400ss I have bravo 3s. One lower has lower water pick up the other does not. The one with out lower water pick up reads between 5-6psi at idle. The one with lower water pick up is 10-11. New transom assembly’s 4 years ago and haven’t seen salt water ever. Never had this issue till this year when I just dropped it in. Back flushed. Retighten all hose clamps related to sea water system. Pull inlet hose off pump from (goes from transom to pump) to check water flow. One with low water pick up def flows a lot more then one with out lower water pick. Both engines running at idle the one with low water pick the collectors are ice cold. The one with out low water pick up the collectors get warm. To me seems like a issue between drive/transom/and pump. I did have the drive serviced over the winter. Also brand new water pump and fuel pump assm. Is it possible a seal between the lower or upper and transom assm is dislocated or am I shooting a pipe dream. Any help would be appreciated. Thanks

So are you saying they both use to be 10-11 lbs @ idle ?

yes! Both collectors were always cold at idle too

new water pump ? circulation pump or sea pump

new sea water pump. Just put a complete assm on it that was rebuilt. I changed the water pump on the old assm thinking that was the issue. Wasn’t. So just put a whole spare assm on and that didn’t change it

10-11psi of water pressure at idle is a ton. Mine at idle is about 2 psi. More important is what is the water psi while running?

I agree your water pressure at idle is very high. By backflushing you did the power steering cooler, oil cooler, the heat exchanger and the fuel cooler all individually? The fuel cooler is directly aft of the impeller housing, many people don't realize it's down there and address the other coolers.

the water pressure has been like this since day one of owning the boat (5years). I back flushed every cooler and hose and tightened all hose clamps. With the inlet hose off the water pump. (Hose from transom assm to water pump). The one side is less water flow in with boat just sitting in the water. At cruise that engine is 3-4 psi less. At full throttle I have 47psi on the port and 29psi on the starboard. The starboard one is the one I’m having an issue with.

Sometimes you can back, flush tighten all you want. No change. I has similar issue few years back. WP at higher RPM's wasn't bad just a little off, and volume was a bit less.
Turned out to be lizard skeleton pieces stuck in the oil cooler holes. I had a SST flat little rod pushing it through all the holes everywhere, problem went away and I found a few little bone looking things. I used an old windshield wiper blades metal fillings, Nice and strong, but flexible and rounded corner. Now I always have one in the boat.

Can’t get videos to upload to show the difference of water flow from inlet hose off transom assm