Water pressure question

[glrichards]

I just put a water pressure gauge on my boat. Out on the water it idles at 2-3 lbs at 2000 rpms it has 10 lbs any more rpms it falls to 8 lbs and stays there is this normal or should it have more pressure at higher rpms. Its a 90 hp 3 cyl. 2 stroke mercury

 

[rickdb1boat]

Re: Water pressure question

Every one I have had increases as RPM increases. You may have a water pump that is getting weak. Best to install a new pump and then you will have good baseline readings to refer to later on...

 

[glrichards]

Re: Water pressure question

Thanks Rick I guess it is time to go to work. I have a manual and it is probly about time to change the impeller the motor is 2 years old and has been on the water almost every other weekend.Thanks again

 

[Silvertip]

Re: Water pressure question

At idle the water pressure should be in 5# range and at wide open throttle it should be in the 15 - 17# range. My water pressure gauge is plumbed like this:

 

[glrichards]

Re: Water pressure question

Thanks Silvertip I did mine the same as yours after seeing your post over the weekend. Am leaving now to find a new pump assy. thanks again

 

[glrichards]

Re: Water pressure question

Put in a new water pump assembly and now the motor has 5-6 LBS at idle and about 10 lbs at 2000 rpms above 2000 it go's up to 13 LBS then drops back to 10 lbs and stays there.
Is there somthing that I am missing here could the thermostat or poppet valve be bad.
I even tried a seperate gauge and it had the same readings even changed the hose from the motor to the gauge with no differance.
Is 10 LBS enough pressure will it hurt the motor at this pressure

 

[Silvertip]

Re: Water pressure question

No two motors are the same so not getting idential numbers is not unusual. However, yours do seem to be ok up to 2000 rpm. Up to that rpm the bypass valve is still closed and what you are seeing is after that the valve opens which would drop the pressure some but it should build up again. I suspect there are differences in impeller quality (OEM vs Aftermarket) but how much that affects pressure I can't say. If you changed just the impeller and the housing has some grooves in it, that would account for the lower readings.

 

[glrichards]

Re: Water pressure question

Thanks again Silvertip
I just changed the impeller and used an O.E.M impeller from the merc dealer.
The inside of the housing was smoth as a babys butt and the plates were in specs.
If I can find one I will change the whole pump assembly. Need to find one tommorrow so we can get out on the water this weekend.
I am retired so it gives me somthing to do. Have plenty of time and no money.

 

[gss036]

Re: Water pressure question

I think you are just fine w/your pressures. The drop in pressure is the poppet valve opening, dumping excess water through the exhaust. Sounds like your pump housing is just fine. Go boating and enjoy yourself.
My 200HP Merc only gets 2-3 # @ idle and 10-12 at 42-4400 rpm.

 

[Silvertip]

Re: Water pressure question

I agree. If the pump housing was fine and you used an OEM impeller, you should be good to go. The only other aspect would be if the water tubes did not seal well when you reinstalled the lower unit. That would allow some leakage. My only concern with this that my 75 HP and your 90 HP engine share the same block. I would expect the numbers to be a little closer but not necessarily identical. Your numbers represent a 33% difference from mine.

 

[glrichards]

Re: Water pressure question

Well I fixed it. Checked the thermostat and poppet valve just for maint. and the thermostat was on the out side of the gasket instead of in the grove in the head and the poppet valve spring was curled up inside the diaphragm.
checked the thermostat and put a new spring in the poppet valve installed them accourding to instructions in the manual and fired it up.
the motor always did run a little cool now it comes right up to temp. and the water pressure idles at 6lbs and at 2000 it holds at 14 lbs a lot of differance.
the motor is only 2 yrs old bought it new first time thermostat cover has been off must have been a bad day at the factory.