Water pressure gauge question

[Blue Gray in PA]

My water pressure gauge didn't match the others so I put a new one in this past week. Now the pressure is showing much lower than the previous gauge did. My previous gauge used 1/8" OD nylotube for the feed and I swapped the fitting on the new gauge from the 1/4" barb fitting it came with and used the 1/8" nylotube. Prior to hanging the motor, we installed a new impeller and it seems to get a decent stream of water out. My concern is that it shows no pressure at idle and about 8 PSI at the same JP and trim settings as I previously saw 15 PSI.

I know the water does not come all the way through to the gauge and pressure is pressure but could the tube size be causing the difference or is it possible the new gauge is more accurate?

My next step would be to swap to the 1/4" tubing and try it but I thought I would solicit any thoughts here first. Any suggestions or ideas guys?

 

[froggy]

Blue Gray,

There is such a thing as pressure drop through piping. 1/8" tubing would have substantially more pressure drop (I'm assuming the tubing runs from the engine to the dash or about 10' - 15' ) than 1/4" tubing. If the gauge is designed for 1/4" tubing it is likely calibrated for the lower pressure drom VS 1/8" tubing.

Just my guess.

froggy

 

[chad202]

I would think it's just difference in gauges. In a closed system where the water in the tube isn't passing the gauge(just up against it), the size shouldn't make a difference.

 

[FlatOutAlly]

Sounds like a hose leak somewhere.

 

[Blue Gray in PA]

I was wondering about that too but the hose is all new and the fittings are tight. When I first started it, it showed no pressure but slowly increased after a few minutes. I guess I should try the 1/4" hose and see what it shows...

 

[newell]

Since I am going through this right now but caused by a different issue. I couldn't be coincidence that your impeller is starting to go as well could it? That is the way my gauge acts right now. But, I really don't have pressure.

Just a thought.

 

[Blue Gray in PA]

We put a new impeller in prior to hanging the motor a few weeks ago. I don't think that is the problem since it showed good pressure with the old gauge and much lower pressure with only a gauge change - unless the old gauge was giving false high readings. I was hoping to swap to the 1/4" hose last night to find out for sure but other things took precedence. Maybe Friday when I get back home for the long weekend...

 

[Blue Gray in PA]

Used the 1/4" hose this time and it didn't seem to make much difference on the pressure readings. Water is still coming out the back but not with as much force as I have seen with my smaller motors. I am thinking there may be more water passages that have to fill and this would affect how much water can get out at idle. I have now swapped out a CLE w/Bob's nosecone lower unit I have and installed a complete new water pump unit prior to closing it up. Hope to have more to report after the weekend is over.

 

[fishnfireman]

As stated above... there will be no friction loss in the tubing since the water is not moving..and pressure is pressure regardless of the size of the tubing..
I have had a lot of different boats over the years and the variance between gauges has been substatinal...
A regular pressure gauge works the same as a speedo. it has a bourdon tube that is made of brass and is curved. As the water pressure incresses it straightens the tube. thus moving the needle. They are not very accurate and can be affected by temp and age.

With this said, if you see a change with the same gauge. I would say odds are you have a problem... If you change gauges there is a good chance it is just the difference between the gauges.

So long is you have water out the pee hole and it's not getting hot.. I would say your OK !! These new mtrs' will tell you when they get hot with a warning buzzer long before any damage is done.

 

[Blue Gray in PA]

Thanks FNF. I ran it a little yesterday and the pressure hovered around one or 2 PSI at idle, 5PSI at 3K and I got around 10 PSI at 6K. The temperature gauge never went above 130 degrees so I guess that's going to be the more important gauge to monitor. I did witness water coming out both pee holes, even at idle so I know it's getting flow. It's interesting to know how the gauge works since the older one probably is more weak from years of bending back and forth and would therefore move more easily. I guess as long as the temp doesn't go above 180, I'll keep flying her high and watching for debris - or jetskis...

 

[www.stvowners.com]

On the 280 if water PSI isn't above 12psi(from memory) above 5k then the ECM backs the engine down. It's in the service manual.

 

[Blue Gray in PA]

I remember you telling me that Larry and that's what keeps me from worrying too much. Hopefully I will have time to check the owners manual to see what the minimum pressure is before it shuts off and not find out the hard way.

 

[mtolley]

a one piece plate and hipo motors dump so much water that you wont see alot of pressure idling. non of the 280's or 260's i have show much pressure at idle. they only read @ 12-13psi at 7-8000rpms also. ecm only cuts off consumer motors for low water pressure, race motors dont have the pressure sensor, but both will shut off before they get too hot and will not start back up until they cool down. mt

 

[Blue Gray in PA]

I'm not sure what if any impact it has on pressure, but my mid-section has the 3/4" water tube not the smaller one. I found this out when swapping the stock Sportmaster LU for a CLE and the pump housing had to be changed. The pump top is shorter and the output diameter bigger but they use the same impeller and lower guts as the other unit. I do see water coming out both sides even at idle and the temp is staying down so I will keep an eye on the temperature more so than the water pressure.

 

[mtolley]

i would think that the big water tube would allow more volume and less pressure than the small tube does. my hipo motors also have the big tube.
just for the info, when its time to replace the upper pump housing, a regular v6 housing can be modified in 10 minutes for the big tube instead of paying the 100 bucks or so for the hi-po specific one. also i saw on one of the boards that someone was selling the the water tube guides from aluminum. merc wants a premium for these as well. mt

 

[Blue Gray in PA]

I was wondering about that too. The OD of the smaller tube housing is a little smaller than the big tube housing. The sleeve that fits over the housing instead of inside like the smaller one is a little loose on the other taller housing. SSTom makes the metal tubes but only for the small one right now - he did say he is working on making the collar style soon. Other than that, I guess you could just cut about 1/2" off the housing and slide the sleeve over it with some RTV and it should work fine.