Getting hot on flush muffs/hose, but NOT in the water

enginesilo

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Jun 9, 2008
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I thought my boat had an issue because I started it on the muffs this year and after about 5-7 minutes the temperature started to climb real high. After lots of feedback I was ready to pull the riser/exhaust manifold but I decided to put the boat in the water for a test ride in the harbor before I did any wrenching. The engine ran cool and the riser never got over 100 degrees, and the manifold and block weren't hotter than 155 anywhere while in the water. The old gauge I had was reading about 20 degree's to hot, so I put in a new one and it maintained around 165-170. However, I was only out for about 20 minutes and I did not rev higher than 2000 rpm's since it was a no wake area.

I brought it back home to flush it on the hose and again it ran cool for about 5-7 minutes with the temperature maintaining around 170 stable on the gauge like it was in the water, then the temperature started to climb and I turned it off before it hit 190+. I know some say you shouldn't stay on the flush muffs for more than 5-10 minutes since they are not efficient at providing the engine with water the way it is designed to, but I have seen posts where people say they can run 30 minutes or more and never experience high temps.

Any idea why this may happen? My next test is going to be to run it hard to see if the temp climbs while cruising at high speeds and working the engine.
 
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As long as you don't rev the engine too high on muffs you can/should be able to run the engine as long as you want without hurting anything.

How old is the impeller in the drive?
 

Starcraft5834

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Jun 2, 2013
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Water pressure? At house? Sometimes not turned on high enough for proper flow. Or kink in hose? If u ran fine in water......impeller last changed?
 

mr300z87

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Sep 15, 2010
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I agree with above. Just the other day we started my friend friend 5.7L for the first time this season. His water pressure from his hose was to say the lease lame! He had a brand new sei alpha 1 drive with brand new water pump installed from factory and it ran hot. We had to shut her down just under 200 degrees. We the. Set the drive in a bin full of water up over the intakes and it ran fine at 145 degrees. At my house (well water) I can run my 5.7L b1 set up all day on muffs with out an issue. In the fall when the well water is cold and the air temp is cool I can barley get the engine operating temps.
 

alldodge

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I thought my boat had an issue because I started it on the muffs this year and after about 5-7 minutes the temperature started to climb real high. After lots of feedback I was ready to pull the riser/exhaust manifold but I decided to put the boat in the water for a test ride in the harbor before I did any wrenching. The engine ran cool and the riser never got over 100 degrees, and the manifold and block weren't hotter than 155 anywhere while in the water. The old gauge I had was reading about 20 degree's to hot, so I put in a new one and it maintained around 165-170. However, I was only out for about 20 minutes and I did not rev higher than 2000 rpm's since it was a no wake area.

I brought it back home to flush it on the hose and again it ran cool for about 5-7 minutes with the temperature maintaining around 170 stable on the gauge like it was in the water, then the temperature started to climb and I turned it off before it hit 190+. I know some say you shouldn't stay on the flush muffs for more than 5-10 minutes since they are not efficient at providing the engine with water the way it is designed to, but I have seen posts where people say they can run 30 minutes or more and never experience high temps.

Any idea why this may happen? My next test is going to be to run it hard to see if the temp climbs while cruising at high speeds and working the engine.

If you have an EFI or closed cooling engine the 165-170 is OK, if you have a carb and open cooling you should be running a 140 degree thermostat and 165 is to high.
 

enginesilo

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Jun 9, 2008
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Thanks everyone. I changed the impeller last season since I thought the boat was running hot but it turns out my gauge is 20 degrees too hot. In the water it stays cool but on the muffs it tends to creep. My water pressure I would say is pretty good, city water and it comes out strong. I am using the square shaped muffs since they seem to feed more than the crappy circle ones. Maybe I'll try again and see what happens but the last 3 or 4 times I ran on the muffs it only stayed cool 1 time for about 10 minutes, all other times it starts going too hot after about 7....strange.
 

ktbarrentine

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Dec 12, 2011
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Pressure is one thing, flow is another (and more important when on muffs). How much flow do you have at the muffs? How much flow out of the idle-relief ports? What is the temp of the water exiting the idle-relief ports?

Are you following the instructions in service bulletin 99-09 for using muffs?
 
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enginesilo

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Pressure is one thing, flow is another (and more important when on muffs). How much flow do you have at the muffs? How much flow out of the idle-relief ports? What is the temp of the water exiting the idle-relief ports?

Are you following the instructions in service bulletin 99-09 for using muffs?


The hose runs strong I would say, if you turn it on full blast with no nossle on the end, it would probably flow about 10" holding it vertical, so I don't think pressure or flow are the issue from the actual water hose. What are the idle relief pots, the 2 exhaust ports on the bottom side of the transom/bell housing area? One flows much stronger than the other. They definitely don't flow that much when the engine stats to get hot, but when it is running cool they seem to flow more. I don't think the muffs shifted or anything like that. Where can I find the bulletin on using muffs?
 

ktbarrentine

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The hose runs strong I would say, if you turn it on full blast with no nossle on the end, it would probably flow about 10" holding it vertical, so I don't think pressure or flow are the issue from the actual water hose. What are the idle relief pots, the 2 exhaust ports on the bottom side of the transom/bell housing area? One flows much stronger than the other. They definitely don't flow that much when the engine stats to get hot, but when it is running cool they seem to flow more. I don't think the muffs shifted or anything like that. Where can I find the bulletin on using muffs?



I hope this attachment comes through

(post-upload-edit) so....doesnt look like PDF attachments get to come through, either... I hope they get this fixed. Meantime, do a google search on "Mercruiser Service Bulletins" and it will take you to the boatfix site where you can look at and download all the bulletins by year. Lots of good intel there!

And yes, those are the idle relief ports.

What is the history of your thermostat? Cant remember if you said anything about that, but I would definitely spend a few bucks and change it out.
 
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enginesilo

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Jun 9, 2008
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I hope this attachment comes through

(post-upload-edit) so....doesnt look like PDF attachments get to come through, either... I hope they get this fixed. Meantime, do a google search on "Mercruiser Service Bulletins" and it will take you to the boatfix site where you can look at and download all the bulletins by year. Lots of good intel there!

And yes, those are the idle relief ports.

What is the history of your thermostat? Cant remember if you said anything about that, but I would definitely spend a few bucks and change it out.
The PDF attachment did not come through, but I found it and will post an image for others to reference. Thanks for pointing this bulletin out, it makes perfect sense.

My idle port relief valves at idle were very low flow when I tested it out last week, and I revved it a few times to get the water flowing. Definitely not a ton, so idling higher may do the trick which is obvious, but in the past I always flushed at idle without any problelms.

I usually bump my RPM's up to around 1100 when flushing but after a while I drop it back down to idle. Maybe that is where I am going hot? I'm going to test again keeping the throttle bumped just slightly per the service bulletins instructions and see if I can run 10 minutes. My next test after that will be running it out in the speed zone to see if it maintains cool.

I actually changed my thermostat last week while trying to diagnose this all so that is all set, and the impeller last season.

Here is the bullet in graphic format for anyone looking to reference this in the future.

EDIT: Attached image showed up tiny for some reason after iboats forum processed it. Here is a link:
http://www.boatfix.com/merc/bullet/99/99_09.PDF
 
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