Yes, I just did a rebuild on a 1993 - 3 cylinder 90 , and it too used the
poppet valve approach. A lot of the older stuff I work on has no thermostat
or poppet. The cooling flow is the cooling flow, and at high rpm's the pump
just can't push more water than the block will pass.

On my late 80's 115, the water comes in, fills the exhaust plates and then
spills over into the top of the block area. The block "top fills" from the
bottom up. When the water gets to the top of #1, the peeing starts.
Amazingly, this takes only about 10 seconds or less.

As a quick side note, I advise anyone running an earlier Merc inline 6 to
plug off the hole where the pee hose goes (with a plastic plug) and to move
the elbow fitting to the very top of the block over #1.

Mercury *finally* did this very late in production. It cures two problems:
It becomes impossible to get an air pocket up in the top, and you won't get
a "pee reading" unless the block is truely full to the very top.

-W

"Ken" wrote in message
...
For a pressure controlled system Merc uses a spring loaded poppet valve.
When pump pressure comes up the poppet is pushed open and allows for
greater
water flow. The thermo itself just doesn't flow enough water to keep
things
cool at WOT.

Unless I've overlooked someone else mentioning it in the thread here, I'll
say a water pressure gauge is a good idea. There is a spec on the pressure
you should have at high rpm's and as the impeller wears this will drop off.
I've noticed on my '88 200hp merc that, after cranking, it takes a minute or
two to see a stream out the back, but the wp shows maybe 10psi, at idle,
within 10 seconds after cranking--long before I see water out the tel-tale.
I put the fitting for the wp at the top of the block where there was a plug.
If you do look at the gauge it can be valuable in saving your engine. I've
gotten in the habit of looking at that gauge always after cranking. If I
don't see pressure after 10-20 seconds, I'll shut it down and find out why.
LD

"Ken" wrote in message
...
For a pressure controlled system Merc uses a spring loaded poppet valve.
When pump pressure comes up the poppet is pushed open and allows for
greater
water flow. The thermo itself just doesn't flow enough water to keep
things
cool at WOT.

If I remember right, the poppet is located in the same housing as the
thermo
on your engine (it has been a while since I've seen a 75 Merc and at my
age
memory is not always my friend). The other place Merc puts the poppet is
at
the bottom of the block, stbd side, just forward of the head.

I agree with you, the setup has always struck me as unusual, but then
again
it does work well.

Ken

"Andy C" wrote in message
...
thanks ken,
yes, the hose is on the back of the thermostat housing as described, so
i
guess this is normal... i thought there would be some sort of bypass to
keep
the telltale going all the time the engine is running
it just seems like a strange set up... presumably in very cold water the
thermostat could open and close constantly and you would not be able to
tell
if the water pump was working or not...

It should, though, discharge
some when you rev them while cold. The system is also pressure based
which
should override the thermostat system.

i don't quite follow that, are you saying the water pressure will force
the
thermostat open above idle ?

thanks !
andy

"Ken" wrote in message
...
A lots of Mercs and Mercury-Force place the tattle-tale after the
thermostat. You can actually trace the hose to the back side of the
thermostat housing. With those setups the thermostat does have to open
before it will discharge a stream while idling. It should, though,
discharge
some when you rev them while cold. The system is also pressure based
which
should override the thermostat system.



"Andy C" wrote in message
...
mercury force '97 75hp
hi there,
does anyone know if it is normal for there to be no flow from the
tell-tale
until the thermostat opens ?
there doesn't seem to be any cooling problems but it seems weird for
nothing
to come out the tell-tale for the first 2 or 3 minutes... the local
dealer
thought this was normal but didn't seem sure...
many thanks for any comments !
andy