http://<a href="http://s1324.photobu...[/IMG]</a>

Hopefully this link works to get the shot of the thermostat housing posted...

http://i1324.photobucket.com/albums/...psa9ubtkee.jpg

Well that didn't seem to work. Let's try this one!

With thru prop what comes out each side is meaningless. They Y together right before going out. Pull the 2 hoses going to the exhausts. Compare water flow. If the same, pull the manifolds and hook up a garden hose to inlet at bottom and flush out, watching the exit holes at the top for rust flakes and such. Also, make sure you didn't block the wet joint holes with silicone when installing riser.
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The Y Pipe has an internal flapper on each side. If you overheat once you can burn one and it can drop down in the pipe and block water flow. They are a pain in the ass to get to. The rubbers that connect the riser to the down pipe and the rubbers that connect the down pipe to the Y pipe are Very hard to get off. I use these red plastic scrapers that I found at Harbor Freight to "Break the Seal". The pic of your manifolds lets us know you DO NOT have the check ball system so that eliminates that.

Chris

just worked on a boat same issue, was a piece of old impeller in hose feeding it

Alright so since my last post I have not had the boat in the water to run it however I did pull the hoses feeding the manifolds, flushed the hose and found nothing clogging it, the inlet to the manifold or the outlet from the thermostat housing. I did not run it on the hose yet with the hoses removed from the manifolds to compare flow but will do that next time it is run in the driveway.

As for removing the rubbers that connect the risers to the Y pipe.... can I access the internal flappers in the Y pipe by simply removing the rubbers? Would I be able to see these flappers? And can these replaced without tearing all sorts of stuff apart? :/

In regards to the comment on the impeller fragment UI assume you are referring to the impeller in the out drive. If that's the case I replaced the impeller last fall and the old impeller was intact, hence I eliminating this possibility I think?

Thanks everyone for your feedback!

Pull the rubber bellows connecting the riser to the y-pipe. The bellows is the retainer for the pivot pin for those flappers. They will be right there at the top of the y. If one burnt and gone, straighten a coat hanger, put a hook in it and play go fish. Or fire up a shop vac and feed the hose down the y-pipe. You might need to fab a skinnier hose to fit. A friend dropped a socket down y-pipe. Said eff it. 2 years later socket against aluminum, rotted a hole in the y-pipe.

Thanks. Will have a look at it first chance I have.

https://www.harborfreight.com/4-piec...set-95832.html

You can thank me later lol..

Chris

check balls in the T stat stuck that will not allow water pressure to open them up ? If your T stat housing has those. I have no idea what you have plus I did not see what Manifolds and risers set up you have ? Is all that OEM stock Mercruiser ?

Flapper Valve aka water shutter(s) broken off and stuck in the Y pipe ? Do you have those in the set up and if so are they installed and installed correctly ?

Restricted water jackets in the manifolds and or risers

355 CID what app is this ?