Mercury Outboard Overheating
 

My 1995 Mercury 135HP 2 StrokeOutboard has decided to overheat whenever the
RPMS are around 1200-2000. Either side it cools off but in this range it
warms up fast. The water pump is working because it streams water ok.
Nothing seems to be bloking the intake.

I have only had the boat for a short while. The previous owner barely put 20
hours on it in 2 years. It was recently serviced by the previous owner as
well.

Any ideas?

Any way I can get hold of a workshop manual in digital format to pull down
and check the water pump?

Regards,

Mercury Outboard Overheating
 

Running lean?
You should probably throw an impeller at it if you don't know the status of the
one you have. There are 5 bolts right near the cavitation plate, 4 down and one
up from the bottom inthe back. That drops the foot. Then the impeller is in a
small can around the shaft.

Mercury Outboard Overheating
 

Ya, I'd suspect a carb thing leaning it out.

-W

"Greg" wrote in message
...
Running lean?
You should probably throw an impeller at it if you don't know the status
of the
one you have. There are 5 bolts right near the cavitation plate, 4 down
and one
up from the bottom inthe back. That drops the foot. Then the impeller is
in a
small can around the shaft.

Mercury Outboard Overheating
 

StrokeOutboard has decided to overheat whenever the
RPMS are around 1200-2000. Either side it cools off but in this range it
warms up fast. The water pump is working because it streams water ok.
Nothing seems to be bloking the intake.

When you say overheating, do you mean the audible alarm comes on? Do you have a
guage that reads hot?
Does it have a water pressure guage? if so what is the pressure at this rpm?
The impellor should be changed yearly, so that a logical place to start.

More questions. What hull is the motor on?
I assume your hull is "mushing' at that speed. Not on plane, just moving a lot
of water.
Some boats were known to get hot running at those rpm's. The hull diverts the
water creating an air pocket allowing temps to rise.
You might try changing the trim angle and see what happens.

BR

Mercury Outboard Overheating
 

Listen to Clams--sounds lean in mixture to me.

Mercury Outboard Overheating
 

I drop the foot and check out the impeller as well as get the mixture looked
at.


"Greg" wrote in message
...
Running lean?
You should probably throw an impeller at it if you don't know the status
of the
one you have. There are 5 bolts right near the cavitation plate, 4 down
and one
up from the bottom inthe back. That drops the foot. Then the impeller is
in a
small can around the shaft.

Mercury Outboard Overheating
 

The temp guage goes way up into red. No alarm yet, I usually alter speed
before it gets too hot.

The funny thing is the range that it heats up on.

Just above idle its fine. A little faster and it heats up quick, then after
that if you get the speed up it cools off to normal. Thinking about it, what
could be happening is that at higher speed the ram effect is forcing water
thru the cooling system. Slow down and it relies on the pump which is not
performing optimally.

Regards,

"Boatriggr" wrote in message
...
StrokeOutboard has decided to overheat whenever the
RPMS are around 1200-2000. Either side it cools off but in this range it
warms up fast. The water pump is working because it streams water ok.
Nothing seems to be bloking the intake.

When you say overheating, do you mean the audible alarm comes on? Do you
have a
guage that reads hot?
Does it have a water pressure guage? if so what is the pressure at this
rpm?
The impellor should be changed yearly, so that a logical place to start.

More questions. What hull is the motor on?
I assume your hull is "mushing' at that speed. Not on plane, just moving a
lot
of water.
Some boats were known to get hot running at those rpm's. The hull diverts
the
water creating an air pocket allowing temps to rise.
You might try changing the trim angle and see what happens.

BR

Mercury Outboard Overheating
 

That is why we all suggest thrwing an impeller at it. If this is new to you it
should be done anyway, just so you know where you are at but it might be your
whole problem. I have an example in my garage of a bad impeller that looks
perfect. The hub spins and that was RPM sensitive. It drove me crazy until I
just made up my mind to replace it.
A thermostat is also not a bad idea. They are cheap.
One thing you could try is have someone feel the discharge water when it says
it is hot. If the water is "MacDonalds coffee hot" you are overloading the
cooling system. (not enough water pumped, lean mix etc) If the water is cool
you probably have a sluggish thermostat. That is not likely if the engine runs
out good and then fails when you slow down. If it is a morning sickness problem
the thermostat is suspect.

