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why did my engine stall when I unplugged the MAF sensor

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I unplugged the mass air flow sensor on my 2014 Scion Tc (30,000 miles) and the engine stopped immediately. From what I know normally the engine should not stop. I unplugged it to check its operation because the idle is a bit low and unstable. All the rest of the mechanics is in perfect condition. Yesterday the mechanic checked with his scan and found no errors. Ideas why the engine stop?

3 Answers
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Why unplug the MFS? I would check for a vacuum leak. The throttle position sensor and it's linkages are prime targets for bad idle. The things to look at on a scan tool is fuel trim.

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I unplugged the mass air flow sensor (...) and the engine stopped immediately

You are lucky you didn't fry the ECU!

because the idle is a bit low and unstable

  • Start the car (Do not drive during the process)
  • Warm up the engine
  • Turn off head lights and turn off AC
  • Shift To N with parking brake engaged
  • Rev to a bit over 4K RPM for a bit over 45 seconds with a stable and constant pedal pressure

Does It idle correctly now?

I am not a mechanic. When I look for the cause of a problem I ask my mechanic. According to the mechanic disconnecting the ECU is a way to check its operation. If the engine stops, there is an air leak except on some cars where the ECU takes a few seconds to return to its original values (like my Scion Tc). In this case the motor stops. The check engine lamp remains lit following this procedure.

After trying the procedure you suggest and after restarting the car the check engine light went out (?). As for the idle, I will test more tomorrow. I am however curious how your procedure affects the RPM.

I’m sure the mechanic did NOT tell you to unplug the ECU -
there are only 2 possible results, the car dies and you need a new ECU, the car you’re lucky you didn’t need a new ECU.

You are right. My mistake when I wrote the comment (french from Quebec). The mechanic told me to disconnect the Mass Air Flow Sensor with the engine running. If the engine stops, there is an air leak except on some cars where the ECU takes a few seconds to return to its original values (like my Scion Tc). In this case the engine stops. But according to the mechanic it will not dammage the ECU.

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I tried with the engine off. I unplugged the Mass Air Flow Sensor then started the engine. It works. The ECU is probably a little slow before compensating for the disconnect. I will test with another Mass Air Flow Sensor for bad idle.

 

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