Another Overheat Question - This One May Baffle You

[Bakeys]

Glastron G1700
Mercury 3.0
Alpha One Gen II w/ new 140? thermostat.
Cayuga Lake, NY

This one even has my local marina boys stumped.
Had the boat out a few times this summer. I can go for miles with the boat at a comfortable cruise speed of 3,000 to 3,500 RPMS. The temp gauge will maintain a steady 140?. If you watch the gauge very closely you can see the needle swing ever-so-slightly as the thermostat does it's job. With no change in the boat operation, the temp will suddenly climb. It'll climb until you throttle down to about 1,500rpms. Then the temp will return to it's 140? mark.

I have an impeller kit, so yesterday I pulled her out of the lake and proceeded to open her up. What I found was peculiar, to say the least. Basically everything above the metal impeller housing had experienced a meltdown. The plastic water feed tube was nearly unidentifiable. If it wasn't for a lighter brownish color, I wouldn't have known what it was. The plastic fitting that attaches to the housing for the water feed tube was partially melted. The exhaust flap had melted down and resembled a gnarled shoe heel, the stainless plate that once was affixed to the flap fell out when I separated the top and bottom drive halves.

The impeller was in beautiful shape. (The marina guy asked me if it was new.)
After examining the top and bottom halves for any more melted material, I replaced the melted components, ( not the exhaust flapper), reassembled the drive, and refilled with Merc gear lube.
Took it down to the launch and spent maybe ten minutes checking everything out with full throttle, 3/4 throttle, etc. Temp maintained a constant 140?. Everything seemed to be back to "normal". By the way, the thermostat was new last week in the event that the problem was that simple. Put the boat back on the hoist.

A few hours later I have my wife and folks in the boat for a ride down the lake. Temp remained at 140?. Seven miles down the lake, I watched the temp walk up and up. At 175? I throttled back and sure enough it cooled back down to 140. Nursed the boat home with 1/4 and 1/2 throttle and put it back on the hoist where she sits now.

So what did I miss?

This may be worth noting as well. When the temp starts to climb, the exhaust sound changes. There's a bit of a louder, low pitched rumble (for lack of a better word). So even if you're not watching the gauge, it'll let you know something is amiss. After it returns to 140, the exhaust rumble returns to normal.

Also, and this is when things seemed to change. Couple weeks ago I had the boat on the hoist and was running the engine to blow the floating weeds away from the shoreline. The outdrive was immersed but the weight of the boat was on the hoist. Ran the engine probably in the 1,500 - 2,000 rpms range for 15 - 20 minutes. It was after this that the problem started.

Thanks all for the anticipated insight.

 

[alldodge]

Is the exhaust boot soft?

 

[Bakeys]

Is the exhaust boot soft?

That's a good question. Never thought to feel it.

 

[Bt Doctur]

With a undamaged impeller and all else melted the flow of water was cut off but not from impeller failure. Only other connection to water flow is the transom hose. A internal wall collapse can shut water off under high rpm and then relax at low rpm

 

[Bakeys]

A internal wall collapse can shut water off under high rpm and then relax at low rpm

Not sure I'm following.
Internal wall collapse?
What wall is collapsing?
The seawater inlet hose?

Pardon my lack of understanding. Can you explain, please?

 

[Bt Doctur]

The hose from the gimbal to the transom.

 

[kenny nunez]

Another way to check the pump output is with the boat in the water , remove the hose from the thermostat housing that supplies water from the out drive pump. With the engine idling try to see if you can stop the flow with your thumb. If you can you are losing pressure which could be the hose from the bell housing to the gimbal housing as said before. Since you mentioned melted plastic the pocket cover in the top of the driveshaft housing may be the reason as it is not sealing to the gasket surface.

 

[Bakeys]

The plot thickens...

Taking Kenny Nunez's post to heart, I disconnected the inlet hose to the thermo housing. I started the boat and my dad (sucker) put his thumb over the hose to determine if he could hold back the pressure. (He needed a good soaking anyway.) He could hold his thumb over the hose and nearly stop the flow. Shut down, reassembled, checked the thermostat to ensure nothing funky was going on there.

We then took the boat out to replicate the overheat condition and see if we could gain more insight.

That freakin' boat wouldn't overheat. 25 minutes of run time, 3,000R's, 4,000R's, 4,400R's (WOT), and I couldn't get it to go over 140 degrees on the gauge.

I find it VERY difficult to think that whatever was the problem has miraculously fixed itself. While I believe in my heart of hearts in Divine Healing, I haven't prayed about the overheat condition in my boat, So I'm still stumped.

Think I'll have to get in the water, raise the drive and see if I can see anything going on with the inlet hoses.

