the ECM and / or (b) the other interconnected sensors, bypassing a single line wouldn't necessarily mean anything as far as fault diagnosis is concerned.
 

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Mambeau / Admin
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Discussion Starter #19
So what do you recommend I do?
Unless you can figure out how to check out the entire harness and / or E7 connector, I don't see how you have much choice except to continue swapping out sensors.

It's conceivable that a messed-up (e.g., shorting out) TPS has been causing the problems you described, but it's not a certain thing.

I still tend to think it's most likely to be a wiring fault.


SIDE NOTE: Is the new ECM throwing any codes? If so - which one(s)?
 
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