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Author Topic:   Flushing 90 HP 4-stroke Mercury - water exit?
Northskiff posted 10-12-2005 10:43 PM ET (US)   Profile for Northskiff   Send Email to Northskiff  
I have a 2005 90HP Mercury 4-stroke on a Montauk 170. I boat in clean fresh water and do not flush during the season.

However, as part of winterizing the engine I hooked up the garden hose and turned it on with moderate flow, as directed in the Service Manual. The engine was vertical. Water was running out by the prop, as I would expect.

I started the engine, and water continued to run out of water/exhaust exit at the prop. No water came out of the tell-tale.

I was alarmed, and turned the engine off.

Is it normal to NOT have water come out of the telltale when flushing the engine?

fishin8tor posted 10-12-2005 11:41 PM ET (US)     Profile for fishin8tor  Send Email to fishin8tor     
I have the same 90hp engine and had the same result when I was flushing the engine. I guessed it was just normal to not have the tell tale coming out of the engine if you use the flushing attachment.
bigjohn1 posted 10-13-2005 07:14 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
You are only supposed to start the engine when you are flushing with the traditional style muffs over the lower unit. When you use that hose attachment, the engine is not supposed to be running. What you experienced is normal; do not start the engine when using the hose screwed into the back of the engine. If this is in conflict with your manual, contact Merc and request clarification on this.
mikeyairtime posted 10-13-2005 09:04 AM ET (US)     Profile for mikeyairtime    
The manual says to run the engine with the hose plugged in. The socket that the hose screws into is the same one the tell tale comes out of. When I remove the hose from mine the tell tale sometimes still has enough pressure to run a little.
high sierra posted 10-13-2005 10:29 PM ET (US)     Profile for high sierra  Send Email to high sierra     
On the 90 and 115 merc's , when you hook up to the pee hole, you are flushing the motor in reverse and should not be running the motor. When you put muffs on it, you run the motor. That's the way I interpret the factory manual. high sierra
jimh posted 10-14-2005 12:23 AM ET (US)     Profile for jimh  Send Email to jimh     
It is not a good idea to run the motor if the water pump is not immersed in water. The rubber impeller is lubricated by the water, and running it dry is not recommended, even for very short periods.
surfkast posted 10-14-2005 02:05 AM ET (US)     Profile for surfkast    
On the 2006 90hp Mercury 4S/EFI manual, it clearly states to flush the cooling system by attaching a hose to the rear fitting and run the engine at idle in neutral for at least 5 minutes, and even has a picture of the hose going straight into the engine. It says "Important: The engine must be run during flushing in order to open the thermostat and circulate water through the water passages."

This is how the dealer told me to flush it as well, and he gave me a quick-disconnect for the hose fitting to make it easier. Yes, water just runs down the propeller when I do it.

swist posted 10-14-2005 05:44 AM ET (US)     Profile for swist  Send Email to swist     
"This is how the dealer told me to flush it as well, and he gave me a quick-disconnect for the hose fitting to make it easier. "

I hope he really "gave" it to you as opposed to selling it to you. The flush fitting is a standard garden hose female connector, but it is awkward to connect a hose because the tightening ring does not turn, so you have to turn the whole hose instead.

The Mercury "adapter" costs $40 but you can make one yourself with a few bucks worth of hardware store hose fittings.

swist posted 10-14-2005 06:01 AM ET (US)     Profile for swist  Send Email to swist     
The diversity of replies to this question is a bit unsettling. Are people suggesting that the procedure in the Mercury manual is wrong and will damage the engine and or impeller?
surfkast posted 10-14-2005 05:39 PM ET (US)     Profile for surfkast    
Yes, he gave it to me free, just as a convenience item. I asked him about the ear muffs and he said I didn't need them. I agree the difference in responses here is a bit unsettling. I'd like to do it the best way.
Barney posted 10-14-2005 07:50 PM ET (US)     Profile for Barney  Send Email to Barney     
jimh said "running it dry is not recommended". That is correct and we aren't running the pump dry when using the flush attachment. Water is flowing when using the attachment and the owner's manual is correct. Barn
Barney posted 10-14-2005 08:05 PM ET (US)     Profile for Barney  Send Email to Barney     
old discussion here:
Barney posted 10-14-2005 08:20 PM ET (US)     Profile for Barney  Send Email to Barney     
I should have said "factory flush port" versus flush attachment. Barn
mikeyairtime posted 10-15-2005 12:47 AM ET (US)     Profile for mikeyairtime    
Like I said...Screw the hose in the fitting and run it. Sheesh!!!
bigjohn1 posted 10-17-2005 10:42 PM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
Yes, much diversity in the replies here. In the end, we will all do what we think best...I personally think Merc was out to lunch when they printed your manual. It makes no sense to me for a person to run the engine when using the rear hose attachment as the impeller is running dry with no lubrication from water flow. My powerhead is Yamaha (as is every other late model Merc 90/115 owner here) and Yamaha says not to run the engine when using the hose screwed in to the rear of the engine. What do I do? use flush muffs and run the engine...tough call, do what you feel is best.
95Outrage17 posted 10-17-2005 11:09 PM ET (US)     Profile for 95Outrage17  Send Email to 95Outrage17     
I flushed my '04 Merc 90 4-stroke with muffs today. I ran it for about 5 minutes with the motor in the vertical position of course. The whole time I kept my eye on the telltale which was strong. I've flushed the motor a few times this way and it seems to work perfectly, I see no reason to use the telltale hookup unless the motor needs to be flushed while the boat is in the water.

