470 Overheat
 

I tried just about everything...

My boat runs great except a very slow heat up after 3200 RPMs until boil over.

I replaced:

1) Thermostat
2) Water pump and housing (twice to be sure)
3) Removed the 3" heat exchanger and replaced with 4"
4) Flushed out the seawater circuit
5) Removed the riser and flushed and cleaned

I installed a new 160 degree thermostat. For some reason, my temp will stay at 120 at rest and increase past 200 very slowly at 3200 rpms

Can the thermostat be installed up side down? I have the springs facing down. One forum I read states the sealoc manual shows this install incorrect.

My only other option is head gasket?

Any thoughts?

do a compression test and look closely at the plugs, find the one that looks brand new. make sure you don't have old impeller pieces stuck in the power steering cooler

Impeller parts
 

I don't have power steering and I removed the voltage regulator as I have the upgrade alternator kit. My seawater circuit is very streamline and I back-flushed the circuit.

It has to be a head gasket or riser? My mechanic told me you sometimes can't tell if the riser is bad just by running water through it.

I've had a brand new thermostat not open up fully causing an engine to run slightly hotter. Replaced it and everything was good to go.

what brand of water pump did you use, I've been burned before using aftermarket pumps. you can plumb in a garden hose y valve between the heat exchanger and the riser, run another hose off the y valve to dump overboard, while running, crack the valve leading to the over board discharge, if the temp starts going down, you have a restricted riser. Be careful, don't starve the exhaust of water or you'll burn the hose

Pump
 

I used Mercruiser parts.

On the muffs, I can't get the temp gauge over 120.

In the water, temp gauge will stay at 120 at idle.

At 2800 RPMs, I can run all day around 170 degrees.

As soon as I bump the throttle to 3200, the temp will slowly rise to 200.

If I open it up, it will quickly jump past 200 and boil.

My recovery tank makes all kinds of noise...I am not sure if it's air or boiling coolant?

If it's air, then I know it's the head gasket.

My research tells me you can't find a small leak with a compression test because it's not enough pressure at low RPM's to find. Also, a leak down test may not work either.

I don't want to pull the head and find it's something else.

I do notice the riser gets hot as well as the heat exchanger. This still points to seawater not flowing fast enough?

It should be running hotter than 120 at idle and lower rpm. It sounds like a thermostat issue

Thermostat Issue
 

Does anybody know which way the thermostat drops in?

I have the older 470 with iron manifolds. I installed the thermostat with the springs down.

If the thermostat is upside down, I am assuming it will cause a high speed overheat.

Thanks for your help...

The spring should be facing the heated water.

A simple test to see if your t-stat is working properly: Take your stat and put it into a saucepan full of water. Bring the water to a boil. Remove the stat with a pair of tongs and look at it. Make sure it's fully open. Then, put a piece of string into the opening that occurred when the spring retracted due to the heat. Let the stat cool and capture the string. Then place the stat, suspended by the string, back into new, cool water and bring it to a boil again. Using a candy thermometer, measure the temp of the water as it's heating up. When the spring opens and the stat drops off the string look at the thermometer reading. That will tell you exactly what temp your stat is opening at. Check it against the reading stamped on the stat housing to see if it's within specs.

Spring facing the heated water
 

Then that tells me my thermostat is installed upside down.

The heated water is on top from the engine - the spring should face up in that case.

Another thread showed the exact same thing. When the person fliped the t stat, everything ran great.