1987 Silverado 350 TBI MAP sensor problems

Discussion in 'Chevy Truck Forum' started by 87Silverado, Mar 25, 2006.

  1. 87Silverado

    87Silverado New Member

    Joined:
    Mar 7, 2006
    Messages:
    4
    Likes Received:
    0
    I had placed a previous inquiry on my 87 Silverado and I have made some progress however, I have run into an interesting problem and maybe some of y'all can help.

    I replaced the coolant sensor as well as the O2 sensor which didn't resolve issue. I unplugged the MAP (Manifold Absolute Pressure) sensor and the vehicle ran great. It idled perfectly and didn't stall. I purchased a new MAP sensor and installed. The vehicle wouldn't idle again so I unplugged. The vehicle will idle and run well without the MAP sensor being connected however, obviously the check engine light remains on due to the disconnected MAP sensor. The vehicle never stored any codes regarding a MAP malfunction when it broke down. I have already replaced or checked the following:

    New Fuel Filter
    New Distributor cap/rotor Cap
    New Plugs and plug wires
    New coolant sensor
    New O2 sensor
    New MAP sensor
    Verified EGR is functioning properly
    Verified Coil is functioning properly
    retorqued TBI/Intake manifold bolts

    Any ideas?

    Regards,

    87Silverado
     
  2. BornToRide

    BornToRide Senior Member

    Joined:
    Mar 24, 2006
    Messages:
    86
    Likes Received:
    1
    there is another sensor for idleing is there not ? i cant think of the name of it at this point in time. this would control the fuel thats going into the tank otherwise it may surge. making it not idle properly.
     
  3. mrjohnwayne

    mrjohnwayne Full Member

    Joined:
    Apr 15, 2005
    Messages:
    4,062
    Likes Received:
    101
    I'm not sure if those years have an idle air control or not (IAC) Might give it a look.
     
  4. 87Silverado

    87Silverado New Member

    Joined:
    Mar 7, 2006
    Messages:
    4
    Likes Received:
    0
    MAP sensor issue

    Unfortunately on the 5.7 with fuel injection (TBI) the idle/air control is controlled by the ECM (computer). The vehicle seems to run well without the MAP sensor being connected however, once it is connected it will not idle and runs very rich. Any other ideas? Any GM techs out there?


    Thanks,

    87Silverado
     
  5. funnnyfarm

    funnnyfarm New Member

    Joined:
    May 11, 2010
    Messages:
    1
    Likes Received:
    0
    Hello 87Silverado my friend has has the same problem on his 1989 silverado step-side you do on your 1987 Silverado. Did you finely figure out what the problem is? If so could you tell us. Thanks funnyfarm
     
  6. wheels18f

    wheels18f Senior Member

    Joined:
    Jan 26, 2013
    Messages:
    74
    Likes Received:
    25
    You have a leak between the MAF sensor and the intake. All intake connections after the sensor MUST be airtight, otherwise the ECM is reading a wrong value due to unmetered air going to the engine causing it to try and adjust to airflow that is not correct. Other than all this..you have a bad sensor. Just a thought:idea:....BJ
     
  7. chevychase

    chevychase Senior Moderator Staff Member

    Joined:
    Oct 15, 2010
    Messages:
    22,147
    Likes Received:
    12,690
    First thing a 87 ,or 89 has no maf sensor. obd11 96 chevy started the maf.. Some tpi had these. First thing i would do is check the vacuum line from back of tbi to the MAP SENSOR.
    The map is a sensor that ecm/computer uses for timing,and fuel. Also the older tbi,s do not like the newer map sensors.. The newer maps for 96 up will not work...
    what brand MAP did you use? here is a chart to test the map circuit
    did you check the color key to make sure it is the same? was the old one original?
    [​IMG]

    A closed throttle on engine ... would produce a relatively low MAP voltage output..., while a wide open throttle would produce output high voltage.. Manifold absolute pressure is the OPPOSITE of vacuum.... When manifold pressure is high..., vacuum is low. The MAP sensor is also used to measure barometric pressure under certain conditions,.... which allows the ecm/computer to automatically adjust to different altitudes. The control module sends a 5 volt reference signal to the MAP sensor. As manifold pressure changes,... electrical resistance of the sensor also changes. By monitoring the sensor output voltage,... the control module knows the manifold pressure. A high pressure, low vacuum high voltage condition requires more fuel..., while a low pressure..., high vacuum low voltage requires less fuel.
    The control module used the MAP sensor to control fuel delivery and ignition timing.

    so at high vacuum(idle) we should see a low voltage output,
    and low vacuum(wot) we should see a high voltage output
    You can vary the vacuum and with the key on engine off see if the voltage changes.
     
    Last edited: May 22, 2013
    1 person likes this.
  8. chevychase

    chevychase Senior Moderator Staff Member

    Joined:
    Oct 15, 2010
    Messages:
    22,147
    Likes Received:
    12,690
    Just thought of sooomething imagine that......coolant temp sensor maybe bad..map calling for more fuel..but ecm does not get reading from the coolant sensor??Also if map is good..can do the ole squeeze test..nine foot Indian showed me this trick...with engine running map connected squeeze the map in flat area where vacuum connects..should stall ..or almost stall the engine. Put pressureon the crystal/quartz in the map..changes voltages.
     
  9. wheels18f

    wheels18f Senior Member

    Joined:
    Jan 26, 2013
    Messages:
    74
    Likes Received:
    25
    Sorry my bad CC I meant MAP but my fingers typed MAF instead. anyway you are right I am pretty sure of it..... GM all
     

Share This Page