Exhaust hose overheat

[SquamInboards]

I'm cross-posting this here, I had been following a couple threads on CCFan about this topic and I thought I'd see what you guys think. I thought I posted this here when I had the problem, but I couldn't find the thread. So here goes.

My boat does a lot of idling, often for night cruising when it's chilly, so we don't want to go fast. Now I've started taking my son for "boat naps" where he sleeps while I drive around the lake (he's almost 2 years old), but again, more idling. Last weekend it was over 3 hours straight.

The problem I had two years ago (and now again) was the exhaust hoses were getting extremely hot, like 250-300 F (they are rated for 250 F). To be clear, we're talking about the 3 1/2" ID hoses that connect the risers to the steel elbows that are in the floor of the boat.

I narrowed it down to an internally collapsing hose, which was preventing water from escaping the riser and cooling the hoses at low speed. At higher RPM the water was forced through, so cooling was no problem. It was worse on one side, and I was sure I had found the culprit because the problem followed the hose when I switched sides. So I replaced the hoses and had no problems thereafter. That is, I checked them multiple times that summer after long idling periods, and they were fine (under 200 degrees at all times). I don't think I even thought about the problem last year, since I had no symptoms.

I was out in the boat this weekend and noticed the hoses getting hot again at idle (I smelled burning rubber in the exhaust fumes), but no problem at speed. The risers are normal temps, all the time - never over 160. I think that because I do a lot of prolonged idling, the hoses get a little too hot over time, and break down around (basically melt into) the channels that allow water to pass from the risers to the hoses. Then the problem is self-perpetuating, as less and less water passes through at lower speeds, and the hoses get worse over time. The engine temp is always dead on normal, and I've cleaned out my manifolds and risers, there's no flow issue in there, I'm certain of it.

I don't think there's a back pressure issue, because the engine always runs flawlessly, and plenty of water comes out the exhaust when I run it on the trailer. The problem is the same on both sides, and my boat has full dual exhausts. The problem went away COMPLETELY with the new hoses, and has slowly returned.

Since the hoses get hot, but never more than 300, I am going to put the higher-temp silicone exhaust hose (rated for 350F) in place of the old stuff, since I now have to replace it anyway, and hopefully that will be the end of it. I hate to mask the problem with higher-tolerance materials, but I genuinely think there is no design issue, and no water flow issue, it's just that when you idle for SO LONG, it slowly weakens and collapses the inside of the hoses, until they get less and less water through the channels, and it's downhill from there.

Anyone have thoughts on this? Am I crazy?

[TitanTn]

I don't think you're crazy. I don't do the same amount of idling as you do, but my hoses are only about 3 years old and I've noticed that they started to droop on the inside from what looks like excessive heat. I'm still doing okay, but they obviously won't last too many more years as they seem to be on the fringe of tolerating normal heat loads. I follow your line of reasoning that your extended period of idling is not running the expected amount of cool water through the hoses and the heat is eating them away. And thus I think your proposed solution is a good one - I mean the alternative is to replace your hoses every year with the factory type of hose - so what's the difference? You're not going to do anything different to the engine because the block and the manifolds are within the expected temp range.

[SquamInboards]

the alternative is to replace your hoses every year with the factory type of hose - so what's the difference?

I think this is the final straw for me - I have to replace the hoses I have. So why not spend a little more on hoses that might last longer? I have a very healthy cooling system with no leaks or clogs that I can find. I've never cooked an impeller in this engine and I've cleaned the manifolds and risers thoroughly so there shouldn't be any blockage. The engine and all its parts run at normal temps. Except those hoses, after long periods of idling.

I will also TRY not to idle for such long periods, I'm sure it's not the best thing for the engine. But sometimes, you gotta use the machine the way you want, and it should just do whatever you ask of it.

[CJD]

This is an interesting condition. I wonder if it isn't less a problem of the amount of heat, but rather poor distribution of the cooling water in the riser at low speeds. Like, possibly, the water collects at the bottom, so the top of the hose overheats? If that's the case, the higher temp hose would be the solution, as you are not going to be able to increase the water flow while idling.

[SquamInboards]

I wondered that too, although the way the riser gasket has a port blocked on the side nearest the bottom of the hose, it's intended to force water up and over the top of the riser, which it seems to; the top of the riser is cool enough. BUT the way the connection of the riser to the hose is designed, water could certainly favor the bottom, and then of course no matter how it enters the hose it will favor the bottom, especially lower down on the hose. I never thought to check the temp separately on the bottom side of the hose.

If money were no object, I'd be using silicone junctions between the risers and a section of pipe, rather than the hoses. But getting two sections of stainless pipe made up seems rather prohibitive. The pipe would have to include a reduction to 3" diameter, because the pipes in the floor are 3", while the engine has 3 1/2" output. Right now I use the reducers that PCM sells, and two sections of hose. The 3 1/2" hose is by far the longer section.

Hey, maybe there is back pressure caused by the reduction? But if that's the problem, then the solution is ripping up my entire rear floor and putting in new exhausts through the transom. Which will no doubt lead to a complete floor rip-up and therefore new stringers. Yeah... not this year.

[92SupraComp]

you can buy the 3" outlet risers. They bolt right up to the same manifold. Also, for our CC, they had TIGHT bends with the 3" wire reinforced hose and it got overheated and collapsed. I replaced it with 4 sections of 3" non-reinforced hose and 4 3" stainless steel 90* elbows off ebay. The SS elbows were only a $100 free shipping! Looks sweet under the hood and runs like a top again.

[Jetlink]

Seeing as how you are more at low engine speed when running the boat, I wonder if changing the diameter of the pulley on the raw water pump to make it run slightly faster to increase water flow at low engine speeds to make it behave like the engine is running faster might help.

[chris young]

I dunno about that one. When I put my intake into a 5 gallon bucket at idle, it empties it in about 20 seconds, that's a lot of water at idle. Additionally, you'd be pumping more water at higher rpms.

[Blackntan90]

What about running the bilge blowers? You are supposed to run them at idle speed, and every one does it- right? But maybe a little more airflow could reduce the dog house temps, and in turn the exhaust might not be such an issue at idle...just a thought.

[lively]

Touch your risers and see if they are hot at idle at first start up and then to operating temp . Timing could have play here as well and heat range in plugs . You need to verify that they are in fact the right ones


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