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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


    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

    Attached Files:

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

    from USA

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

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

    mt shasta steve

    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


    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


    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


    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

    from Nicasio Ca

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


    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


    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


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


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


    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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