Cooling system plumbing 1986 vs 1991 and newer Accords

Discussion in 'Accord' started by JP, Jan 14, 2009.

  1. JP

    JP Guest

    My son called the other day and was telling me about changing the thermostat
    in his 1991 Accord. He said this involved removing the lower radiator hose
    where it connected to the thermostat housing. I knew that on my 1986
    Accord, and on every other vehicle I have worked on that the thermostat is
    at the upper radiator hose, restricting the flow back to the radiator. This
    is the same as the schematic shown in the link from "how stuff works" below.

    I thought he was either mistaken or something was connected up wrong, but I
    looked in the 1991 service manual, and sure enough, the hose connections are
    the reverse of what I expected. I confirmed this by looking under the hood
    of a friend's 1991.

    This plumbing arrangement doesn't make sense to me. Surely, the water is
    still being pumped from the radiator bottom hose, but now flow is being
    restricted into the engine. It seems that this could result in cavitation
    or running the water pump dry.

    Is there an explanation as to why this would be a better than the old


    JP, Jan 14, 2009
  2. JP

    Tegger Guest

    That's an "ode sku" cooling system! My dad's '70 Ford had such, as did
    all of my previous cars.

    I do believe 1990 was the first year for no carburetors in North

    You'd think so, but there are bypass hoses.

    So long as the thermostat remains closed, the bypass hoses bathe the
    bottom of the thermostat with a steady flow of coolant recirculated from
    and to the block, so as the block warms, the thermostat opens. Once
    fully open, the thermostat blocks the bypass hoses, so all the flow is
    past the thermostat bulb.

    The heater hoses also bypass the thermostat, by the way.

    The change seems to have coincided with the advent of universal fuel
    injection and of more restrictive emissions controls. I'm guessing
    having the thermostat at the inlet end of the lower hose means better
    control over block temperature. Remember that excessive combustion
    temperatures result in excessive NO emissions. Better control over block
    temps mean less likelihood combustion temps will exceed 2,500F, which is
    NO territory.
    Tegger, Jan 15, 2009
  3. JP

    jim beam Guest

    tegger got it right. in the mean time, ask yourself whether it matters
    which end of a pipe you put the faucet.
    jim beam, Jan 16, 2009
  4. However, when the thermostat is in the upper hose it is responding to the
    cylinder head temperature, and the cylinder head is the main source of heat
    (especially when an aluminum head is mounted to a cast iron block). When
    mounted in the upper hose there is usually no flow past the thermostat, it
    just is bathed in the coolant of the hottest part of the engine.

    There has apparently been a design decision to heat the block rather than to
    leave it as part of the cooled fluid return path. I'd be curious to know why
    the change... possibly emission related or to manage the expansion of some
    or all the engine parts?

    Michael Pardee, Jan 16, 2009
  5. JP

    jim beam Guest

    except that there is no flow if the thermostat is closed, only
    convection. that leads to considerable local temperature differences
    within the head and block. keeping coolant actually flowing, as the honda
    design does, is a very smart solution.

    it is part of a flow path, right from start-up. tegger exactly describes
    the flow mechanism - the thermostat is opened "from behind" and mixes hot
    with cold, not just waits for the whole assembly to heat before
    arbitrarily getting hot, then circulating cold coolant, etc.
    thermal consistency. honda engineers were not dumb with this design. not
    by any stretch.
    jim beam, Jan 17, 2009
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