Overheating at WOT

TomB985

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Good evening everyone.

Have a 2011 Larson with a 4.3L/Alpha 1 drive. Bought the boat new, and the impeller has been replaced twice. Most recently was replaced this spring with an SEI kit, and the quality seemed to be equivalent to the Mercury part I installed three years prior.

No running issues until today when I was on the way back to the dock.. After two minutes at WOT temperature alarm went off and the gauge was around 225 degrees, so I immediately backed off the throttle. Once the RPMs came down temperature dropped rapidly to 150, and then slowly moved back to 175 where it normally runs. Tried running at 4,000 RPMs and temps stayed at 175, but at 4,700 RPM WOT temps started quickly rising after about 45 seconds.

Best guess is that it's somehow sucking air which causes the temps to rise quickly, but as RPMs come down it starts sucking water again and it quickly cools down with the thermostat wide open. This boat goes on plane at 2,600 RPMs, and it spent over 2 hours on plane today with no temperature issues whatsoever. If the seal between halves of the seawater pump were imperfect, wouldn't it suck air whenever the boat was on plane? If that's not the cause, what would cause it to act up only at 4,700+ RPMs?
 

HT32BSX115

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Howdy,

If this happened "all at once" I would suspect some sort of debris that might have been sucked up the raw water pickup and is causing a low flow of cooling water.

If it happened right after an impeller replacement, something is wrong with the reassembly of the pump/drive "lower" etc. You could install a clear vinyl hose in the raw water supply line from the transom to the t-stat housing and see if there's bubbles in the raw water supply.

If it's happened gradually(and you're operating in salt) , you may be getting to the EOL (end of life) of your manifolds, risers or both (and they're getting clogged with rust)

You might also consider getting yourself an infrared temp "gun" and do some checking of temps. See where your risers are running (temp) and even "shoot" the thermostat housing to verify your gage. (although if the alarm is going off, it's separate from the gage)

Regards,


Rick
 

tpenfield

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If the water intake were sucking air, it would tend to do that at the 4,000 rpm speed as well. So, I would think it is more a restricted water flow as Rick suggested.

Had you run at WOT on your previous outing? Or is this the first time in a while that you ran at full throttle?
 

TomB985

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Thanks for the quick replies!

We were on the Mississippi river when this happened, so it stands to reason that we could have sucked something up. We had about 20 miles left to get back to the dock, so I did some experimenting after that. Ran at 4,000 RPMs for about three minutes with temperatures staying steady at 175, and then later on tried another WOT run. Perfectly normal for about thirty seconds, and then it was like flipping a switch...the gauge started rising fast. Back down to 3,500 RPMs and it shot right back down to 175.

Most of my boating is on smaller lakes up here in MN. 4,700 RPMs in this boat is about 50 MPH, and I don't typically have the room or water conditions to go that fast for more than 30-40 seconds at a time. Boat sees WOT every time it runs, but rarely for more than a minute or so.
 

tpenfield

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I got my fingers crossed that it is a water flow issue that can't quite keep up to the heat of WOT.

A head gasket problem can also cause similar symptoms, but not as likely the issue with a 5 year old engine.
 

TomB985

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Thanks, Ted. I'm somewhat boat stupid, but fairly mechanically inclined. The water intake on the side of the drive is completely clear, where would I start looking for a complete restriction that only affects at WOT? The way the gauge moved, /!; especially how it overcooled below 160 when it first recovered sort of implies to me that something completely obstructed the cooling system, and then started working normally once RPMs came down and the cooling system was overwhelmed with cold water before the thermostat could shut.

Never thought about head gasket, but it makes sense. Peak cylinder pressures are when a failing gasket would lift, and on a car the pressure would be vented out the radiator cap. No radiator or cap on the boat, so combustion gasses would go out the water outlet. definitely hoping this isn't the case.
 

tpenfield

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There are probably a series of oil coolers that the incoming water passes through before it reaches the engine block up at the thermostat housing. If you get some marine growth sucked up through the outdrive pick-up holes it will often block the flow at coolers and thus the engine too. It may be a small enough restriction that only at full throttle the cooling system can't keep up.

