2003 Mercruiser 4.3 clogged manifold.

[Kronoss]

Hello guys, I just bought a used boat with 60hours and a 4.3 v6 (2003) obviously it has been sitting all of its life, as I have seen demo boats with more hours than this.

I took it out on the water, and noticed one of the manifolds was way hotter than the other, and when i turned it off, the engine started dieseling.

Brought it home, hooked it up with ear muffs, and same thing happened.

Water was coming out on only one port or the two on the transom exhaust holes. (the other port was just dripping very hot water)

Engine looks brand new, manifolds and risers look the same. I took the riser down, and seems to have a pretty good wall left on it, at least for 1 more year. Exhaust flapper was so burnt, it looked like coal.


I blew compressed air via the hoses that goes to the manifold (disconnected them from thermostat), starboard side, perfectly fine, and port side was definitely clogged.


Is there any way I can unclog one of the manifolds without replacing it ? Im pretty sure its rust that settled at the bottom of the manifold blocking water from entering it.

 

[NHGuy]

Inspect them well, they could be done. If you tap them with a hammer it should feel solid. They're cast iron. If they are soft, replace them. Mine rotted through low on the outside on the curved part.
If you can save it, take the manifold right off the motor and flip it upside down to dump out loose rust. You can then reach in there with tools to loosen other built up stuff. Maybe use wire brushes on it. As I recall one member had good results using gun brushes in this kind of a situation. You want round brushes on a flexible firm extension, maybe attached to a power drill driver. There's a company that makes fancy ones called FlexHone, they might be over kill for this job though. With those you can break down the rust as long as the metal is still usable.

 

[Kronoss]

Took manifold and riser off, filled the manifold with Muriatic Acid, left it over night, and bam, Manifold flows water again. It was 98% clogged with rust and gunk.

Installed it back, and started the boat, and I can actually see water come out the two exhaust ports via transom.

It seems its still a bit clogged, but not enough to overheat the engine.

I used a laser temp gauge, and the manifolds on both sides are same temp, but the riser on the side that i took off seems 10-20 degrees hotter than the other, is that acceptable ?
I didn't clean the riser with acid, as I didn't think it was that bad.

 

[alldodge]

It's normal for one side to be a bit hotter then the other, just works out how water flows, don't worry about it.

Now a note on your original question and now their clean. If a lot of the stuff that came out was rust, I would suggest looking into replacing them. May be ok now, but come next season you may be looking into water ingestion. Just a thought

 

[tlcnate]

, they might be over kill for this job though. With those you can break down the rust as long as the metal is still usable.

I am dealing with the same/similar problem on my 2000 Mercruiser Capri 5.0 liter engine. I am having mine cleaned and "dipped" at a radiator shop. And I am reviewing posts for precisely the answer to this. How do I know if they are "still usable"? They seem solid with only about 1/8" or so of rust scale that will be gone when I get them back. A boat shop guy made me paranoid and said I'll be lucky if I get one outing before they fail and ruin my engine. He said this without looking at the manifolds. It seems the radiator shop guy (referred from a VERY reliable mechanic) would have told me not to bother if he thought they'd be ruined by the cleaning process.