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Unknown Overheating Cause, (have checked literally everything... please prove me wrong)


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So I have hit a wall and would greatly appreciate some advice/ fresh perspective on my 2007 VLX w/Indmar monsoon 340. Purchased the boat roughly two months ago, 376 hours, sat for roughly two years before I purchased. After swapping the cap and rotor, plugs, and impeller water pump assembly the boat ran like a top. Shortly thereafter someone else was driving (I was not on the boat) and the water circulation hose burst. Which led the boat to overheat to roughly 215 degrees and melted the 90 degree elbows on the top of the exhaust manifold from lack of cooling. Long story short, cleaned the melted nylon out of the threads and put new fitting on as well as replaced the circulation hose. When I took it back out to test I had a misfire in cylinder 3. A compression test revealed that I had nearly no compression on cylinders 3 &5 (blown head gasket). So I did a head gasket swap, all new seals throughout and replaced the thermostat, thermostat housing, water circulation pump, and serpentine belt. Also note that the exhaust manifolds were replaced when I purchased the boat. After putting the boat back together it has 167-180psi compression on all cylinders and fired right up. But as I ran it on the hose I watched the temps climb to 170 and I killed it. Thinking I somehow had a bad new thermostat I opted to remove it to see if that kept my temps down, and it did the trick. Stays right at 130 degrees as I would expect. However, when I put it in the water and ran it at 3500 rpm the temps creep up to 170 with no thermostat! Cruising at lower speeds the temp seems to fluctuate from 140-160 (a little high for not having a thermostat in). So my question is what could be causing this engine to overheat? I have checked the transmission cooler and it is clean.

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Are you sure all the hose clamps are tight? That's a lot of work to do on a motor, one hose clamp not tight will suck air and not let the system cool. Look for an air leak, recently I found heater hoses rubbing on pulleys causing air leak on a 2015 6.2L Indmar, and I found a hose clamp running from thermostat to starboard exhaust manifold not tight on a 2015 Ilmor 5.7L both were sucking air and overheating. 

On a side note, your motor has a re-plumb kit available from Indmar. It helps and updates the cooling system.

Edited by skurfer
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I went through my hose clamps to make sure they were tight... but I am not opposed to going over it once more tomorrow! Are there any other tell tale signs of an air leak? Like bubbles in the cooling hose etc? 

My next step is to purchase an infrared thermometer so I can determine if my temperature sending unit is functioning properly. But temp seems to be inversely proportional to oil pressure so I believe it to be accurate. 

Also, I have heard that there is a re-plumb kit. Going to get one on order tomorrow but my understanding is that this will only affect the exhaust manifold temperatures so I am doubtful it will solve my problem. 

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Check the hose fittings on your raw water intake especially well, all the way from the inlet screen to the raw water pump.  All of those fittings are under suction and can easily draw air if a hose is loose or poorly fitted. 

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Should be running a 160 degree thermostat, so 170 doesn't seem overly concerning if you temp gauge is not 100% accurate.  

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  • 2 weeks later...
  • 1 month later...

@justgary - Sorry for the super slow reply, I solved my overheating issue (which is actually somehow back but that is another story..) and didn't check my profile. 

Anyways, I usually go boating in blackwater river, or along the gulf coast in a few of the inlets. HBU? 

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27 minutes ago, Christian said:

@justgary - Sorry for the super slow reply, I solved my overheating issue (which is actually somehow back but that is another story..) and didn't check my profile. 

Anyways, I usually go boating in blackwater river, or along the gulf coast in a few of the inlets. HBU? 

We're usually in the bayous around Choctawhatchee Bay. 

I do go with a friend from time to time to the private Skiwatch Estates community just north of I-10 and west of blackwater river.  Watching some of those folks make a slalom run look so easy is very humbling....

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