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Any GM 7.4 TBI wizards here?

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I think this is an intake airflow problem.....Mine throws the code too, but at 400K I'm not interested in chasing it down. If I where to chase it down, here is where I'd start;

Gasket under the throttle body, then the airflow sensor, might just need a throttle body rebuild kit. 

I was also told that if you ran/run a cooler thermostat, that they will throw this code, do to a slightly higher idle.

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It's definitely an 02 sensor related code which refers to an open circuit or a period of no activity.


Trouble Code 13 indicates that the exhaust stream oxygen-content sensor (O2 sensor) is not responding as expected. When cold, the sensor is 'biased' by the ECM to about 450 millivolts. Before it warms to at least 600 deg F (315 deg C) it acts as an open circuit and when the ECM reads it, it reads the 450 mV bias. The ECM expects the sensor to warm in a short period of time and begin sending its own voltages. The general conditions for this code getting set are:
?   engine running at least 2 minutes
?   coolant temperature at least 50 deg C (122 deg F)
?   O2 voltage not fluctuating (i.e steady between 350 and 550 mV)
?   TPS signal above idle
?   all above conditions met for 60 seconds

Typical causes may include:
1.   Defective or degraded O2 sensor
2.   Deposit-contaminated O2 sensor (running leaded fuel, RTV silicone deposits etc)
3.   Corroded/defective O2 sensor connection
4.   Defective sensor ground circuit
5.   Defective connection at ECM
6.   Defective ECM
NOTE: If codes 21 and/or 22 are also present cause '4' above should be checked first.



I'm not at work,  or I'd have one of our mechanics access ShopKey and bring up a schematic by VIN number.  


I did search for Google for a schematic.   When you read it,  any intersection with a dot means a circuit connects at that junction,   any that just cross without the dot indicates that a circuit does not connect at that junction.    It might be something as simple as a damaged wire,  or an oxidized terminal.   Might be worth trying to reseat the ECM's connector.  I can try to get more info in the AM.




Edited by PsychoSane
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I spoke with one of our good techs, he said that a period of no activity means a period of no cross counts. 450 milli volts would be median voltage, and a cross count is when the voltage goes above and below the median voltage. He said that if the o2 sensor is confirmed to be in good condition, and he can't find a wiring fault, a period of no activity can indicate a period of full rich or full lean.

He said that measuring the o2 mv will tell you which is the case, but that the average home owner type multi meter tends to be too slow to accurately observe the real picture.

He mentioned that a vacuum leak or other enengine malfunction could be a potential cause, or anything else that mechanically could cause a very rich or lean condition.

He said that he would get back to me and try to give me more info, whatever that entails.

Good luck though, I love 454's and have owned exclusively GM vehicles my entire life.

Edited by PsychoSane
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