2002 496HO-MAG(200hrs) closed system, freshwater all it's life, all last summer noticed I had water PSI near 40 which from what I'm told is too high. Looking into it now... if impeller failed at onetime where is the most likely place for restriction? I purchased with only 170hrs. No faults

thanks

the first cooler that water hits from the raw water pump.

Easist and best thing to first check is the Heat Exchanger. Have you replaced the antifreeze in the motor yet? It should be bright pink, and Mercury says the AF is only good for 5-7 years.

I suggest buying new heat exchanger end cap gaskets. I guarantee you need new ones. They always fail, and sometimes cause water flow issues. Buy those and the grommets for the heat exchanger to the crossover. Those 4 pieces should be replaced every 5 years or so. Drain the antifreeze out of the block, and pull the HE off to inspect and clean it out. Chances are you have some restrictions in there.

Backflush and change the rubber endcap seals on the heat exchanger. You should be fine.

At some point, with all these higher hour 496's coming into higher hour lifecycles, somebody needs to discuss backflushing these motors with 70/30 or 50/50 water muriatic. Maybe BUP or somebody will chime in on what is acceptable or typical for descaling engines. I fashioned some pvc to the raw water pump line and used a water transfer pump from harbor freight and ran a 5 gallon bucket of 70/30 water muriatic thru mine and tons of crap came out. Then immediately ran it on the hose. Dont say "but its fresh water" salt water engines develop salt deposits and freshwater engines develop calcium deposits over time which are actually much harder to remove.

first and foremost what is water pressure reading with just key on - engine off ? that is always the first test to do regardless for process of elimination of the issue at hand. Next is what should that normal psi reading be KOEO ? anyone what to chime in what that reading should be with just key on engine off. I am not asking for myself but asking so others can learn here. Also this this is called (KOEO) key on / engine off.

Next is what is water psi at idle 650 rpms -- anyone want to answer for the readings of what is the spec for water psi at idle speeds ? What does water pressure raise to off idle rpms. ? does it default that reading ? and do you have scan fault codes for water pressure and or cooling faults ??

In most cases there is a restriction after the water pressure sensor. First place to go is the OIL COOLER next is the heat exchanger and any water feed hoses next is the end cap gaskets of the heat exchanger and next is the exhaust side. These all are process of elimination steps to find your issue(s) and where that area is causing the issues.

I had one that was driving me nutz,,damn thing would trip the overheat alarm.did all the obvious,,then for schits and giggles,brought heat exchanger to a radiator shop and had it boiled out(best 20 bucks I've spent,except for that oriental massage ).
engine never overheated again.
btw,there was no room to move around these motors in a searay w/ v-drives.almost got stuck once in there.

Question what happens to the HE gaskets? Which I have new ones ready
My layout pump-PS cooler- OC - then HE

Now data from rinda recordings all PSI
KOEO .2
660 RPM 8.3
750 4.3
2500 26
3100 34
3800 39
4000 42
4300 44

I can see the PSI for 660 and 750RPM fake because the PSI at 660 should be less than at 750??
could that indicate a flaky sensor?
Also I'm sure the coolant was never changed, but it shows no discoloring
does it just use the GM stuff which is pink/red colored?
I think I was told the WOT PSI should only be 30+/- a few

Today's I removed the input hose to PS cooler and will back flush from HE to check flow. Will the HE gaskets like this cause restriction?
And trying to figure out water floe in HE as from what I can see is each side section is divided into 3 and no opening between them? All
internal tube are open except one which has rubber particles in.

Flush one cooler at a time. The PS cooler is just an open tube with no "honeycomb" tubes like the HE and oil cooler has. Those gaskets are very likely your problem. They are in bad bad shape. I check mine every other year and replace as needed. They should be flat rubber gaskets. Blow out your HE tubes with compressed air. Coolant flows around the tubes while seawater flows thru them in the HE. Dexcool and distilled h20 in your cooling system.