so i just inherited a boat with a mercruiser 190. the motor was locked up when i got the boat and after i pulled the plugs i noticed that all the cylinders had anti freeze in them. i pumped out all the anti freeze and let the motor soak with marvels for about to weeks. after a new set of plugs the boat runs real well. i pulled off the riser tonight to do a visual but it seems fine the only thing i noticed is the small ports in the bottom of the riser (the ones that run parallel with the manifold) were full with mill scale and rust. im not sure if that is my problem or not kinda wanted some second opinions. also the engine oil is clean as a whistle.
Well I pulled off the exhuast manifold and upon inspection it has multiple cracks. Looking online I noticed that some of these 3.7 motors came with a aluminum exhaust manifold can i switch that out with my cast one?
As long as you have the later style manifold and not the Log Manifold (square exhaust manifold) yes you can swap it. But you say your engine is a 190 and that should have an aluminum exhaust manifold and a 4bbl carb. I am assuming you are talking about the manifold that bolts to the engine head and not the Riser that bolts to the manifold itself.
As long as the exhaust manifold is NOT square with a tank on one end and the riser on the other, a later aluminum manifold should fit. As a matter of fact all exhaust manifolds types will fit any engine, the problem is where the riser outlet to the lower exhaust pipe is located. Look here at the exhaust section. It is manual #3 (early) and combines several engine types. Manual #8 is the later manual just for the 470.