Mercury Outboard Overheating
 

Yeah I guess you are right, how do I get to the thermostat to check it?

Also is Mercury AF or Metric, in Australia I am used to metric? Dont want to
strip bolt heads.

I will drop the lower leg and chech the pump. One problem I foresee though,
is to get into the small hole above the trim tab to loosen the bolt, it is
damn deep and I am going to need some kind of thin extended socket to get to
it.

Thanks and regards,


"Greg" wrote in message
...
That is why we all suggest thrwing an impeller at it. If this is new to
you it
should be done anyway, just so you know where you are at but it might be
your
whole problem. I have an example in my garage of a bad impeller that looks
perfect. The hub spins and that was RPM sensitive. It drove me crazy until
I
just made up my mind to replace it.
A thermostat is also not a bad idea. They are cheap.
One thing you could try is have someone feel the discharge water when it
says
it is hot. If the water is "MacDonalds coffee hot" you are overloading the
cooling system. (not enough water pumped, lean mix etc) If the water is
cool
you probably have a sluggish thermostat. That is not likely if the engine
runs
out good and then fails when you slow down. If it is a morning sickness
problem
the thermostat is suspect.

Mercury Outboard Overheating
 

The foot bolts are American SAE, The powerhead may be metric if it is recent.
(They are swapping stuff with Yamaha)
I think the thermostat housing bolts are 10mm

Mercury Outboard Overheating
 

"Chunky" wrote in message
u...
My 1995 Mercury 135HP 2 StrokeOutboard has decided to overheat whenever
the
RPMS are around 1200-2000. Either side it cools off but in this range it
warms up fast. The water pump is working because it streams water ok.
Nothing seems to be bloking the intake.




Assuming that the thermostats are operational and the pressure relief valve
on the side is not sticking, then you may need the heavy duty impeller that
Merc sells, but it is not the one included in their water pump kits. The
following is an excerpt from a friend who "chased" a similar problem on his
V6 Merc for quite a while.

"As for my intermittent overheating problem, we finally cured it -- but I
had
to call an engineer at Mercury for the magic bullet. It was my starboard
engine that was overheating at displacement speeds
(approx. 1200-1800 rpm),

The Merc guy said to substitute what he called the "standard" nylon-hub
water pump impeller (Merc #47-89984T-3) that came with the rebuild kit
(#46-96148A) with the "heavy duty" brass-hub impeller (#47-89984T-4). It's
the same impeller, just different hub materials. He couldn't explain why,
but said it usually works with problems like I was having. Son of a gun, he
was right. I switched them out and the problem is gone. Hallelujah."

Hope that this could be of help to you.

Bill Grannis
service manager

Mercury Outboard Overheating
 

The Merc guy said to substitute what he called the "standard" nylon-hub
water pump impeller (Merc #47-89984T-3) that came with the rebuild kit
(#46-96148A) with the "heavy duty" brass-hub impeller (#47-89984T-4). It's
the same impeller, just different hub materials. He couldn't explain why,
but said it usually works with problems like I was having. Son of a gun, he
was right. I switched them out and the problem is gone. Hallelujah."

Hope that this could be of help to you.

Bill Grannis
service manager

Bill, just a warning, The metal hub impeller was the one I had that was
spinning. It is sourced from Sierra and they have (had?) a problem with the
bonding of the brass to rubber. It was RPM sensitive but I replaced it before I
had a well developed pattern. It wasn't until I actually had it in my hand that
I found the spun hub.

Mercury Outboard Overheating
 

Well I changed the impeller although the old one was in good shape anyway.
Both have a brass hub.

Checked the thermostats and the pressure relief valve and they are seem
fine.

Ran the boat again today again same thing. Idle no problem. Bring up a few
revs to get 5 or 6 knots and the temp goes to the red zone. Open the
throttle a little more and still the same until the RPM is about 2000 and
the temp drops. Crank it up to 20 knots + and the temp drops to 150F and all
is well.

One thing I noticed is that the tell tale is more of a dribble now at low
speeds, and hot.



"Billgran" wrote in message
. ..

"Chunky" wrote in message
u...
My 1995 Mercury 135HP 2 StrokeOutboard has decided to overheat whenever
the
RPMS are around 1200-2000. Either side it cools off but in this range it
warms up fast. The water pump is working because it streams water ok.
Nothing seems to be bloking the intake.




