Merc Optimax 150 O/heat problem

[Flywest]

I have a 2005 Merc Optimax 150 HP 2 stroke OB with 1500 hours that is having an overheat problem.

I suspect maybe a thermostat jammed?

It has been doing this overheat thing on and off for maybe 5 months , mostly only when you come off plane from WOT, and start to idle she seemed to get warm quickly if you aren't moving forward at any sort of speed.

Once moving she seems to cool down quickly to an operating temp around 54 degrees C. When she does get warm, idling after a run at WOT etc, she will get up to 80 degrees, when the alarm starts to sound and if you don't either shut down or move along a bit, she will overheat to 100 degrees C.

Heres the funny thing - even at that temp on the smartcraft gauges, - the water coming out the tell tale hole is still only warm to the touch!

So - I gave her a full 1500 hours service - new plugs & filters, and I dropped the leg and replaced the impeller!

The old one didn't seem to be badly worn at all - and had no cracks in the impeller rubber fingers, no chunks missing or anything!

I put the new one in just as I have with my other Honda outboards (which use Merc lower units so identical water pumps impellers and replacement methods - not like I haven't done a LOT of these!).

So - here we are with a new cleaned hull - and serviced outboard - I even emptied a sealed compartment underfloor when I repaired the bilge pump that had not worked since we got the boat! Must have taken out ~1000 pounds of weight of water thats been trapped in there.

WOT Speed went up from 10 or 12 knots on this 19 ft commercial aluminum center console hull - to 28 knots!
Now we are talking!

But I still have the overheat alarm problem, if anything worse now than before, soon as you slow down from WOT, or back off to idle, the overheat alarm is not far away!

I put new seals in the water pump as well as the impeller!

I am thinking there is some restriction in the top of the motor near the heat sensor (like a malfunctioning thermostat maybe) maybe blocked with sand or grit - thats letting water heat up in the top galleries of the engine at low rpm's and causing the O/Heat alarm to go off.

Anyway to check this out?...

It ran a treat on the muffs at idle after the new water pump impeller, so I don't believe there is a problem with water supply from the water pump before or after replacement...

Just can't figure why the overheat alarms all the time at idle and low rpm's after WOT runs - when the tell tale water stream temp isn't hot to the touch?

Last thing I want to do is overheat the engine - the tell tale is peeing well - and doesn't seem to be any grit in it blocking it at all..

This is a commercial boat thats lived in the water on a swing mooring (with engine tilted up when not in use) since 2005 when the new Optimax was fitted so it has never got flushed with fresh water in it's life - which is why I suspect grit and maybe corrosion of the thermostat to be the most likely cause of the problem.

Just curious if anyones come across this and can walk me thru what to do next?.

I live on an island, so the boats our main transport - I need to get it working right and have spent quite a few $1000's on all the servicing gear and new safety gear - and this is the last of the problems I need to solve on her - I even fixed the leaking helm seal.

Anyone can help out I'd be much obliged - I can take closeup photo's and post to the web to show anything required in detail.

I really don't think this is water pump related - having had problems in the past with this overheat alarm and now again same thing...after fitting a new impeller..

Very frustrating to say the least.

Cheers.

 

[Steve S]

Is the sender temp bad ?

 

[Flywest]

I suspect that maybe it's not so much a bad sender as maybe a blockage/ restriction in the gallery / thermostat area of the engine near the sender.

You see the temp gauge seems to accurately represent the real temps and they vary so widely so quickly at slow rpm's that I suspect more of a blockage / pooling of warm water near the sender due to a restriction, maybe of the thermostat.

The history / usage (commercial) on a swing mooring suggests never been flushed with freshwater in 1500 hours / 3 years and the big tidal movement 20 Ft twice a day means silt laden water more often than not.

So I am thinking "partial blockage" somewhere near the sender more than a crook sender itself, and once that hot water gets past the restriction and mixes with water out the tell take it is just warm not hot to the touch...

But I could be wrong and it could be something as simple as a crook sender!

Its a nuisance because when it does happen, the computer cuts the rpms and leaves you almost stranded until you shut down and let her cool...which is a royal PIA.

Cheers

 

[Steve S]

Do you remove the sender ,thermostat and flush ?

 

[wileyE]

I'd at least pull the stat(s) and take a look, being you run it in salt. Next time it alarms lift the cover and see how hot the head is. You should be able to lay your hand on it, and the water out the weep spray shouldn't be all that warm.

 

[w9awx]

Check with your merc dealer. I think there is a TSB on this. I also believe that it was the 2005 models that were having so many problems with blown power heads due to thermostats sticking and overheating the engine. Have you checked to see if your engine has any recalls on it?