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.

http://www.howstuffworks.com/cooling-system.htm

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
plumbing?

TIA

John

 

"JP" <xjj_potterx@xhotmailx.com> wrote in
news:0dednTJ02uJp-vPUnZ2dnUVZ_vKdnZ2d@earthlink.com:

> 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.
>
> http://www.howstuffworks.com/cooling-system.htm
>



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



> 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.



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



>
> 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.



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.



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


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

The Unofficial Honda/Acura FAQ
www.tegger.com/hondafaq/

 

On Wed, 14 Jan 2009 14:32:19 -0800, JP wrote:

> 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.
>
> http://www.howstuffworks.com/cooling-system.htm
>
> 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
> plumbing?
>
> TIA
>
> John

tegger got it right. in the mean time, ask yourself whether it matters
which end of a pipe you put the faucet.

 

"jim beam" <spamvortex@bad.example.net> wrote in message
news:XsTbl.1776$ye3.603@fe12.news.easynews.com...
> On Wed, 14 Jan 2009 14:32:19 -0800, JP wrote:
>
>> 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.
>>
>> http://www.howstuffworks.com/cooling-system.htm
>>
>> 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
>> plumbing?
>>
>> TIA
>>
>> John
>
> tegger got it right. in the mean time, ask yourself whether it matters
> which end of a pipe you put the faucet.
>

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?

Mike

 

On Fri, 16 Jan 2009 05:23:01 -0700, Michael Pardee wrote:

> "jim beam" <spamvortex@bad.example.net> wrote in message
> news:XsTbl.1776$ye3.603@fe12.news.easynews.com...
>> On Wed, 14 Jan 2009 14:32:19 -0800, JP wrote:
>>
>>> 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.
>>>
>>> http://www.howstuffworks.com/cooling-system.htm
>>>
>>> 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
>>> plumbing?
>>>
>>> TIA
>>>
>>> John
>>
>> tegger got it right. in the mean time, ask yourself whether it matters
>> which end of a pipe you put the faucet.
>>
>>
> 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).

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.



> 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.

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.

> I'd be curious
> to know why the change... possibly emission related or to manage the
> expansion of some or all the engine parts?
>
> Mike

thermal consistency. honda engineers were not dumb with this design. not
by any stretch.