Question about Chevrolet Blazer

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My Blazer will idle rough on startup but is ok when the engine is speed up beyond idle.If I disconnect the MAP sensor the engine runs smoothly.Also once the engine has been run for a while,it will idle fine.

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  • bjfield Apr 05, 2011

    Hi thanks for that we spent a small fortune at the local garage having all that done.I believ the ECU and airflow meter have checked out fine.

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Lately there has been alot of questionable fuel going around, I'd start by running a good fuel system cleaner through it and change fuel filter. Another question for you... when was last tune up? plugs, wires, all fluids & filters? I would get the basics on a "ground zero playing field" before unplugging electrics to diagnose - that will usually have you chasing ghosts and throwing parts at an easy fix.

Posted on Apr 05, 2011

  • Mical Caterina
    Mical Caterina Apr 05, 2011

    Stop by any good local parts store IE: Autozone, Advance, etc... and have them do a OBII scan (fault scan) and see what codes they pull from your OBC. They do this FREE and will give you an idea where to look. Most issues as you've described are simple things like O2 sensors, cats, etc... but can get totally twisted with cheap gas or dirty connections.

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What is a solution to fixing the mass air flow system, with code po121


DTC P0121: Throttle Position (TP) Sensor 1 Performance
Circuit/System Description
The engine control module (ECM) uses the following information to calculate an expected airflow rate:
?€¢
The throttle position (TP)


?€¢
The barometric pressure (BARO)


?€¢
The manifold absolute pressure (MAP)


?€¢
The intake air temperature (IAT)


?€¢
The engine RPM


P0121 The predicted air flow and the predicted MAP combined are outside a calibrated range for more than 3 seconds.
Circuit/System Testing

  1. Inspect for the following conditions:
  2. ?€¢
    Vacuum hoses for splits, kinks, and proper connections as shown on Vehicle Emission Control Information label


    ?€¢
    Inspect thoroughly for any type of leak or restriction


    ?€¢
    Air leaks at throttle body mounting area and intake manifold sealing surfaces


    ?€¢
    Throttle body for dirt, debris, and coking--Refer to Throttle Body Cleaning.

  3. Allow the engine to reach operating temperature. Observe the MAP Sensor Voltage parameter with a scan tool. Voltage should be more than 0.8 volt and less than 4 volts.
  4. ?‡'
    If more than 4 volts or less than 0.8 volt, refer to DTC P0106.

  5. Idle the engine. Observe the MAP Sensor kPa parameter with a scan tool. Increase the engine speed slowly and then back to idle. The MAP sensor kPa should change smoothly and gradually as engine speed is increased and returned to idle.
  6. ?‡'
    If the MAP sensor kPa does not change, refer to DTC P0106.

  7. Take a snapshot of the engine data list while performing the actions listed below. The mass air flow (MAF) sensor g/s should change smoothly and gradually as the engine speed is increased and is returned to idle.
  8. ?€¢
    Idle the engine.


    ?€¢
    Increase the engine speed slowly to 3,000 RPM, then back to idle.


    ?€¢
    Exit from the snapshot and review the data.


    ?€¢
    Observe the Mass Air Flow (MAF) Sensor parameter frame by frame with a scan tool. The MAF sensor g/s should change smoothly and gradually as the engine speed is increased and is returned to idle.


    ?‡'
    If the MAF sensor g/s does not change smoothly and gradually as the engine speed is increased and is returned to idle, refer to DTC P0101 or P1101.

  9. Inspect the throttle body for the following conditions:
?€¢
Loose or damaged throttle blade


?€¢
Broken throttle shaft


?€¢
Any throttle body damage


?‡'
If any of these conditions exist, replace the throttle body assembly.

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1 Answer

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map sensor has a low speed air passage in it, in this passage is a temp sensitive diode, this controls idle fuel mixture it would be unwise to drive with this disconnected as connection also has main air flow circuit which may damage other components if it'll run at speed at all

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Do you know what engine scan data is ? Disconnecting sensors is not the way to find a problem ! What is the computer seeing as data input signals . Is the vehicle in closed loop fuel control , is the EGR opening when it shouldn't be , Is the PCM controlling the idle air control valve or is the vehicle drive by wire (electronic throttle control ?
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1 Answer

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1 Answer

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