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Technical BBC With an odd overheat issue. Ideas ?

Discussion in 'The Hokey Ass Message Board' started by Mac_55, Sep 19, 2015.

  1. Mac_55
    Joined: Mar 10, 2008
    Posts: 688

    Mac_55
    Member

    Ok Fellas , heres the scoop . Looking for ideas on an odd overheating issue on a big block.
    396 +.060 topped with a 6-71 . This is the issue .

    When i first fire the car i let it warm up a little before i hit the road , it sits at 180 at idle . As soon as i take off headed down the road it will start to climb . I always stop and shut it off as soon as the needle hits 220-230 With the car off ,the gauge will climb quickly and hit 250 obviously with the hot engine and the coolant stationary. As soon as it hits 250 i start it and it will drop down to 175-185 and will not go any higher the entire time i drive it .
    It only does it when i let it cool off , if i pull into a gas station and just leave it off for 20 minutes or so , it wont do it . Ive replaced thermostat a few times , drilled holes in the t stat as well . The cooling system is up to snuff as well , the biggest Be Cool aluminum i could fit in a 55 chevy with a 7lb cap and twin 2500 CFM shrouded Pushers . The whole process " overheat, shutoff , restart and cool " only takes about 15 seconds , sometimes i just shut it off as im rolling down the highway , count to ten and restart it and like i said , it cools right off and wont act up again . Its not the gauge , i actually added a second gauge and put it out under the hood as well as the one in the car " Both Mechanical " . Im pulling one temp from the head and one from the back of the intake .

    My first thought was that it was possibly sucking the bottom hose closed on initial start and holding it , then when it warmed up and the system was pressurized " when i shut it off " the water pump could not collapse the hose due to the pressure , however i checked the lower hose and it is not collapsing . So im down to guessing that it is an airpocket possibly trapped somewhere in the blower intake or head . The intake has two large ports on the back that i did open to let bleed the air but there are no such ports at the front .
    Just figured i would post and see if anyone here has had a similiar problem . My next step is to buy one of those vacuum air lifts to pull air from the system and see if that works.

    Engine in question
     

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  2. henryj1951
    Joined: Sep 23, 2012
    Posts: 2,304

    henryj1951
    Member
    from USA

    nice car....brb...:cool:

    7# cap might need more
     
  3. mt shasta steve
    Joined: Mar 26, 2010
    Posts: 270

    mt shasta steve
    Member

    Every 1 lb pressure of the cap raises the boiling point 3 degrees. 7 lb cap raises boiling point 21 degrees. Plus coolant raising boiling point to about 250. My coupe hits 220 before the thermostat opens, then I can watch the gauge drop to 180 for the rest of the day. If your gauge is showing 220-230, I think you are safe.
     
  4. Mac_55
    Joined: Mar 10, 2008
    Posts: 688

    Mac_55
    Member

    Once it does it little overheat thing and i restart it , the normal operating temp drops to 175-185 which is actually on the cool side for a blown big block but im fine with that . I just did a little test , i took it out into the pasture and put it on the steepest grade i could and let it cool completely down , I started it and it didnt do its little issue . Thermostat opened and closed like it should . Maybe it pushed a bubble past.
     

  5. donno
    Joined: Feb 28, 2015
    Posts: 421

    donno
    ALLIANCE MEMBER

    It may have "burped" itself, but doubtful. Temp sensor should be as close to the thermosat as possible. ( Intake manifold). You can tap the housing and put a plug / petcock in it to bleed the system.
     
  6. Mac_55
    Joined: Mar 10, 2008
    Posts: 688

    Mac_55
    Member

    The problem with the blower intake is the thermostat sits low on the intake and if i had to pick a place for there to be an air pocket , id guess its in the cavity directly about the t stat . Think i may drill it right there and put a bleed in.
     
  7. blowby
    Joined: Dec 27, 2012
    Posts: 7,191

    blowby
    Member
    from Nicasio Ca

    Put a hand held laser temp gauge on it.
     
  8. stimpy
    Joined: Apr 16, 2006
    Posts: 3,547

    stimpy

    do you have a line to recirculate around the thermostat ? if you removed the heater hose the water could be stagnanting around the thermostat and not allowing it to get hot till its shut off and heat soaks , you should have 1 line that allows the water to circulate the crossover water otherwise the pump is just cavitating till the thermostat opens when it sits . and finally sees hot water . this is why they put a bypass hose on the water pump to the manifold the little 1/8 hole is not enough flow .
     
  9. 325w
    Joined: Feb 18, 2008
    Posts: 5,440

    325w
    ALLIANCE MEMBER

    Use a radiator pressure pump. Pump it up for awhile. If you drilled that 1/8 hole in the tstat it will bleed itself. The GM style bleeder valves can be bought from regular auto store thru the Dorman brand. Metric tap required.
     
  10. Mac_55
    Joined: Mar 10, 2008
    Posts: 688

    Mac_55
    Member

    No Heater , The Blower intake does not have the provision for the bypass.
     
  11. thirtytwo
    Joined: Dec 19, 2003
    Posts: 2,639

    thirtytwo
    Member

    Any coolant passage higher elevation than your fill cap?
     
  12. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,284

    Kerrynzl
    Member

    When the temp climbs because the engine is switched off, it is because there is no coolant circulation.
    If the block is partially filled with "concrete" you will see a more dramatic temp rise when the engine is switched off.
    Try an electric water-pump tee'd into the heads and the lower radiator.[Audi/VW make a small inline pump for their heaters]
     

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