The other common cause is if the exhaust manifolds or elbows start to cake up inside with rust and corrosion, which I think Rick mentioned. That will restrict water from exiting the engine, and the only way cool water enters the engine is for hot water to exit the engine.
 

TomB985

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If it makes any difference, the boat has seen salt water only once in its life, and that was for about fifteen minutes at the mouth of the Connecticut river before heading back to Hartford. Fairly certain all residual salt was flushed out on the ~60-mile return trip through fresh water.
 

HT32BSX115

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If it makes any difference, the boat has seen salt water only once in its life, and that was for about fifteen minutes at the mouth of the Connecticut river before heading back to Hartford. Fairly certain all residual salt was flushed out on the ~60-mile return trip through fresh water.
Pretty unlikely that would do much to your manifolds or risers. Fresh water manifolds and risers can last up to 20 years!

I don't think you have an oil cooler but you may have a power steering fluid heat exchanger that could be clogged. You also could've sucked up some sand/silt that might still be in the block. That would reduce the cooling capability of the block possibly making it overheat at maximum power output.

If it isn't, you might look at the thermostat too. It's also possible that you could have either a stuck stat or some debris in there......
 

TomB985

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Thanks, guys.

When I go to winterize I'll check this out. So far my list includes pulling the lower drive and checking the impeller and water inlet, and check the thermostat. What else can I check if these two are normal?
 

stonyloam

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If you go to the trouble of checking your impeller, just replace it, it may be new but could have been damaged by the overheat.
 

CV16

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I'll add that if you replace the impeller, go with OEM. I have had the rubber spin on the hub a few times with aftermarket.
 

TomB985

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Thanks, guys. I'll be towing it back to the house this weekend and taking a look, hopefully something obvious will show itself.

I've monitored temperatures on a lot of different engines over the last ten years, mostly while towing things, and this one bothers me. The behavior of the gauge really indicates that the engine isn't slowly being overwhelmed...it's as if water flow is completely interrupted. On the second run it went from 175 to 200 degrees in about six seconds, after holding 175 at WOT for 30 seconds. It recovered in as many seconds after dropping to 3,500 RPMs. The only explanations that make sense to me are something that's only getting sucked up and obstructing under maximum water flow, or a head gasket that's starting to lift. Kind of concerned about this...

Thanks again for all the helpful replies!
 

Rick Stephens

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Testing for exhaust gas presence on a closed cooling system is so easy.... wonder if anyone has found a way of testing an open system?
 

tpenfield

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Testing for exhaust gas presence on a closed cooling system is so easy.... wonder if anyone has found a way of testing an open system?

With an open cooling system, probably the way to test for exhaust gases in the cooling water would be to temporarily install clear hoses for the ones that exit the engine and go to the exhaust.
 

TomB985

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Just pulled the impeller, think this may have something to do with it?



Also, the seal that sits on the top of the housing is cracked. Gonna replace the impeller and the seal, anything else while I'm in here?
 

TomB985

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Replaced the impeller and the seal above the pump and put it back together, then took it for a test run. Ran it across the lake at 4,000 RPMs and temperature stayed steady, then turned around and went WOT. About 3:30 altogether at WOT until I ran out of lake, and the gauge didn't budge. Not sure what exactly happened to cause the overheat, I'm starting to think the vanes may have gotten misaligned as I was working the assembly off the shaft. But I don't really care as long as it doesn't happen again.

Playing with the old SEI impeller, it looks like it needed to be replaced anyway:



Won't be buying one of those again.
 

Rick Stephens

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I've never heard bad about SEI parts. Your impeller looks like it has some set. And never saw one that didn't flip all the vanes around first time it got up to speed. Doesn't look to me like it should have caused you your problem. No matter, glad you got it fixed.
 

TomB985

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Thanks Rick. Had it out yesterday, and no issues. Ran WOT until I ran out of lake, which was about two minutes at that point, and it's working fine. Still not sure about what caused the issue, but will continue to keep an eye on it going into next season.
 
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