2004 Mercury 50hp 4 stroke pisser weak at higher rpm’s

[JordonDunlap95]

So I got my boat out after being stored for winter and noticed the pisser was a little weak.. so I changed the impeller and gaskets and now it is pretty strong at idle, but when out on the water and giving it some gas at higher rpm’s it gets weak and like there no pressure really behind it and it will sound the alarm after to long I’ve changed the thermostat as well and it still does it.. but at lower rpm’s it won’t sound the alarm and idle it’s good.. has anybody had this issue? I’m really getting frustrated with it!

 

[Texasmark]

On larger engines like my 90 and 115, they have a high volume thermostat bypass "poppet" valve forced off its seat by water pressure caused at the higher RPMs because the thermostat opening isn't large enough for the required flow.

If I had your conditions with my engine, I would go directly to the poppet and inspect it for "smoking gun" characteristics and change it.

If you have no bypass then possibly you have clogged water passages, or the stat you changed had failed you......not opening far enough, or hole where it mounts is clogged with debris, or debris (sand-weeds) in other parts of the block cooling cavity.

 

[JordonDunlap95]

Ya I originally thought poppet valve but I looked all over the internet and couldn’t find anything on these smaller motors having a popper valve and I always use salt away when flushing with water I’m thinking I might redo all the gaskets again on the water base and impeller side