Assuming that the thermostats are operational and the pressure relief
valve
on the side is not sticking, then you may need the heavy duty impeller
that
Merc sells, but it is not the one included in their water pump kits. The
following is an excerpt from a friend who "chased" a similar problem on
his
V6 Merc for quite a while.

"As for my intermittent overheating problem, we finally cured it -- but I
had
to call an engineer at Mercury for the magic bullet. It was my starboard
engine that was overheating at displacement speeds
(approx. 1200-1800 rpm),

The Merc guy said to substitute what he called the "standard" nylon-hub
water pump impeller (Merc #47-89984T-3) that came with the rebuild kit
(#46-96148A) with the "heavy duty" brass-hub impeller (#47-89984T-4). It's
the same impeller, just different hub materials. He couldn't explain why,
but said it usually works with problems like I was having. Son of a gun,
he
was right. I switched them out and the problem is gone. Hallelujah."

Hope that this could be of help to you.

Bill Grannis
service manager

Mercury Outboard Overheating
 

Are you sure your impellor is spinning?

Alan

"Chunky" wrote in message
u...
Well I changed the impeller although the old one was in good shape anyway.
Both have a brass hub.

Checked the thermostats and the pressure relief valve and they are seem
fine.

Ran the boat again today again same thing. Idle no problem. Bring up a few
revs to get 5 or 6 knots and the temp goes to the red zone. Open the
throttle a little more and still the same until the RPM is about 2000 and
the temp drops. Crank it up to 20 knots + and the temp drops to 150F and
all
is well.

One thing I noticed is that the tell tale is more of a dribble now at low
speeds, and hot.



"Billgran" wrote in message
. ..

"Chunky" wrote in message
u...
My 1995 Mercury 135HP 2 StrokeOutboard has decided to overheat
whenever
the
RPMS are around 1200-2000. Either side it cools off but in this range
it
warms up fast. The water pump is working because it streams water ok.
Nothing seems to be bloking the intake.




Assuming that the thermostats are operational and the pressure relief
valve
on the side is not sticking, then you may need the heavy duty impeller
that
Merc sells, but it is not the one included in their water pump kits. The
following is an excerpt from a friend who "chased" a similar problem on
his
V6 Merc for quite a while.

"As for my intermittent overheating problem, we finally cured it -- but
I
had
to call an engineer at Mercury for the magic bullet. It was my starboard
engine that was overheating at displacement speeds
(approx. 1200-1800 rpm),

The Merc guy said to substitute what he called the "standard" nylon-hub
water pump impeller (Merc #47-89984T-3) that came with the rebuild kit
(#46-96148A) with the "heavy duty" brass-hub impeller (#47-89984T-4).
It's
the same impeller, just different hub materials. He couldn't explain
why,
but said it usually works with problems like I was having. Son of a gun,
he
was right. I switched them out and the problem is gone. Hallelujah."

Hope that this could be of help to you.

Bill Grannis
service manager

Mercury Outboard Overheating
 

Yes definitly spinning


"Grumpy" wrote in message
u...
Are you sure your impellor is spinning?

Alan

"Chunky" wrote in message
u...
Well I changed the impeller although the old one was in good shape
anyway.
Both have a brass hub.

Checked the thermostats and the pressure relief valve and they are seem
fine.

Ran the boat again today again same thing. Idle no problem. Bring up a
few
revs to get 5 or 6 knots and the temp goes to the red zone. Open the
throttle a little more and still the same until the RPM is about 2000
and
the temp drops. Crank it up to 20 knots + and the temp drops to 150F and
all
is well.

One thing I noticed is that the tell tale is more of a dribble now at
low
speeds, and hot.



"Billgran" wrote in message
. ..

"Chunky" wrote in message
u...
My 1995 Mercury 135HP 2 StrokeOutboard has decided to overheat
whenever
the
RPMS are around 1200-2000. Either side it cools off but in this
range
it
warms up fast. The water pump is working because it streams water
ok.
Nothing seems to be bloking the intake.




Assuming that the thermostats are operational and the pressure relief
valve
on the side is not sticking, then you may need the heavy duty impeller
that
Merc sells, but it is not the one included in their water pump kits.
The
following is an excerpt from a friend who "chased" a similar problem
on
his
V6 Merc for quite a while.

