impeller water pressure

[djvan]

Mercruiser 3.0 alpha 1

If I were to put a low pressure guage in the hose that goes from the outdrive to the therm. housing what kind of pressure should I see?

Would this be a good way to keep track of impeller condition?

Thanks

DougV>

 

[achris]

Re: impeller water pressure

Now that's a question I've never been asked...

The problem with a pressure gauge in the cooling system is that there are many other things that will affect the reading.... As the exhaust elbow rusts up it will restrict flow, raising pressure, as the impeller becomes older and less efficient the pressure will drop.. these together will make it look like everything is fine... which would not be the case....

Chris..........

 

[45Auto]

Re: impeller water pressure

My 120 HP outboard and 425 HP I/O both read just over 3 PSI at idle and around 12 PSI at full throttle. I think it's an excellent indication of impeller problems or cooling system problems if you start getting low readings or see a sudden change.

 

[djvan]

Re: impeller water pressure

Thanks guys I guess I'll add a low pressure guage to my want list.

I'm a sucker for gizmos and gadgets.

DougV>

 

[79Glastron]

Re: impeller water pressure

I think a measure of volume would be a better method. Something like liters per second, or gal per second. I know OMC had values for checking the flow from the main feed hose from the drive to the thermo housing for engine idling. I would figure merc would have a similar value found somewhere.

 

[JustJason]

Re: impeller water pressure

79 Glastron is on the right page with this one.
I know i've seen pressure readings somewhere in some book, but its not how merc wants you do diagnose flow, so it's not commonly refered to.
Its all about the GPM baby. and being free of air bubbles.
The other thing is, is that merc makes 2 types of pumps, high pressure low volume, and high volume low pressure. So if you work on a lot of different boats you would need to know what kind of pump it has.
its to much work to do it that way.

Now... a water pressure gauge in your helm can be a good thing if your a guy that needs to have everything.
It should always be used in conjuction with a water temp gauge.
Water temp is always more important. but water pressure can be usefull especially if you boat someplace there's a lot of weeds. Its more of a bass boat thing but you can put one on anything.
The way it works is it obviously shows the psi of the incoming cooling water. what your looking for when you look at it is a drastic drop of that reading from what it normaly is. if you get one that usually means the raw water pickup passages are blocked. It's a neat little gauge to have because it will show a problem instantly, whereas the water temp gauge may take a few moments to register.

 

[achris]

Re: impeller water pressure

When setting these sorts of things up it's good to go 'back to basics'... That said, what is pressure? Fluid pressure is a restriction of flow. To explain... You may have a massive flow, but if it doesn't come up against a restriction it's pressure is 0. Conversely you can have a very small flow and a high restriction which would give you a high pressure....

Not sure what i mean? Go out to your garden hose and try it....

The upshot is that pressure is almost meaningless.... without knowing what the flow is.

Chris...........

 

[45Auto]

Re: impeller water pressure

The upshot is that pressure is almost meaningless.... without knowing what the flow is.​

Funny, I find oil pressure to be one of the more useful indicators of an engine's overall condition. Have no idea what the flow is, never worried about an oil flow spec that I can think of.​

Knowing your pressure in your cooling system is analogous to knowing the oil pressure in your lubrication system. Chances of something clogging up the outflow causing pressure to rise at the same time something clogs the input causing pressure to fall are about the same as getting hit by an astoroid. I don't worry about either. Either abnormally high or abnormally low pressure is a sign that something is wrong.​

The primary reason gauges measure pressure is because it's easier to measure than flow. If you know the size of your restriction and EITHER flow OR pressure, you can calculate the other one if it makes you happy. But as I said, within an operating engine system it's much easier (and cheaper) to measure pressure than flow.​

When I was running an outboard I found myself looking over my shoulder all the time making sure water was coming out of the pee hole (lots of shallow, muddy bottom and weeds to clog the intakes around here). I found it much easier just to glance at the water pressure gauge once I installed it in the dash to verify that the motor was getting water. I prefer to find out something's wrong with my cooling system BEFORE the overheat buzzer goes off! My I/O has a buzzer that goes off if the water pressure is out of spec, but I'm not sure what that spec is.​

 

[djvan]

Re: impeller water pressure

Can I run a guage to my console? Electric or mechanical? I was thinking of just installing on the front of my doghouse and glancing at it occasionally. But if it could be put by my other guages BONUS!!!!

I was thinking if I could establish a normal reading, if it one day started running high or low I could start to investigate.

I like the flow idea also, hadn't thought of that. Is there a relatively easy way to guage flow? I haven't looked yet but I don't remeber seeing flow guages anywhere in my travels.

DougV>

 

[achris]

Re: impeller water pressure

Flow gauges tend to be quite a bit more expensive and subject to problems in an unfiltered environment... One of the best systems I've seen is one the commercial operators use... Run a piece of thin tubing up to the console and point the end overboard... You see water, no problem... water stops, stop and investigate.....

Chris............

P.S. The reason engine oil pressure gauges work so well is because they are in an environment that is not subject to the restriction of flow changing... like rusting up (Exhaust manifolds and elbows do rust up)

 

[abj87]

Re: impeller water pressure

exhaust temp gauge <--- first indication of a flow problem is the exhaust system overheats.

 

[45Auto]

Re: impeller water pressure

Here's an article on a guy putting one on an outboard. Same process for an I/O, you just need to find a handy fitting to screw a Home Depot pipe-thread barb fitting into. My I/O had pipe-thread holes with plugs in them that were intended to be used with a heater. Since I don't have a heater in the boat, I took one of the plugs out and used the hole for the water pressure fitting.​

http://www.diy-boat.com/index.php?option=com_content&task=view&id=151&Itemid=49​

The pressure gauge will come with about 30 feet of small diameter tubing that you run from the fitting on the engine to the back of the gauge at the dash. I found a gauge that matched the rest of my gauges, looked factory once it was installed.​