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Mass Air Flow/ ECM

  #1  
Old 12-17-2014, 08:47 PM
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Join Date: Dec 2014
Location: Port Angeles, Wa
Posts: 2
Angry Mass Air Flow/ ECM

I just rebuilt my 91 B 2200, was on my first out of town trip and the check engine light comes on. MY 91 is EFI and displays a 8 error code. I have tested the Mass air flow and have a mill-volt pulse signal voltage at the sensor or ecm pin 2O, when I checked the burn off it has no voltage present before or after ign. is turned off, all I get is a mill-volt pulse.
Checked voltages at ecm pins 2P .3 ohms resistance, pin 2O mill-volt auto-range only, 1I ign on 12V present.
Does the ecm supply sensor voltage or does the ecm, ecm supply grounds as I remember.
What do I test next or am I to believe the ecm is bad as Mitchell would indicate, it says if the voltages are correct then replace sensor.
If all voltages are not present would that mean replace the ecm, or is the sensor bad because it receives a 12V input and all grounds test good?

Retired ASE Master lost, no definitive answer from Mitchell. HELP
 
  #2  
Old 12-18-2014, 07:03 AM
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Join Date: Aug 2007
Posts: 1,324
Default

It is possible that the code 8 is just in the memory from the overhaul for whatever reason. The truck has a mileage counter that sets a check engine light. It is a reminder to service emission items. The large wire harness on the drivers side as it goes into the cab under the dash has a single pin connector plug into one of two similar connectors. Unplug that and see if the light goes out. If this is not the case then it is possible that the AFM flap bound up at some point ( move the flap by hand and see if it binds) or the boot from the AFM to throtllebody is not air tight. You can remove the ECU,disasemble it and inspect the board/components for damage. You don't mention any driveability problems.
 
  #3  
Old 12-18-2014, 03:41 PM
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Join Date: Dec 2014
Location: Port Angeles, Wa
Posts: 2
Default Mass air flow sensor or ecm?

Well I did not mention that I have poor fuel mileage, missing, poor acceleration, idles at 400 rpm and the spark plugs are fuel fouled. I really believe that there is a problem, I just don't know if its the Mass air flow sensor or the ecm? When I clear the code it returns immediately.
Does anyone have a schematic of the internal parts of the mass air flow sensor? The 12 Volt supplied to the mass air flow sensor I believe should produce the 1-5 volts the ecm is looking for?
 
  #4  
Old 12-18-2014, 06:19 PM
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Join Date: Aug 2007
Posts: 1,324
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The fuel injected models were sold with California emissions. I worked for Mazda 20 years in Houston. I think i might have seen one. It either has an air flow meter or a mass air flow sensor. They are completely different in the way they work. If it has a flap that moves as more air enters the engine then it is an air flow meter. If it does not then it has a hotwire that heats and cools as the air volume increases and decreases. This changes the resistance of the wire and reports it to the ECU. I will see what alldata has on it tomorrow but would like to hear about the overhaul. What you found/did to correct it/why the overhaul/etc.

Those trucks were bad about sticking EGR valves. If they stick open the EGR will suck on cylinder 3 intake runner and miss. Also there is a wire you must ground to bypass the ECU to set the ignition timing with a timing light.
 

Last edited by hixx; 12-18-2014 at 06:21 PM.
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