I replaced my 1986 330 merc with a 502, i'm using the T stat housing from the 330...if you look at the 330 parts diagram, it shows the sleeve above the t stat,..I found a service bulletin, 2001-03 that describes how to install the t stat in an HP500it has the sleeve below the t stat.. I've had some cooling issues, engine staying cold, and am replacing the sender and t stat. both diagrams have no bypass around the tstat. my housing has a bypass...

I'm trying to understand the flow diagram...If I look at the 330 flow diagram it shows raw water coming in through the sea water pump, thru oil cooler HX, and splitting 1) up on the angle into the T stat housing. from here some water goes to the water pump ( through heads and back up intake to T stat housing) , 2) into a splitter at front and top of tstat housing then to the exhaust elbows and out. the t stat is located above the spacer and has a bypass port that allows water to go to the underside of the exhaust manifolds. when the t stat opens it allows more water to the manifolds in paralel with the bypass.


the drawings show the bypass blocked off by a gasket, should this be blocked off? I'm wondering how anything ever comes up to temp when so much cold water is going thru the block and manifolds without going through the t stat.


Am I way off? I was told the tstat is in the wrong place, that it should go on top of the intake (like in a car) so when the block gets hot it opens and dumps water to the upper and lower exhaust manifolds...but this contradicts the parts diagram and the bulletin...


is there a different alignment i should be using? should i use a 502 tstat housing? the difference is the 502 housing only has water going into the bottoms of the exhaust manifolds, not into the elbows like the 330...or go by the HP500 bulletin?