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Need help with spontaneous overheating!!!

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by foxman, Apr 4, 2002.

  1. foxman

    foxman 1/2 ton status

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    At an idle in park my 90 K5 warms up to 195 without overheating but when it warms up while driving it reaches well past the 260 mark and then shoots back down to the 190 range. It does this several times while driving on the expressway until it heats all the way up. Am I looking at a sticking thermostat? Fan clutch? perhapse it is because the truck has a 7 lb. radiator cap on it??? Anybody got any Ideas?
     
  2. Waxer

    Waxer 1/2 ton status Author

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    you should have a 13lb rad cap on it if I'm not mistaken. Also it sounds like your thermostat is in fact sticking. I would never let it get to 260 and keep driving it. You got some troubleshooting to do definitely.
     
  3. TX Mudder

    TX Mudder 1/2 ton status

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    Replace that radiator cap with a 16 lb. one.
    That would be the first thing to do.
    If it still does it, buy a new thermal clutch for your fan.
    -- Mike
     
  4. Blazer1970

    Blazer1970 1/2 ton status

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    I have seen this happen when there is a leak in the system, such as from one of the plastic tanks on the radiator cracking. This can be very hard to see. I would have someone pressure check it to make sure.
     
  5. TopOff

    TopOff 1/2 ton status

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    Mine did this when it was LOW on water/ radiator fluid.

    If it hit 260, you probably already cracked a head... /forums/images/icons/frown.gif I hope I am wrong.

    Mine did this while sitting in a fast food drive-thru... I rev it up, then the temps SHOT down... idle... it shot up to 250...

    Good Luck...
     
  6. txbluethunder

    txbluethunder 1/2 ton status

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    when i first purchased my k5 it did this all day all the time (never got quite that hot). replace the thermastat and made all the differenct in the world.
     
  7. Nrose07

    Nrose07 1/2 ton status

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    Well there are a number of us with Vortec heads that have that issue. You didn't specify your engine setup so I don't know. But in my case at least upgrading to a quality high flow thermostat helped but even then didn't solve it %100. So I'm left to belive that from my Vortec head swap, and not drilling out the "optional" steam holes in the block, I have steam pockets that cause this fluxuation in the temperature. This flux is not seen on the factory gauge just the mechanical unit installed in the intake. Which gauge are you oberserving this by? Hope it helps.
     
  8. arq

    arq 1/2 ton status

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    I'm using the vortec heads (pre86 350 block, .030 flattops)and it gets like 120 in the summers here, no problems with over heating.

    make sure you HAVE a thermostat, if it's not there your water pump will builld up too much pressure and shoot it out the cap.

    make sure your thermo is not on backwards, i've noticed this causes irradic fluctuations in temperature, similar to the ones you are seeing. i think.

    make sure that your water pump/fan/clutch are good.

    also some of the new pumps run the opposite dirrection that the old water pumps did. make sure you got the right application.

    AND, make sure it's not the guage. is it actually over heating or is the wire/guage hitting the manifold/shorting out causing the temp to shoot up.

    good luck,

    ARQ.
     
  9. fr8train

    fr8train 1/2 ton status

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    Have you replaced the temp. sending unit?
     
  10. MudbogAD

    MudbogAD 1/2 ton status

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    I would first check the thermostat and the temp gauge. My engine was running really cold so I thought the thermostat was stuck open, sure enough, it was in about 5 pieces. Yours may be sticking.
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    What brand t-stats are you using? These "temp spike" problems have been a product of the Robertshaw (Mr Gasket) brand t-stats by some people...apparently quality control on these aren't quite up to snuff.

    I have yet to hear of anyone buying an AC Delco T-stat and having problems. It may cost more, you also probably don't have to buy three to get one good one lol
     

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