Water pressure gauge decision Mercury outboard

bobdec

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I want to add a water pressure gauge to my dash and would like feedback on what pressure range to purchase. Its a 1996 Mercury 150 HP 'Offshore'. The service manual says pressure varies from around 3 PSI at idle to +12 PSI at WOT, so I was thinking either 0-15 PSI or a 0-30 PSI gauge. What are you Merc OB owners running, any recommendations. Is a 0-15 to low for higher RPM operation.
 

Texasmark

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Re: Water pressure gauge decision Mercury outboard

Back when I was industry, the norm was to size a gauge so that it ran near half scale in normal operation. That gave you room for unnatural occurrences. O-30 may be easier to find.

Mark
 

Silvertip

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Re: Water pressure gauge decision Mercury outboard

Both 15 and 30 PSI are easy to find. If the gauge is normally reading near full scale (12 psi on a 15 psi scale), you have more precise indication of slow degradation of the impeller. If you are gradually seeing 1 or 2 PSI drops, that is more noticeable on a 15 PSI gauge than it would be on 30 PSI gauge. In other words, half the scale on a 30 PSI gauge would never be used. Temperature gauges have a similar issue. Unless the scale starts at something under 100 degrees, (the best but hard to find start at 60 degrees) the needle hardly gets off the cold peg.
 

Rodbuster

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Re: Water pressure gauge decision Mercury outboard

I want to add a water pressure gauge to my dash and would like feedback on what pressure range to purchase. Its a 1996 Mercury 150 HP 'Offshore'. The service manual says pressure varies from around 3 PSI at idle to +12 PSI at WOT, so I was thinking either 0-15 PSI or a 0-30 PSI gauge. What are you Merc OB owners running, any recommendations. Is a 0-15 to low for higher RPM operation.

I put one on last year. 0-30 Mine runs about 6 psi at idle 12 psi running. I think I am in a good range if something goes wrong I will see it.
 

H20Rat

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Re: Water pressure gauge decision Mercury outboard

So on gauges that you are concerned about a low reading, you want as small as a scale as possible. I don't care that it is reading 18 psi, but I DO care if its 5 when it used to be 15. (and of course, reverse the logic for gauges that you need to monitor for the high end... think exhaust gas temp.)
 

Frank Acampora

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Re: Water pressure gauge decision Mercury outboard

I will be the lone dissenter here. You are not worried about a gradual drop since you will notice it before any real damage is done. You are worried about a sudden loss of pressure--think a plastic bag blocking the intakes.

Typically, if you can arrange it, you set up analog gauges so that at normal operation all pointers are the same or centered.

That way, when you glance at the dashboard, if all is normal, you don't need to interpret. All needles pointing the same---everything good. It takes only a glance. Much less time and thought required.

SO: If your tachometer and speedometer are relatively centered at whatever speed you normally run at, then get a gauge with a range that will match the others.
 

Texasmark

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Re: Water pressure gauge decision Mercury outboard

Thanks again Frank,
Mark
 

smokeonthewater

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Re: Water pressure gauge decision Mercury outboard

I'M gonna take yet another path.... I say get the 0-15.... if for some reason you make 17 psi and peg it... who cares.... but if it starts to fall you want to absolutely see it moving... If you want the needles pointing the same direction like Frank (and a LOT of speed freaks) then rotate the gauge body so that the needle in normal use points in whatever direction the others point.... 0-15 is the CORRECT scale for a 3-12 operating range.

IMHO of course :D
 

smokeonthewater

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Re: Water pressure gauge decision Mercury outboard

BTW on mine I'm gonna set it up with a 0-15 gauge AND a 2-3 psi pressure switch with a light and audible alarm
 

bobdec

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Re: Water pressure gauge decision Mercury outboard

Thanks for all the responses... What prompted wanting a water PSI gauge is my temp goes up from 160*F to over 180+* F when running at low RPM (1000-1200) pulling a load . Towed two boats in last season a 20 ft pontoon about a mile and an 18 ft runabout a bit less. Temp was still climbing until I backed off at the dock(s), then it cooled down when I dropped RPM down to idle w/o the load. Done this many times in the past with I/O's and my inboard and never had any temp problems. Opened up a thread in Mercury OB forum and was advised that is normal for a two stroke loaded down at low RPM. I addition to knowing when I snagged a plastic bag I want to monitor the water delivery system at low RPM. Under normal operation it seems fine heats up to about 145 at cold start , stays there at idle speeds runs 145-165 at cruise and WOT depending on water surface temp. I ordered the Merc 0-15 PSI (p/n 79-858650A) to give a better pic of the low end PSI, let you guys know how it works out. My next question will be what's the best hose to run to the gauge ?

smoke, like the idea of a loss of flow alarm, going to search out low pressure switches..sounds easy to wire something like that up..
 

Silvertip

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Re: Water pressure gauge decision Mercury outboard

I guess the deal with gauges always pointing in the middle of the scale on a boat is rather meaningless unless one is into racing. The tach may be at cruise rpm with an I/O but not with an outboard, the speedometer rarely is, the fuel gauge is only when the tank is half full and that's not accurate either, oil pressure isn't usually at cruise, wot, nor idle, and therefore what use is a 30 PSI gauge when the engine is not capable of putting the needle in the upper half. If the engine is capable of over 15 PSI then this is all moot. Most folks don't look at gauges often enough to notice a sudden drop until the alarm goes off at which point they notice the low/no water pressure. They are more apt to noice subtle drops with each use as the impeller goes bad. I look at the 30 PSI gauge like a 140 or 160 MPH speedometer on most family sedans these days. Pegging the needle is wishful thinking unless you are being pushed by a Corvette, Viper, or a NASCAR Sprint Cup car.
 

dingbat

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Re: Water pressure gauge decision Mercury outboard

Uhh..... the OEM water pressure installation kit for that motor comes with a 0-30 PSI gauge. ;)

The full-scale pressure of the gauge selected should be approximately two times the intended operating pressure. The gauge should have a full-scale pressure such that the operating pressure occurs in the middle half (25% to 75%) of the scale, (12 o'clock position is best). Application specific (scaling) dial faces are frequently used to meet this requirement.


12041-1-ti.jpg

I ran a Merc 175 Hp for a number of years and the gauge is more a diagnostic tool than any thing. The water pressure on the upper end is just as important, if not more important, than the lower end when it come to diagnosing a cooling system problems.

The "typical" operating range may be from 3-12 psi but those are not the min and max. pressures you'll encounter on that motor. Is your your poppet valve kicking out at the right pressure? Partially closed thermostat? Have something restricting stuck in the tell-tale tubing or nozzle?
Silt in the heads?

 
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