"As for my intermittent overheating problem, we finally cured it --
but
I
had
to call an engineer at Mercury for the magic bullet. It was my
starboard
engine that was overheating at displacement speeds
(approx. 1200-1800 rpm),

The Merc guy said to substitute what he called the "standard"
nylon-hub
water pump impeller (Merc #47-89984T-3) that came with the rebuild kit
(#46-96148A) with the "heavy duty" brass-hub impeller (#47-89984T-4).
It's
the same impeller, just different hub materials. He couldn't explain
why,
but said it usually works with problems like I was having. Son of a
gun,
he
was right. I switched them out and the problem is gone. Hallelujah."

Hope that this could be of help to you.

Bill Grannis
service manager

Mercury Outboard Overheating
 

I have the same problem on a merc 225. Goes up to 175 deg at 1200 rpm
(about 6-8 knots) and is o.k. above and below that rpm. I suspect the
bypass (relief) valve is not opening soon enough. I never get into the red
zone and never set off the audible overtemp alarm so I have been living with
it for 10 years with no problem.

I have been tempted to drill a hole thru the bypass disc but haven't done it
yet. Might make the engine idle too cold.

A lot of people without temp gauges probably have the same problem and don't
even know it.

"Chunky" wrote in message
u...
Well I changed the impeller although the old one was in good shape anyway.
Both have a brass hub.

Checked the thermostats and the pressure relief valve and they are seem
fine.

Ran the boat again today again same thing. Idle no problem. Bring up a few
revs to get 5 or 6 knots and the temp goes to the red zone. Open the
throttle a little more and still the same until the RPM is about 2000 and
the temp drops. Crank it up to 20 knots + and the temp drops to 150F and
all
is well.

One thing I noticed is that the tell tale is more of a dribble now at low
speeds, and hot.



"Billgran" wrote in message
. ..

"Chunky" wrote in message
u...
My 1995 Mercury 135HP 2 StrokeOutboard has decided to overheat
whenever
the
RPMS are around 1200-2000. Either side it cools off but in this range
it
warms up fast. The water pump is working because it streams water ok.
Nothing seems to be bloking the intake.




Assuming that the thermostats are operational and the pressure relief
valve
on the side is not sticking, then you may need the heavy duty impeller
that
Merc sells, but it is not the one included in their water pump kits. The
following is an excerpt from a friend who "chased" a similar problem on
his
V6 Merc for quite a while.

"As for my intermittent overheating problem, we finally cured it -- but
I
had
to call an engineer at Mercury for the magic bullet. It was my starboard
engine that was overheating at displacement speeds
(approx. 1200-1800 rpm),

The Merc guy said to substitute what he called the "standard" nylon-hub
water pump impeller (Merc #47-89984T-3) that came with the rebuild kit
(#46-96148A) with the "heavy duty" brass-hub impeller (#47-89984T-4).
It's
the same impeller, just different hub materials. He couldn't explain
why,
but said it usually works with problems like I was having. Son of a gun,
he
was right. I switched them out and the problem is gone. Hallelujah."

Hope that this could be of help to you.

Bill Grannis
service manager

Mercury Outboard Overheating
 

I get up close to 180+ but never tripped the alarm so far.

I have a suspicion the popper or bypass as you call it is the problem. The
spring is too strong and not opening soon enough.

If you left it out I guess it would make the motor run too cool at low RPMs.

Regards,

"Bill Sc" wrote in message
. ..

I have the same problem on a merc 225. Goes up to 175 deg at 1200 rpm
(about 6-8 knots) and is o.k. above and below that rpm. I suspect the
bypass (relief) valve is not opening soon enough. I never get into the
red
zone and never set off the audible overtemp alarm so I have been living
with
it for 10 years with no problem.

I have been tempted to drill a hole thru the bypass disc but haven't done
it
yet. Might make the engine idle too cold.

A lot of people without temp gauges probably have the same problem and
don't
even know it.

"Chunky" wrote in message
u...
Well I changed the impeller although the old one was in good shape
anyway.
Both have a brass hub.

Checked the thermostats and the pressure relief valve and they are seem
fine.

Ran the boat again today again same thing. Idle no problem. Bring up a
few
revs to get 5 or 6 knots and the temp goes to the red zone. Open the
throttle a little more and still the same until the RPM is about 2000
and
the temp drops. Crank it up to 20 knots + and the temp drops to 150F and
all
is well.

One thing I noticed is that the tell tale is more of a dribble now at
low
speeds, and hot.



"Billgran" wrote in message
. ..

