Mercury outboard water pressure help

[codyclyatt]

I have a 1995 Mercury 90hp outboard on a little Skeeter that I just bought. I put a new impeller in it and a new water pressure gauge with new tubing, that is hooked in the block at the designated plug above the spark plugs. Does anyone know what pressure I should be running? It pees great but no matter what rpm range I am at it will not break 10psi at the gauge. Idle is about 6 and wide open is about 9-10. I have been scouring the web and cannot seem to find a number to put to it. Thanks in advance for any help.

[codyclyatt]

I ran the boat today and it stayed around 8-9 psi at wot and 6 at idle. The cavitation plate is covered with water at wot.
Not sure if it matters but I pulled the thermostat a minute ago and tested it in a pot of water, it opened around 130-135. I saw online where the t-stat could effect the water pressure, not sure how much but I figured it would not hurt to check. I also checked checked the lower unit again and all passages are clear.
I am lost and do not know what else to check. I keep seeing articles on comparable models that I should see at least 10psi if not 12 at wot.

[Ba$$n]

My 1996 Merc Tech Manual specifies a min of 10 PSI at 5,000 RPMs for a 3 cylinder 90 HP engine.

[codyclyatt]

Not as bad a thought but still not where it needs to be... I have got to figure this problem out but am completely stumped at this point. I keep going back to the impeller change that I did, I just changed the impeller and gasket instead of the whole $100 kit, maybe I should have done that, but the housing looked great and the metal plate under the impeller was smooth as glass... hell the old impeller looked to be in good shape I just changed it for precautions not knowing the age. I wish I would have had the pressure gauge installed before the impeller swap.

[EuropeanAM]

Really should be able to maintain 10 psi (or better) at WOT.

I'd definitely go back to the SOURCE: The Water Pump. When servicing the water pump, you should always replace ALL GASKETS and impeller (at a MINIMUM). Especially important to replace the gasket UNDER the impeller plate.

[codyclyatt]

Ok, I will try redoing the water pump, I just bought the entire repair kit, including the housing, plate, seal, gaskets, ect. I also got a new thermostat kit. I will update after all of that is installed and tested. I also forgot to add that when I throttle up for the hole shot it does jump past 10psi for a second but as the boat levels out on plane it stays 8-9 psi as mentioned. That made me think that the manual jackplate was to high, but the cavitation plate is at least 1" under water while at wot, so that can't be the problem. Midrange RPMs and WOT have about the same pressure. I just thought it was wierd that the pressure tries to go past 10 psi.

[codyclyatt]

uh oh... I may or may not have found the issue. Does the impeller metal housing suppose to have a seal on top of it? When I took the old one off it did not, the mercury parts diagram for my motor(1090412RD) also does not show one, but the kit came with one.

[Ba$$n]

Kit usually fits more than one model. All parts in the kit may not be needed for your engine. I don't believe the collar would drop water pressure, its used to keep exhaust gases from entering the water pump.

[EuropeanAM]

uh oh... I may or may not have found the issue. Does the impeller metal housing suppose to have a seal on top of it? When I took the old one off it did not, the mercury parts diagram for my motor(1090412RD) also does not show one, but the kit came with one.

Really need an engine SERIAL NUMBER to lookup parts and/or answer this sort of question. Serial numbers are formatted:

Digit, Letter, Six Digits (examples: 0G123456, 0T123456, 1B123456, 2B123456).

Also, while you have it apart, inspect ALL water INLET PASSAGES (below the pump, down to the water inlets) for WOOD OR DEBRIS (or pieces of an old impeller). Additionally, look up through the water supply tube with a bright flashlight, to ensure there's no debris (or chunks of an old impeller) trapped in the tube.

[codyclyatt]

Sorry about that, that was the model number... I will get the serial when I get home this afternoon. I will also check all of the passages as you described.

[codyclyatt]

Installed new water pump repair kit and it has 6 psi on the hose at idle like before. I really won't know anything until I can run it on the lake this saturday. When I did the install I checked everything mentioned and all passages were clear. Hopefully the new pump does the trick.

[Ba$$n]

Mercs tend to have very low water pressure at idle compared to Yammie and others. Pressure from the hose also affects the reading. Pressure while running is what counts.
Some gages have higher scale ranges than others making the idle pressure hardly read any pressure. A 0-15 PSI scale is better for a Merc than a 0-30 PSI scale.

[EuropeanAM]

A 0-15 PSI scale is better for a Merc than a 0-30 PSI scale.

For this PARTICULAR Merc... Perhaps.

That does NOT apply to most V-6 models, many of the newer 4-strokes, or Verado models, which commonly have WOT pressures that EXCEED 15 psi (exceed the maximum pressure of the above mentioned gauge).

[codyclyatt]

Ok... I got to run the boat today. It performed the same... 6 psi at idle, then when I throttled up it jumped over 10 but went back down to about 9-10 psi running, whether it was at 3000 or 5200 rpms it stayed 9-10 psi. To me it acts like the motor is to high on the boat, but the cavitation plate is 1" under water at wot, so that can't be it. Could it be the poppet valve maybe? IDK

On top of that I had other issues.... at the start of the morning wot maxed out at 4400rpm, later in the day it ran fine at 5200. I notice the voltage to the battery was very low when not running, will replace it before I go out again. Would a bad battery effect the ignition?

Thanks for all of the help... I am seriously starting to wish the boat would just burn to the ground.

[Ba$$n]

How old is the gage? It may be a gage problem and not a WP problem. Have you asked the previous owner about what pressure the gage read in the past?

Sounds like you may have a stator problem which could affect the ignition and charging systems.

You need to buy the factory service manual or take it in for diagnostics. Throwing parts at it is costly and not effective.

Don will need the serial number to be of assistance.

[codyclyatt]

Gauge is brand new... boay didn't have one before I bought it.... I am thinking stator also.... battery shows 13.2v throughout rpm range when running though, 11 when off

[Ba$$n]

I would have the battery checked, it may have a bad cell.

[EuropeanAM]

Has this engine ALWAYS (continuously) been on this boat, or was it recently MOUNTED to this boat?

Previously had WHAT for water pressure?

Please provide your engine's serial number in your next response. Thank you!

[codyclyatt]

I don't know if it is the original motor or not... just bought it a couple months ago, I installed a water pressure gauge after buying the boat

[codyclyatt]

Sorry about the serial number... finally got it..... 0G141912