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Author Topic:   Mercury 90 FOURSTROKE Cooling System
chuck21401 posted 05-08-2011 11:06 AM ET (US)   Profile for chuck21401   Send Email to chuck21401  
I have a 2008 170 Montauk with the 90-HP FOURSTROKE engine. When I flush the engine I connect a garden hose to the rear fitting as described in the owner's manual. Yesterday I flushed the engine after the first run of the season. In the past I seem to remember a small amount of water, along with exhaust, coming from the idle exhaust boot which is located just below the rear fitting. This time I only observed dry exhaust gasses coming from the idle exhaust boot. Is that right?

The idle exhuast boot can be see in this picture: jpg

And this picture (thanks to SCJoe, the original photographer)

I have the owner's manual and the service manual; I didn't find any useful information in manuals to answer this question.

GreatBayNH posted 05-09-2011 11:03 AM ET (US)     Profile for GreatBayNH  Send Email to GreatBayNH     
I use ear muffs to eliminate these types of questions. That way you know how the system should work because you're not by-passing it with the hose fitting.
jimh posted 05-09-2011 10:41 PM ET (US)     Profile for jimh  Send Email to jimh     
Is there any sort of pressure regulating device in your hose adaptor connection? If not, maybe the water pressure in the hose is variable, resulting in a variation in the volume of water introduced into the cooling system.
L H G posted 05-10-2011 04:04 PM ET (US)     Profile for L H G    
I don't think the engine is supposed to be running when you use the hose flushing port. At least I don't when I'm flushing my 200 EFI's. I just turn up the water pressure until the pressure gauge shows 10PSI and let the water run for about 5 minutes. That's all that is needed.
bben posted 05-10-2011 09:48 PM ET (US)     Profile for bben  Send Email to bben     
I would only use the earmuffs. You may get a little wet but it will flush top and bottom as if the boat were in the water, talking in fresh water from the muffs. I was told never to use the hose fitting for flusing even though the manual says it is ok.
chuck21401 posted 05-11-2011 10:08 AM ET (US)     Profile for chuck21401  Send Email to chuck21401     
>>> I was told never to use the hose fitting for flusing even though the manual says it is ok.

Why? It seems to me that this fitting was specifically designed by Mercury to facilitate flushing the engine?

chuck21401 posted 05-11-2011 10:16 AM ET (US)     Profile for chuck21401  Send Email to chuck21401     
Here is a link to an excerpt from the owner's manual for the 75/90/115 FourStroke models regarding maintenance. maintenance-75to115FS.pdf

The document includes a section on "Flushing the Cooling System." See page 4 of the PDF document.

The legal department must have helped write the manual; Merc recommends removing the propeller each time the engine is flushed.

chuck21401 posted 05-23-2011 12:40 PM ET (US)     Profile for chuck21401  Send Email to chuck21401     
Update: I used the boat this weekend. Water does not exit from the idle exhaust boot when the engine is runing with the boat in the water.

Again, I wasn't sure if this was normal or not so I checked youtube for videos of other 90 FourStroke engines running.

I found this video; it appears that my concern is a figment of my imagination.
(after 26 seconds)

sapple posted 05-24-2011 08:26 PM ET (US)     Profile for sapple  Send Email to sapple     
If it matters, I have this engine on my 170 Montauk. I flush it with the garden hose all the time, running it at idle speed for 10-15 min., just like the owner's manual recommends. If this results in any sort of problem, I am not aware of it.
whale posted 05-25-2011 06:54 PM ET (US)     Profile for whale    
I was told by the dealer, If I wanted to flush the entire system, I must use the muffs. The flushing connection is designed to be used at the marina while the boat is in the water. it flushes the upper portion only and not the lower half.

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