"Chunky" wrote in message
u...
My 1995 Mercury 135HP 2 StrokeOutboard has decided to overheat
whenever
the
RPMS are around 1200-2000. Either side it cools off but in this
range
it
warms up fast. The water pump is working because it streams water
ok.
Nothing seems to be bloking the intake.




Assuming that the thermostats are operational and the pressure relief
valve
on the side is not sticking, then you may need the heavy duty impeller
that
Merc sells, but it is not the one included in their water pump kits.
The
following is an excerpt from a friend who "chased" a similar problem
on
his
V6 Merc for quite a while.

"As for my intermittent overheating problem, we finally cured it --
but
I
had
to call an engineer at Mercury for the magic bullet. It was my
starboard
engine that was overheating at displacement speeds
(approx. 1200-1800 rpm),

The Merc guy said to substitute what he called the "standard"
nylon-hub
water pump impeller (Merc #47-89984T-3) that came with the rebuild kit
(#46-96148A) with the "heavy duty" brass-hub impeller (#47-89984T-4).
It's
the same impeller, just different hub materials. He couldn't explain
why,
but said it usually works with problems like I was having. Son of a
gun,
he
was right. I switched them out and the problem is gone. Hallelujah."

Hope that this could be of help to you.

Bill Grannis
service manager

Mercury Outboard Overheating
 

Well I changed the impeller although the old one was in good shape anyway.
Both have a brass hub.

Checked the thermostats and the pressure relief valve and they are seem
fine.

Ran the boat again today again same thing. Idle no problem. Bring up a few
revs to get 5 or 6 knots and the temp goes to the red zone. Open the
throttle a little more and still the same until the RPM is about 2000 and
the temp drops. Crank it up to 20 knots + and the temp drops to 150F and all
is well.

One thing I noticed is that the tell tale is more of a dribble now at low
speeds, and hot.



Did you ever reply as to what hull it is on?
Did you try changing the trim angle?

A cooling system can be tricky as there a a few instances where removing the
powerhead becomes necessary to check some things.
If you are not sure of what is going on it sounds like time to take it to
someone with experience.

BR

Mercury Outboard Overheating
 

yeah i spoke to a couple of mechanics. One suggested drilling a 1/8" hole in
each of the thermostats to allow some extra water to run thru the system
until the poppet opens at 2000RPM. I have drilled a couple of holes as
described and will put it in the water tomorrow. It makes a lot of sense
because whenever the motor gets hot all i do is open the throttle and the
temp drops like a rock.

Regards,

Kind of makes sense
"Boatriggr" wrote in message
...

Well I changed the impeller although the old one was in good shape
anyway.
Both have a brass hub.

Checked the thermostats and the pressure relief valve and they are seem
fine.

Ran the boat again today again same thing. Idle no problem. Bring up a
few
revs to get 5 or 6 knots and the temp goes to the red zone. Open the
throttle a little more and still the same until the RPM is about 2000 and
the temp drops. Crank it up to 20 knots + and the temp drops to 150F and
all
is well.

One thing I noticed is that the tell tale is more of a dribble now at low
speeds, and hot.



Did you ever reply as to what hull it is on?
Did you try changing the trim angle?

A cooling system can be tricky as there a a few instances where removing
the
powerhead becomes necessary to check some things.
If you are not sure of what is going on it sounds like time to take it to
someone with experience.

BR

Mercury Outboard Overheating
 

Well the saga continues:-

Drilled the holes, no difference.

Took out the thermostats completely, still no difference.

The mechanics I spoke to were stumped. Now I was getting really suspicious
of the whole overheating thing. Bought a multimeter and a digital
thermometer. I noticed while using the multimeter that one of the
rectifer/regulators (what takes care of the power coming off the alternater
to charge batteries, ignition, accessaries etc.) was looking scorched. I
examined it further and realised it was burnt out. So the electric system
was limping along on 20 amps instead of 40. Ordered a new regulator.

I also checked the temperature sender with the multimeter to see how it
registered. I checked the ohms it out put along with the table that converts
this to temperature. I also put the sender into very hot water along with
the digital thermometer. The temperature guage in the cockpit matched the
sender and the digital thermometer. Hmmm! So the guage was right and the
sender.