In the meantime, thanks for the assistance. Feel free to chime in with more theories.

 

[alldodge]

Was the exhaust boot soft, the one from the riser to the exhaust tube?

 

[JustJason]

You are probably picking up crap in your local body of water and that crap is partially covering up the water inlets on the lower unit. Whenever there is an "intermittent" overheat (and the engine is actually overheating, and this is not a gauge/sender/wire issue) it is almost always picking up crap in your local body of water. If you want to "test" the theory then install a water pressure gauge on the boat, and see if water pressure drops as the temp increases.

 

[Rick Stephens]

You are probably picking up crap in your local body of water and that crap is partially covering up the water inlets on the lower unit. Whenever there is an "intermittent" overheat (and the engine is actually overheating, and this is not a gauge/sender/wire issue) it is almost always picking up crap in your local body of water. If you want to "test" the theory then install a water pressure gauge on the boat, and see if water pressure drops as the temp increases.

Is an open system, virtually no pressure to measure. Flow is the only thing we can care about.

 

[JustJason]

Is an open system, virtually no pressure to measure. Flow is the only thing we can care about.

​On a properly working system there is roughly 2 or 3 PSI at idle and 10 to 15 PSI at full speed. But what do I know.

 

[Rick Stephens]

​On a properly working system there is roughly 2 or 3 PSI at idle and 10 to 15 PSI at full speed. But what do I know.

And what do I know? :^)

You are literally the first person that I ever heard even mention measuring cooling system pressure on an open cooled system. Never would have occurred to me to check. If you install a gauge, how do you know what is normal at that measuring point, with these particular manifolds, hoses and RPMs? I can see getting readings but not know what they tell me.

 

[Bondo]

Ayuh,.... The flow of the exhausted water is restricted at the risers, so, Yes, there is measurable pressures, like Jason says,....

Bakeys,... Did ya change the Whole raw water pump,..??
Any scorin' in the top or bottom plates will adversely affect the water flows,...

With the in-comin' hose off the t-stat housin', there should be a solid stream of water, 'bout 4" or 5" high, when pointin' the hose straight up,...

 

[JustJason]

And what do I know? :^)

You are literally the first person that I ever heard even mention measuring cooling system pressure on an open cooled system. Never would have occurred to me to check. If you install a gauge, how do you know what is normal at that measuring point, with these particular manifolds, hoses and RPMs? I can see getting readings but not know what they tell me.

​Well..... Once you go to Mercruiser school/Mercury University they will teach you this. Mercruiser actually installs water pressure sensors on EFI engines in order to monitor engine block water pressure. (The raw water side, not the coolant side). As far as "how would you know what is normal. They cover that at both Mercury University in the classroom as well as through the E Skills DVDs. The majority of inboard/IO engines make just a couple of PSI at idle and roughly 10 to 15 PSI at full throttle as long as the impeller is good and the manifolds/risers are not to rusted up.

This is nothing new, they have been doing this for 15+ years. On any Merc engine with a 555 or newer ECM an alarm will sound for low engine water pressure in order to alert the driver and put the engine into guardian mode. They do this with sterndrive engines, inboard engines, Optimax outboards, Verado outboards, and some of the newer 4 stroke outboards as well. (These are all open cooling systems). IMHO a water pressure gauge is a good idea on any boat with any engine. The idea behind it is if you always keep an eye on the water pressure, you can catch problems before the engine overheats, and not wait until the engine actually overheats and the alarm goes off.

 

[Rick Stephens]

​Well..... Once you go to Mercruiser school/Mercury University they will teach you this. Mercruiser actually installs water pressure sensors on EFI engines in order to monitor engine block water pressure. (The raw water side, not the coolant side). As far as "how would you know what is normal. They cover that at both Mercury University in the classroom as well as through the E Skills DVDs. The majority of inboard/IO engines make just a couple of PSI at idle and roughly 10 to 15 PSI at full throttle as long as the impeller is good and the manifolds/risers are not to rusted up.

This is nothing new, they have been doing this for 15+ years. On any Merc engine with a 555 or newer ECM an alarm will sound for low engine water pressure in order to alert the driver and put the engine into guardian mode. They do this with sterndrive engines, inboard engines, Optimax outboards, Verado outboards, and some of the newer 4 stroke outboards as well. (These are all open cooling systems). IMHO a water pressure gauge is a good idea on any boat with any engine. The idea behind it is if you always keep an eye on the water pressure, you can catch problems before the engine overheats, and not wait until the engine actually overheats and the alarm goes off.

Thank you! Good stuff.

 

[JustJason]

You are very welcome.