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Can a MAP Sensor be the cause???

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by DuneRunner, Jan 22, 2002.

  1. DuneRunner

    DuneRunner 1/2 ton status

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    A question that I'm verry curious about. Would a MAP (Maximum Air Pressure) sensor be the cause for loss of power in high elevations?
    I was resently at Lake Tahoe Ca. and I realy noticed the loss of power. My (83's) Q-Jet Runs fine at Sea leve cuz thats where I live. I was reading in 4wheel and off road mag. this months issue and the MAP sensor was given a quick little paragraph. But it was focused on EFI engines. SO the real question is should I replace the map sensor or just leave it? Or is it something else that could be the colprit. (ie. tranny?)

    Later <font color=green>KEN</font color=green>
    Fremont, CA
    DANGIT it's not a bronco!
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  2. BurbinOR

    BurbinOR 3/4 ton status

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    What motor do you have in your truck? If you are talking about an EFI motor, the MAP (Manifold Absolute Pressure) might contribute to that issue at high elevation............as one function of it is to measure the amount of air into the motor (which a MAF equipped car has a MAF sensor for), as well as measuring engine load (which is increased when you are in 'thin air').

    You have a MAP sensor on a Qjet equipped vehicle???? Learn something new every day............

    Seems your issue is just a normal occurrence of carbed vehicles............but I might be wrong.

    '79</font color=blue><font color=red> ONE TON</font color=red> <font color=green>TPI</font color=green><font color=orange> K5 PICS</font color=orange>
     
  3. DuneRunner

    DuneRunner 1/2 ton status

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    I have an 1983 350 with 4 barol Quadra Jet Carb.(sp?) You are correct about the name Manifold Absolute Pressure. I know for a fact that there is a MAP sensor on the firewall near the distributor. and I'm wondering if it ever needs to be changed. Thanks for the response and I hope that this is one of the only things that could cause it.

    Later <font color=green>KEN</font color=green>
    Fremont, CA
    DANGIT it's not a bronco!
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  4. HarryH3

    HarryH3 1 ton status Author

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    All normally aspirated internal combustion engines lose power as the altitude increases. It's caused by the reduction in ambient air pressure as you gain alititude. Lower pressure means that the cylinders don't fill as well as at sea level, so you effectively end up with a lower compression ratio, along with a lower oxygen content. By 6,000 feet above sea level, you've already lost over 20% of the power your engine would have at sea level [​IMG]

    If you want to play with the numbers, check out this calculator:

    <a target="_blank" href=http://rshelq.home.sprynet.com/calc_hp_dp.htm>http://rshelq.home.sprynet.com/calc_hp_dp.htm</a>

    <font color=black>HarryH3 - '75 K5</font color=black>
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  5. Burbinator

    Burbinator 1/2 ton status

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    MAP sensors usually tap directly into the manifold for their readings...at least on EFI engines.

    The thin air of higher altitudes affords the engine less usable oxygen for the combustion process, as Harry said. Sounds like a textbook description of high altitude carburetor performance (or lack thereof).

    Haze gray and underway...<font color=blue>or blue, in my case.</font color=blue>
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  6. Thunder

    Thunder 3/4 ton status

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    Harry's right. You lose a lot of power the higher you go. If you thought your k5 was a little slow you should see what the altitude at Tahoe does to a boat that is tuned and proped for sea level. Some people cant even get up on boats plane without tuning for high altitude and re proping..
    I dont think there is anything wrong with your MAP sensor. if it is bad your truck will run like crap but usually will idle good.
    The lack of power is mainly that your truck was not tuned to run at 6000 ft.

    A sort of rule of thumb for better performance at high altitude is is to advance your timing 1 degree for every 2000 ft of altitude.

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  7. gokartergo

    gokartergo 3/4 ton status Staff Member Moderator

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    I don't think a pre EFI motor as a map sensor...Does it???....Dan

    If it ain't broke.......Don't fix it....
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  8. TwoCrowsDesign

    TwoCrowsDesign 1/2 ton status

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    EFI engines will handle elevation changes more readily than carb-fed motors because of the computers ability to alter fuel/air over a greater range. The MAP sensor (or MAF if equipped) is what tells the computer where you are (elevation wise) by comparing ambient air pressure outside with the air pressure at the manifold. At least that's how I think it works, someone please correct me if I'm completely out of my mind. :-)

    John
     
  9. Gold Rush

    Gold Rush 1/2 ton status

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    Actually a bad MAP sensor will cause a wavering idle.


    David

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  10. DuneRunner

    DuneRunner 1/2 ton status

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    To all Thank you. [​IMG][​IMG]
    I have talked to GoldRush and a few others and I'm comming to the conclusion that it is the ever so popular 700r4 and its time. Yes!!!! time to get her rebuilt[​IMG] SO I'm going to leave the good old MAP sensor alone and drive it till it breaks.

    Again thank you.

    Later <font color=green>KEN</font color=green>
    Fremont, CA
    DANGIT it's not a bronco!
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