I then glued the digital thermometer between the cylinder head and exhaust
manifold near the send and took the boat out. The temp guage worked up to
200+ °F and then I checked the digital thermometer which showed 171 °F.
Tried a few times heating up the engine and checking the temp, 167, 156 etc.
So the guage reads high by 30-40° but it checked out ok when I compared it
on the beach.

I now suspect the loss of one regulator is causing the guage to read high
when the engine is running. When the engine is running fast the temp appears
to go down when in actual fact the motor is simply generating more power.



"Chunky" wrote in message
u...
yeah i spoke to a couple of mechanics. One suggested drilling a 1/8" hole
in
each of the thermostats to allow some extra water to run thru the system
until the poppet opens at 2000RPM. I have drilled a couple of holes as
described and will put it in the water tomorrow. It makes a lot of sense
because whenever the motor gets hot all i do is open the throttle and the
temp drops like a rock.

Regards,

Kind of makes sense
"Boatriggr" wrote in message
...

Well I changed the impeller although the old one was in good shape
anyway.
Both have a brass hub.

Checked the thermostats and the pressure relief valve and they are seem
fine.

Ran the boat again today again same thing. Idle no problem. Bring up a
few
revs to get 5 or 6 knots and the temp goes to the red zone. Open the
throttle a little more and still the same until the RPM is about 2000
and
the temp drops. Crank it up to 20 knots + and the temp drops to 150F
and
all
is well.

One thing I noticed is that the tell tale is more of a dribble now at
low
speeds, and hot.



Did you ever reply as to what hull it is on?
Did you try changing the trim angle?

A cooling system can be tricky as there a a few instances where removing
the
powerhead becomes necessary to check some things.
If you are not sure of what is going on it sounds like time to take it
to
someone with experience.

BR

Mercury Outboard Overheating
 

I am following this closely as I have the identical problem. Please post
results when you install the new rectifier.



"Chunky" wrote in message
u...
Well the saga continues:-

Drilled the holes, no difference.

Took out the thermostats completely, still no difference.

The mechanics I spoke to were stumped. Now I was getting really suspicious
of the whole overheating thing. Bought a multimeter and a digital
thermometer. I noticed while using the multimeter that one of the
rectifer/regulators (what takes care of the power coming off the
alternater
to charge batteries, ignition, accessaries etc.) was looking scorched. I
examined it further and realised it was burnt out. So the electric system
was limping along on 20 amps instead of 40. Ordered a new regulator.

I also checked the temperature sender with the multimeter to see how it
registered. I checked the ohms it out put along with the table that
converts
this to temperature. I also put the sender into very hot water along with
the digital thermometer. The temperature guage in the cockpit matched the
sender and the digital thermometer. Hmmm! So the guage was right and the
sender.

I then glued the digital thermometer between the cylinder head and exhaust
manifold near the send and took the boat out. The temp guage worked up to
200+ °F and then I checked the digital thermometer which showed 171 °F.
Tried a few times heating up the engine and checking the temp, 167, 156
etc.
So the guage reads high by 30-40° but it checked out ok when I compared it
on the beach.

I now suspect the loss of one regulator is causing the guage to read high
when the engine is running. When the engine is running fast the temp
appears
to go down when in actual fact the motor is simply generating more power.



"Chunky" wrote in message
u...
yeah i spoke to a couple of mechanics. One suggested drilling a 1/8"
hole
in
each of the thermostats to allow some extra water to run thru the system
until the poppet opens at 2000RPM. I have drilled a couple of holes as
described and will put it in the water tomorrow. It makes a lot of sense
because whenever the motor gets hot all i do is open the throttle and
the
temp drops like a rock.

Regards,

Kind of makes sense
"Boatriggr" wrote in message
...

Well I changed the impeller although the old one was in good shape
anyway.
Both have a brass hub.

Checked the thermostats and the pressure relief valve and they are
seem
fine.

Ran the boat again today again same thing. Idle no problem. Bring up
a
few
revs to get 5 or 6 knots and the temp goes to the red zone. Open the
throttle a little more and still the same until the RPM is about 2000
and
the temp drops. Crank it up to 20 knots + and the temp drops to 150F
and
all
is well.

One thing I noticed is that the tell tale is more of a dribble now at
low
speeds, and hot.



Did you ever reply as to what hull it is on?
Did you try changing the trim angle?

A cooling system can be tricky as there a a few instances where
removing
the
powerhead becomes necessary to check some things.
If you are not sure of what is going on it sounds like time to take it
to
someone with experience.

BR