Hy 5. That bearing theory sounds interesting. It is possible, but not sure how to check with out swapping the whole thing. I might have to do this. It would explain why the low WP stays with starboard. There is no noise from the pulley and fuel seems good. I will do a test run again and if no luck I will swap the assembly.

Hiy5 you beat me to it, swapping pumps or at least moving one to the other engine will tell you if it is a pump issue. The only place in the diagram I see to allow any real pressure are the headers or the hoses that feed them.

I don't think you would hear anything coming from the pulley and unless you know for sure what your fuel pressure was beforehand and then now, you might not be able to detect only minor slippage since the pump is still pulling water albeit lower pressure. I'd try swapping the pumps, it's not that much more work than changing two impellers and could solve the mystery. If not, I'm not sure I've got much left but I'm working on it. LOL.

Come to think of it, checking fuel pressure might be another indicator, but emphasis on "might", I'd do the pump swap. If the problem moves, then you know for sure it's the pump.

OK.. Before I start wrenching, I will do a test run again first. However, even if the bearing is shot? isn't the pump shaft one peace? and the small pooley adaptor is pressed on or have a key? then the large pooley bolts on. So how could it spin less with a direct one peace shaft with a key for the impeller? Symptoms do indicate that the impeller is spinning slower, just don't see how could it. Just courios, but I will check around if I can borrow pump housing so I don't have to do this in and out thing 4 times. lol

That would cause misalignment in the housing, and reduced efficiency.

You want the whole pump assembly, not just the housing. You should also check / replace the gear lube in the pump too while you're at it (but after the experiment). These pumps are notorious for leaking fuel into the gear oil.

lol.. I know what you meant about the pump housing, (with the fuel pump attached). I have 1/2 doz. plastic impeller housing around. I change them once or twice a year, JIC. I talked to Merc. They don't think it's the pump itself. The shaft is keyed so the impeller can't turn any different RPM then the pooley, but agreed that it can be the last thing to do, so I'm doing it. I hope that's it. I will replace the hose from the transom to the pump first. There may be something I can't see under the motor. A hole or ? But I'm stumped since swapping outdrive intake the LWP remains with starboard. Can't be the hose, can it? lol and GGRRrrr. Mystery of the century here.

:popcorn:

I really hope you solve it. And very curious to know what it was...

On the lighter side. Several years ago I had some heating, water pressure issues. Nothing major but one hellava dilemma. Turned out to be a lizard skeleton in the oil cooler. :) go figure. I been taping and covering the outdrive water pick ups ever since during storage. I'm ready to see some wasp nests somewhere. :)

That was going to be my next suggestion, swap one hose and one cooler at a time look for a change.
a question for those in the know, if the dumps into the headers were eroded or cracked I can see that resuting in lower pressure, but the flow would be there so I don't think that would elevate engine temps but I don't know.