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Overheating SB400

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by mountainexplorer, Jun 20, 2003.

  1. mountainexplorer

    mountainexplorer 1/2 ton status

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    I just recently got my friend's Blazer through a trade. I mainly got it for the new SB400 with only about 2-3 hours of run time on it and hardly any miles.

    They couldn't run it for very long at a time since it would always overheat. The 1st intake they had on it was a warped aluminum Edelbrock performer, now it has a stock style aluminum intake. It's still heating up too much.

    Within a couple minutes of starting it from being cold, the water is just about boiling (sounds like it).

    It is .040 over with flattops, has a brand new radiator. But I'm sure the timing is off and someone said there must be a vacuum leak.

    What else could make it heat up so badly?
     
  2. bigblock44k5

    bigblock44k5 1/2 ton status

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    sbc 400's are different than other chevy's as in they have small steam holes in the block and the cylinder heads in between the cylinder bores. they probably arent 400 heads and dont have the steam holes. ask the guy you got off of if they are 400 heads. if not, you have to take the heads off, and using a 400 head gasket as a template, mark and drill the heads into the water jacket. this is common to people rebuilding 400's for the first time.
     
  3. mountainexplorer

    mountainexplorer 1/2 ton status

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    They should be the original 400 heads. He got the motor as a complete longblock from a reputable local engine remanufacturer. But that doesnt mean they can't make a mistake.

    What I am going to do is pull the motor out, and start over again as a longblock. Then I can check the casting numbers on the heads to verify they are 400 heads, and hopefully discover the problem (whatever it is).
     
  4. BorregoK5

    BorregoK5 1/2 ton status

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    Could be as simple as a clogged radiator or stuck thermostat. My 400 was pretty demanding of the cooling system before I swapped on a new 4 core and electric fan, now its no different than any 350 I've ever had. The steam holes are a likely suspect as well and a common oversight with engine rebuilders. Timing can cause an engine to heat up fast, 400's are generally set at a static of 2 degrees BTDC. A lean condition can also cause you to overheat but more common when driving than idling. When its cold, pull the radiator cap and see if your bubbling or building pressure at a cold startup, could signal a leaking head gastket into the cooling system. I just picked up another 400, love these engines!
     
  5. Mreetz

    Mreetz 1/2 ton status

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    I'm going to a 400, have a 4 core radiator, going to 2-12" electric fans and adding an oil cooler! /forums/images/graemlins/grin.gif
     
  6. imiceman44

    imiceman44 1 ton status

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    By the way you do know that the 400 is already prone to overheating.
    You will probably find a problem and fix it and be able to drive it but remember to provide ample cooling for it later.
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    Check into the easiest stuff, namely, thermostat and radiator.

    A marginal radiator will cool a smaller engine, but a larger engine (more heat) will max out the same radiator. BTDT, had a 403 that I couldn't keep cool. Pulled the radiator, had it flow tested, sure enough, it was flowing half what it should have been. Had it re-cored, problem gone. (It was an expensive radiator, thats why I didn't just replace it)

    Thermostat you can just pull and try without, (I've done it myself, if the radiator is good, the engine will NOT overheat with no t-stat) but from personal experience, the ONE time I had a bad thermostat, it was sticky, and the temperature change was drastic, not constant.
     
  8. HarryH3

    HarryH3 1 ton status Author

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    Make sure that the intake gaskets are installed correctly. If they're backwards, water can't flow to the water pump. /forums/images/graemlins/frown.gif It's pretty easy to check without taking anything apart. Remove the thermostat housing and thermostat, then snake a piece of wire towards each head from the t-stat opening. If it stops at the edge of the head, it's hitting the block-off plate in the gasket, that should be be at the rear of the engine, instead of the front. /forums/images/graemlins/eek.gif
     
  9. Thunder

    Thunder 3/4 ton status

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    It sounds like you are getting no coolant flow at all.
    Besides the checking thermostat and the intake gaskets as said above. Also check the water pump. They may have accidently instaled a newer type reverse rotation serpintine belt type pump. They wont work with regular belts.
     
  10. mountainexplorer

    mountainexplorer 1/2 ton status

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    Well, I started it again this weekend and it still gets way too hot within under 5 minutes from completely cold.

    I do know the following:

    -The radiator is brand new.
    -The fan clutch is wearing out
    -The water pump is good and the proper one (off the old 400)
    -The carburetor is bad, but I can get it to run with it choked 3/4 of the way.
    -Timing hasnt been checked with a light, but it starts ok, and shuts down fine... still, the timing is not dead on.
    -Theres an exhaust leak on the drivers bank header

    It still shouldnt heat up as fast as it does (with no cap on the radiator).

    Other things I have left to check:
    -Intake gaskets
    -Thermostat
    -Timing
    -Heads and head gaskets

    I'm starting to think they goofed on the head gaskets.
     
  11. RustBuket

    RustBuket 1/2 ton status

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    I think the head gaskets or the thermometer are at fault.
     

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