Can you by-pass the tail pipes and put a temporary hose from KE manifold over the side? Just long enough to test the change in WP.
Another easy test is to run a dump hose from a water plug on the intake manifold to a temp hose overboard. We have a permanent bleed/dump hose from intakes to relieve air pockets and to bleed WP.

I'm going to give it a try today. I'm also going to try running without a T-stat to see what that does. I wonder if my Comtic head gaskets could have anything to do with it?

What size are your bleed off lines? Are they in the back of the intakes or up front?

THanks,
Frank

I would just use a 1/2" hose at this point to see what happens. If this ends up as the solution, I will get you all the info on a suggested permanent rigging.

Any plug will work for a test.

Ok, here's what I've done so far. I eliminated the tails (no difference) then I bypassed the water to the headers for a quick start (no difference). I changed out my brand new merc 140 t-stat with a high flow 140 t-stat (no difference).

So that means the problem is somewhere in the motor, or circ pump right? What should I do next? Just run the dump on there?

Thanks,
Frank

If I understand correctly, you have the water from the intake manifold thermostat housing (w/o thermostat) free flowing out dump hoses (by-passing the exhaust) and you are reading 20+ lbs WP at the intake manifold? Is this correct?

That is correct, except I do have a high flow T-stat installed. I haven't ran it without a stat yet. I need to keep it in there for the efi.

Thanks,
Frank

This might be a problem. Mercury used by-pass hoses to the headers (bleed off) with a pretty ugly thermo housing on many engines. How is your water getting past the thermostat when it is closed? If it is just with a few small holes in the thermostat, this might be causing your high WP. Maybe a quick test without the thermostat?

you said that you have 13psi at 1200 not moving the boat, so you should be able to pull the stat and do that even with the efi.

as you are getting that high pressure with no movement, it has to be a restriction from the impeller to the block. (out of the block seems okay based on the elimation of the headers in your one test) your water pickups have no real effect sitting still on pressure

you can put a dump line from the intake front or rear and see if that lowers it.

you can also make a cheap PCV tap to insert anywhere into the water lines. as your water exit is okay, thi sshould show you where on input the restriction/problem is.

you can also make a cheap tap from PCV pipe and insert it into the water lines any place that you like.

Here is a pic of my T-stat housing. I'll try no T-stat tomorrow, but I have a feeling I'm going to have to run a dump valve. Or at least a dump off the intake.