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Author Topic:   Mercury 90-HP: Water Pressure
sapple posted 05-21-2008 11:56 AM ET (US)   Profile for sapple   Send Email to sapple  
I have a 2007 170 Montauk with the Boston-Whaler-supplied Mercury 90-HP FOURSTROKE EFI motor. The boat has a water pressure gauge. What is the normal [water pressure] and how low a reading is a signal that water intake is obstructed?
gbcbu posted 05-21-2008 06:09 PM ET (US)     Profile for gbcbu  Send Email to gbcbu     
I have an 2006 170 Montauk and the gauge is getting replaced. First thing to check with or without a gauge is the [water output from the cooling system confidence stream] to make sure water is circulating. I check mine when I first turn on the motor. At idle the needle will register just to the first mark. I can't remember the actual reading. As you increase the [engine speed] the reading will also increase. I asked this very question last year when I first put the boat in. I got some helpful responses so you might get more and accurate info if you do a search of this site. Good luck
BlueMax posted 05-22-2008 12:08 AM ET (US)     Profile for BlueMax  Send Email to BlueMax     
I have a 2007 170 with 90-HP Mercury FORSTROKE EFI. My pressure readings are 2-PSI and lower at idle (on flush or in the water), between 8 to 12-PSI from 1,000- to 3,000-RPM, 15-PSI and better at 3,600 to 3,800-RPM (my normal running/cruising rate), and up to 18 to 20-PSI at 5,000-RPM.

I did a site search and found a quote from Whaler1234 posted on 7-24-2005:

For reference, these measurements for Water pressure are from the 75/90HP model year 2000+ Merc service manual.
RPM Pump Pressure (PSI)
1000RPM ~6PSI
2000RPM ~8PSI
3000RPM ~12PSI
4000RPM ~15PSI
5000RPM ~17PSI
6000RPM ~20PSI

So it seems my readings are right in line except maybe for idle - which is interesting. I do always check for the stream at any speed (paranoia) - though sometimes I have to hold the muffs on tight by hand when flushing to get the water to come out and the stream is often weak (although it is quite strong when I am actually on the water and idling). I wonder if I am just not picking up water properly when on flush?

jimh posted 05-22-2008 12:31 AM ET (US)     Profile for jimh  Send Email to jimh     
As several have noted, the water pressure reading varies with the engine speed, so there is not a particular lower threshold that indicates a problem. Rather, you should be alert for a water pressure that is lower than the normal pressure you have been seeing. If the water pressure drops below the normal reading, this is a good sign that there is something awry in the cooling system. You can often detect that a water intake has become blocked from the immediate drop in cooling system pressure.

sapple posted 05-22-2008 11:21 AM ET (US)     Profile for sapple  Send Email to sapple     
I always check for the water stream comming out of the motor. Unfortunately, it is a lot harder to see on the 170 Montauk + Merc90 compared to my former 130 Sport + Merc40.

Your replies have ansered my question, thanks for responding.

gbcbu posted 05-22-2008 12:26 PM ET (US)     Profile for gbcbu  Send Email to gbcbu     
As a side note... I was informed that I could run the boat while waiting for a new gauge. The company that services the boat had a technician check it all out and was able to determine that all is well with the cooling system and the system's confidence stream.
I couldn't agree with you more sapple that it is very difficult to check the "cscs" when underway in the 170 Montauk.
swist posted 05-27-2008 08:22 AM ET (US)     Profile for swist  Send Email to swist     
Other threads in this forum have also noted that this guage is a mechanical prssure gauge and is subject to clogging quite easily. When I first got my 170, it was the first time I'd seen a water pressure gauge on a small boat.

Would it not make more sense, if they want to give you more information on the state of the cooling system than the telltale stream, to provide a water temperature gauge, as in a car? Such gauges are completely electrical and probably a lot more reliable. And I might note that these outboards have an overheat alarm so they already have an engine temerature sensor!

Granted they are not measuring the same thing but in this case I'm not sure why anyone would care that much.

Clyde posted 05-29-2008 09:21 AM ET (US)     Profile for Clyde  Send Email to Clyde     
I have both on my boat and wouldn't have it any other way. More info is better in my humble opinion.

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