Saw this on another thread here:..."More than likely the pressure relief valve "poppit" is stuck open not allowing water to get to the top end where the thermostats are, went through the same thing with mine last year. But would be a good idea to flushed and impeller replaced."
And that got me wondering if that could be part of my issue....On a 1994 Yamaha 225 2-stroke ,V-6,I recently rebuilt the water pump and tested both thermostats in hot water with a thermometer,and both of them passed that test fine,BUT,after putting it all back together,I still get barely a trickle of water out the weep hole,even after running it long enough for the Tsats to open...Cleaned out the weep hole and tubing with a wire,but no improvement.Tried starting motor with one of the Tsats out,and water did come shooting up out of there somewhat,but not with much force.(No pressure gauge on this boat to go by)...Question is,Does my older motor have this "poppit" valve,and if so ,where would it be located? The Tstats are right on top of each bank of cylinders.

Looks like you do since there is a part listed for it here.
http://www.simyamaha.com/category_s/972.htm

Should be located on the rear under the right rear head. On my 2000 I know they did a redesign of the poppet valve.

Thanks Detective Friday,You just gave me a great source for all my parts! Got it bookmarked now.

Tip I learned from working on the Yamaha`s. When replacing the impeller use only soap for a lubricant while installing and push on the housing toward the back of the lower unit while tightening the 4 bolts. This will improve the water pressure. The poppet and thermostats need attention periodically too. Some of the 250 HPDI`s need the improved design poppet assembly.

This is good stuff...I learned something here that my service manual never even mentioned at all,the poppet valve.Lesson learned: Don't buy SELOC manuals...They leave out important details! I gotta just bite the bullet and pay the price for a REAL oem service manual!

You can find a digital copy on fleabay for $10.

OK,so after studying this situation a little more,I see that the weep hole should get its pressure BEFORE the thermostats,Not AFTER,like I thought...But I still need clarification on one thing...About that poppet valve,I can understand the WHY,but not so much the HOW.If the pumped water pressure becomes excessive,it's gotta get out somewhere.Fine.But HOW could it ever be TOO MUCH pressure??? How could that rubber impeller even generate enough pressure to become an issue anyway??? Or is it the possibility of extra load on the motor they were worried about,if the system gets "backed-up"??? Or maybe the plastic pipe from the pump blowing-up??? Help me out,I'm grasping at straws here!

All the poppet assembly is for is to force water to the top of the power head for cooling. When it opens it dumps excess water into the mid. I have no idea what your talking about on having too much pressure but I think the impeller would self destruct.

Uncle Jed look here for poppet valve location and replacement

poppet valve

I had an overheating problem on mine with a weak weep hole at idle and the poppet was the problem...put in the upgraded one and everything has been fine

Thanks 03,those pictures were great!

Yw let me know if you need anything else