4.3L Overheat?

[PS94]

So, this is new to me. boat started randomly chirping the overheat alarm to me. Changed the thermostat, no change.
So, I checked a few things. when the boat is running, there is a nice, solid flow of cold water from the leg (raw water) pump. I can blow through the lines to the manifolds, and hear it bubble in the lake. where do I go from here? Leg and circ pump have about 45 hours on them.
Motor is a 4.3L 2bbl carbed engine, raw water cooled, with new(2 years) manifold and elbows on her. 0L072498
If I pull the thermostat completely, it runs so cold the temp gage won't register. When its chirping it's in the 170 range on the dash. thermostats function in a pot of hot water. 160* T-stats.

What next?

 

[alldodge]

Re: 4.3L Overheat?

Did the impeller ever go to pieces, and if so was the oil cooler back flushed?
Did you install the thermostat sleeve correctly?
Why was the manifolds and risers replaced ?

 

[Volphin]

Re: 4.3L Overheat?

You need to confirm the actual engine temp with a temp gun to see if an overheat condition actually exists. Under what conditions are you getting the alarm? Under power? Idle? Full throttle? If engine temps seem normal, look at the temp sending system.

 

[PS94]

Re: 4.3L Overheat?

The boat was a salt water boat at some point in her life. I'm guessing manifolds were done as maintenance or for rust out. I did the motor as it had froze on the previous owner. I did the leg impeller and circ pump to have a worry free boat. The temp sender for the dash, and alarm are on different circuits, and both say hot.
I'm only getting alarm at higher RPM.
Thermostat sleeve has a spline so it is impossible to index it erroneously.

Heres a thought - (it literally just came to me)
Where does the thermostat actually go?! Does it sit down into the intake, with the plastic sleeve above it?! or should it sit UP in the spider looking thing with the hoses(what i would call a water neck), and the sleeve on the engine side of it? Obviously i put the wax ball side down towards the engine, but i blindly replaced the t-stat in the same manner i removed it....

 

[mercs4mark]

Re: 4.3L Overheat?

I agree with volphin if all that checks out ok then move to the water pocket on the outdrive. You can check that by splicing a clear piece of hose on the inlet hose and putting the boat in the lake and see if you are getting air bubbles in the system.

 

[alldodge]

Re: 4.3L Overheat?

Where does the thermostat actually go?! Does it sit down into the intake, with the plastic sleeve above it?! or should it sit UP in the spider looking thing with the hoses(what i would call a water neck), and the sleeve on the engine side of it? ....

Here is a pic


http://www.mercruiserparts.com/Show_pictures3.asp?dnbr=809969 98&ivar=IMAGES/COMMON/14045.png&inbr=2694&bnbr=150&bdesc=Standard+Cooling+System

 

[PS94]

Re: 4.3L Overheat?

Hmm, scratch that idea, t-stat is in the correct spot...
as for clear hose, you're talking pull it off at the hull fitting, then use 1" clear hose to the t-stat raw water intake, and see if I get bubbles on plane?

 

[Bt Doctur]

Re: 4.3L Overheat?

http://www.boatfix.com/merc/Servmanl/18/18A6R2.PDF

 

[mercs4mark]

Re: 4.3L Overheat?

Hmm, scratch that idea, t-stat is in the correct spot...
as for clear hose, you're talking pull it off at the hull fitting, then use 1" clear hose to the t-stat raw water intake, and see if I get bubbles on plane?

Yes...my thoughts are if you had a blockage somewhere it would be blocked at low rpm as well as high....if your impeller where bad usually you get a cool down at higher rpm. In your case you are slightly overheating at high rpm as if you are pulling warm water back into the system....but really check and make sure that its not a sensor.