High RPM Overheat

aero3113

Coming back to the dock on Saturday with my daughter my port engine started to overheat half way across the bay. I shut it down and let it cool. Started back up and the temperature was steady. I put the drive into reverse thinking maybe there was some seaweed or a bag on the intake. Powered back up, cruised around for a little bit and the temp started to rise again. Powered down to idle and the temperature went down and stayed steady. Manifolds and risers were replaced last season. I’m going to trouble shoot some more tomorrow before I pull the water pump and check the impeller. New Hardin pump was installed beginning of last season. What are you thoughts?

Liberty44140

If you are FW, is the heat exchanger full? Coolant level good? Then I’d check impeller, even though it’s new it could have been damaged. 

aero3113

@Liberty44140 , I’m engines aren’t closed cooling.

Aqua_Aura

Thermostat and impeller is my only thought. Could have some junk in the system maybe causing some blockage not providing enough cooling at higher RPM. 

aero3113

@Aqua_Aura , if it was the thermostat, wouldn’t it overheat at all RPM’s?

Aqua_Aura

Not necessarily if it's not warming up enough to open the thermostat. 🤔 Could stay cool enough at low rpm so basically the thermostat would maybe be stuck closed. 

aero3113

Thermostats were new last season also.

aero3113

I did rev the engine in neutral some at the dock and the temp didn’t rise. Not sure if it cleared and fixed itself. But I don’t think I reved it long enough to cause it to rise. I was able to cruise a decent distance when the temps rose again.

Black_Diamond

Could be the raw water intake hose on the outdrive. It is a Z shaped hose and can easily kink under high water draw if not installed correctly. The upgraded hose now has a hard plastic end to solve the problem. 

I had this exact problem. 

Aqua_Aura

Actually I guess on a raw water cooled engine if the thermostat wasn't opened then the "hot side" would be completely dry?

aero3113

Black_Diamond said:

Could be the raw water intake hose on the outdrive. It is a Z shaped hose and can easily kink under high water draw if not installed correctly. The upgraded hose now has a hard plastic end to solve the problem. 

I had this exact problem. 

@Black_Diamond , wouldn’t the engine overheat pretty quickly up on plane if it was the hose? It seemed like once it cooled down I was able to get a couple of minutes before it overheated.

LaRea

Bravo 3?  The water pump is not getting enough water because of corrosion in the thru-transom fitting.  

https://rinkerboats.vanillacommunities.com/discussion/1945/solved-engine-alarm-on-merc-496

aero3113

@LaRea , one of the things I want to check is the intake hose. I’m going to separate the hose at the quick disconnect and hope I see water flowing in. Last year on my first day out, I sucked up some mud on my starboard engine. I removed the quick disconnect and had barely any water coming in. I used a shop vac to blow out the line and was good after that.

Black_Diamond

@aero3113. When it happened to me, It would run for a few min at plane before the high temp alarm.  

LaRea

My boat had a partial blockage -- enough flow to run at 3200 RPM, but not faster.  The static flow rate might not answer the question.  Maybe if you disconnect both sides and compare flow rates.  

aero3113

@LaRea , I remember what good flow is from when I had to pull my starboard side last season. If there is a clog, would I be able to prod with a long wire up the hose to the fitting?

aero3113

If I can’t blow it out with my shop vac.

Liberty44140

I’d still start simple and check that impeller. I know, reading online makes you start to think it has to be complicated as there have been some very complicated and very awesome finds on here…. But most issues are resolved with something simple so start there. 

Aqua_Aura

If you read online you'll tear down you entire engine. Kinda like when you research any medical condition and it's always cancer 🤣

LaRea

If you disconnect the hose at the transom, you can do a visual inspection.  But if it's corrosion of the hose fitting, the only solution is to replace the hose inside the outdrive, which is a haulout repair. 

Liberty44140

Aqua_Aura said:

If you read online you'll tear down you entire engine. Kinda like when you research any medical condition and it's always cancer 🤣

Lol true dat!

davidbrooks

Be careful with that transom hose fitting.  I learned the hard way why a lot of them dont get replaced.  They become brittle and the bolts can easily break off.  The only way to drill out the bolts are to then remove the engine...or buy a right angle drill.  I was also thinking to check and make sure your exhaust flappers are still intact.  That caused enough of a blockage on me that it had a similar reaction.

aero3113

I removed the quick disconnect, there was water flow. I still used my shop vac to blow it out. I went for a 10 minute run and there was no overheating. I don’t know if I blew something off of the intake or I cleared something in the line. We will see how she does on the next trip.

aero3113

I was going to go for a test run before I did anything to see if it cleared when I was reving and playing with it on Saturday. So I’m not 100% sure when it cleared.

PickleRick

If flow is good i think it could still be a sticking (in the open position) thermostat.

The coolant in the engine needs to stop long enough in the radiator (heat exchanger in boat application) in order to cool.  If the thermostat is not stopping or slowing the coolant enough eventually all of the coolant will be over heated. The more load/rpm the quicker this will happen

A thermostat is at least cheap and easy to replace.

When I first got my boat the first thing I did was pull the caps on the heat exchanger to check for corrosion as it was used in salt water. Im sure you already checked this.
 

aero3113

@PickleRick , I’m raw water cooled. No heat exchangers.

PickleRick

So scratch that one, although if it's not opening or not opening fully a 14 dollar thermostat could still be the issue. The shade tree limp home back in the day fix for a thermostat that didn't want to open was to cut the center out if it.

I still can't believe that big boat is raw water cooled!  

GMSLITHO

I agree with the transom hose theory 

aero3113

I did the same run today as I did Saturday and a little more with no issues, temps were stable.

Michael T

I like @PickleRick with the thermostat (cheap fix and if older preventative maintenance) AND @Black_Diamond with intake hoses. A few years ago an American boater pulled into Kingston (Ontario) harbour. He had just made a crossing from New York State on his way to Belleville (Ontario). He was imagining all kinds of expensive scenarios. We replaced his thermostat but the overheating was actually caused by a hose that had degraded and become soft and collapsed progressively under load. We got him sorted and underway the next day.