- Chris

Barney posted 10-18-2005 07:41 AM ET (US)     Profile for Barney  Send Email to Barney     
John, I just want to make sure the correct information gets out.
1. The manuals have been the same for at least three model years, maybe longer.
2. When flushing with the port, water comes out of the water pump intake and the prop.

The pump has to be wet with water coming out of the intake. This setup is different from Yamaha and Honda. Don't ask me why Mercury did it. This subject has been nothing but confusing to folks. Barn

mikeyairtime posted 10-18-2005 09:09 AM ET (US)     Profile for mikeyairtime    
You're correct, the powerhead is a Yamaha but everything else is Mercury. The impeller is not running dry while flushing through the garden hose attachment point that was designed by a Mercury engineer to flush the motor with the engine running. As I said before when you remove the hose from the flushing port the tell-tale will run for several second. Again I say....Sheesh. This board could debate how many toes is the proper amount for the average human.
bigjohn1 posted 10-18-2005 09:12 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
Thanks Barn - and I agree, its very confusing
Whalerider posted 10-18-2005 11:37 AM ET (US)     Profile for Whalerider  Send Email to Whalerider     
mikeyairtime hit the nail on the head.
Barney posted 10-18-2005 01:22 PM ET (US)     Profile for Barney  Send Email to Barney     
Whew, I hope everyone has got it. Barn
Barney posted 10-18-2005 01:54 PM ET (US)     Profile for Barney  Send Email to Barney     
And I probably should have said in the beginning that I agreed with mikeyairtimes. Maybe it doesn't appear that way, but I have since the beginning of this discussion. I have used the port for a couple of years while the boat was on a lift. Barn
Maximus posted 10-18-2005 03:58 PM ET (US)     Profile for Maximus  Send Email to Maximus     

Glad to see you made it through the hurricane.


Having owned a 2003 90 4 stroke mercury for since new, I flush the engine after every use. I simply tilt the motor up, put the ear muffs on and let it run for 5 minutes or so, in the tilted up position. I try to do this as soon as I hit the dock as I want the motor still warm and the thermostat open.

This being said, you may expect the inner cooling passages to be free from salt deposits...well not exactly. At 100 hours, I had the water pump replaced and found the parts to be coated with salt. Not just a light coating, the parts were practically white. Besides learning the water pump didn't need to be replaced at 100 hours (3 years), I found that my religious flushing practice may be all for not.


Whalerider posted 10-18-2005 04:47 PM ET (US)     Profile for Whalerider  Send Email to Whalerider     
Something to think about.

My brother and I are as different as can be. About the only thing we have in common is we are both avid fishermen. He is a hardcore offshore fisherman, I tend to stay more inshore. I am just about as anal as they come when it comes to the condition of my boat and fishing equipment, he on the other hand is NOT. I can spend hours making sure that the caulk line around the latest addition is as perfect as possible, he will just glob the cheapest silicone he can find somewhere CLOSE to the screw and let the excess drip down the side of his boat. GAWD…I cant stand it. I honestly don’t think he has ever washed his boat, I know he has NEVER flushed his engine. Get the picture?

I am about to take my boat in for its 100 hour maintenance, I told him I was going to change the water pump and plugs. He replied….why …are they broken? His current boat has somewhere around 800 hours on the engines….never a new water pump …had the plugs changed about 2 years ago and he said he changed to oil in the lower unit once. Well… I can testify that his engines run almost perfectly, they start up just as easily as mine and run just as smoothly. They have never broken down, he has spent almost nothing on maintenance and even less time. All his boats have been treated this way and he never had any more problems than I have.

I think that our modern engines are ALOT tougher than we think and really don’t need all the things we do for them (expensive oil, fuel additives, etc.). Look at the E-Tec engine, they claim no maintenance for 3 yrs?

I will continue to pamper my boat and do everything I can to keep it as new and as perfect as I can. He WON’T. In the end it seems that his works just as well as mine…his looks like crap but works just as well. Me thinks we are all too worried about the details…as he says…crank it up and go have some fun. I wish I could.


swist posted 10-18-2005 09:11 PM ET (US)     Profile for swist  Send Email to swist     
As in all things, moderation is probably the right approach. Your brother is not a statistically significant sample of outboard owners who do not properly maintain their engines. I'm sure you could also find at least one maintenance fanatic whose powerhead blew after 50 hrs.

I do the stuff in the owner's manual and no more - sometimes I'm a little late - I don't get crazy about stuff that can't really harm the engine - like spark plugs. It is interesting that a lot of owner's manuals don't give replacement frequency for things like batteries, impellers, themostats, fuel pumps, etc. That's why there are so many threads here and in other forums on how often to do this stuff - nobody really knows. The maintenance nuts do them all every two years and claim they'll continue because they never had a problem, not knowing whether or not they would have had a problem if they waited 5 or 10 years....

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