Starboard Engine Overheating Coming Off Plane

aero3113

Last week coming back to the dock I pulled back the throttle levers to idle into our marina and the starboard engine overheated and the alarm sounded. I quickly put it into reverse then idle and shut down. I waited a few seconds restarted it and the temperature went down to normal like nothing happened. I figured it was seaweed or a plastic bag and it came off when I went into reverse.

Today we left to go to our destination, about a 40 minute run. All was good the whole way and temps were fine. Go to power down to get into the marina and it happens again, starboard engine overheated. I did the same procedure and started back up, all was normal again. What do you guys think is going on?

PickleRick

Doesn't your engine management system have a sophisticated flow readout to tell if each engine is getting enough raw water?  

You could also swap thermostats between engines to see if a 15 dollar sticky thermostat is causing an intermittent issue.    

Stuck open might make it run too cool in raw water.  Stuck closed would impede water flow to the likely two small holes on the outside ring of the thermostat. 

aero3113

@PickleRick I don’t have a digital Vessel View system to see water pressure. I was thinking a sticky thermostat but seems too simple…. I just replaced the thermostats this past winter also.

PickleRick

I like starting with simple and cheap....I really like when the problem is the simple and cheap.

aero3113

No overheating coming back to the dock tonight 🤷🏻‍♂️.

LaRea

Mine would do exactly the same thing when it was time to change the raw water impeller.  The pump output was strong at high RPM, but weak at low RPM.  

captkevin

Not sure if it is correct or not but i was told thermostats stick open when they are bad not closed. If thats true you can rule them out.

PickleRick

A thermostat can stick either way.  The corrosion can also make it stick intermittently in either direction although it's more noticable sticking closed as temps skyrocket quickly. 

Many times when it sticks open you'll fail to reach normal running temps unless under heavy load (like a boat).    Modern vehicles will throw a CEL it the engine isn't reaching proper running temp.   

aero3113

LaRea said:

Mine would do exactly the same thing when it was time to change the raw water impeller.  The pump output was strong at high RPM, but weak at low RPM.  

Wouldn’t mine overheat at idle then? It’s runs normal temp at idle.

LaRea

Any flow at all will keep 'em cool at idle because they don't create much heat. 

I'm not saying I understand it.  It's just an observation:  every time mine did what yours is doing, I'd change the impeller, and the problem would go away.  It never happened on an impeller less than a season old.  

rasbury

I like closed cooling ...nothing but coolant in the system other than the heat exchanger. A lot smaller area to check for plugging up or other issues...and the exhaust elbow...

PickleRick

It's been a while since I had raw water cooling but at idle on ear muffs I never could get the motor to normal operating temps.  Even increasing the rpm to 1500 to 1700 it would never get warm enough to open the thermostat.


There are small holes on either side of the thermostat to allow some bypass circulation.   With no load and at idle one could, in theory, run with a stuck thermostat indefinitely so long as you kept it at idle.  





@LaRea may be onto something about keeping fresh impellers as it would likely flow more water than an impeller that has begun to lose efficiency as it ages.  

You're getting plenty of flow at high rpms but after you let off the engine is still extremely hot and while you are pumping water it's not enough flow to keep that cast iron cool. 

aero3113

I’m thinking this issue has to do with the elbows being corroded and plugged up. I’m in the process of replacing them, hopefully I won’t see this issue this season.

aero3113

@LaRea I’ll be replacing the impellers on both engines as well 👍🏻.

LaRea

Yeah, with obstructions in the elbows, who knows what kind of crazy uneven cooling you'd get.  I hope this will do it for you.

rasbury

I feel like I'm an impeller expert as many times I've replaced mine. I also have two water inlets and I avoid running muffs- I don't think the seacock seals right with the suction- burned up the impeller at least 2x if not more. The way that impeller fills the chamber either the veins are there or they are not. What I will add from my experiences is the pump impeller side to side wears the pump housing , especially if you are in an area with lots of sand, well it's like 100 grit paper grooving the housing and losing pressure . Lots of discussions on here about this- I bought kits with ss spacers and the impeller is a 16th" shorter- solved my problem. They wear also but I'm using the original housing at around 350 hours...100 bucks was the last kit I bought.

raybo3

Take the end caps off of your heat exchanger and clean it out. 

aero3113

@raybo3 I’m raw water cooled, no heat exchanger installed.

aero3113

Do you guys feel there’s a shelf life for rubber impellers? I have two kits that have been sitting in a plastic tote in my engine room for the past 5 years that are vacuum sealed. I don’t think there’s an issue but I’ve read some people feel otherwise on other forums.

Aqua_Aura

If they are sealed not sure what the issue would be. Rubber seems to be deteriorated by UV, water, and excessive heat none of that should happen sealed in a box 🤔. 

As long as they aren't dried out or crack when pulling on the fins. 

aero3113

I agree @Aqua_Aura

captkevin

Sure they are fine. If you bought new ones there is no way of knowing how long they have been sitting in the box.

LaRea

If you're sticking to the Merc maintenance schedule, I'd say the risk is pretty low.