blocked manifold? 5.0 mpi mercruiser

[woodguy81]

Just trying to confirm what ive already found before i order parts. Have had an overheating issue for a while now and think ive found it. Boat runs hot at higher rpms. Did the clear hose test. No bubbles. Did notice the last time out the left riser was too hot to touch but the right was barely warm. Pulled the manifold and riser. Removed the bottom plugs from the manifold and it looked solid I can scrape the black material out of the holes and somewhat clear that immediate area but im assuming the rest of the inside looks the same. Replace the manifold? The riser appears clear and has good flow. Put a hose on the inlet at the bottom of the manifold and there seems to be a good bit of resistance preventing solid flow. Also worth noting that I replaced the heads last spring and the block and heads were completely blocked solid with the same black debris. New heads and 2 hours with a screwdriver and a hose to clear the block fixed that. Reasonable to assume both manifolds should look the same and should both be replaced?

 

[alldodge]

Also worth noting that I replaced the heads last spring and the block and heads were completely blocked solid with the same black debris. New heads and 2 hours with a screwdriver and a hose to clear the block fixed that. Reasonable to assume both manifolds should look the same and should both be replaced?

Also notice you said in another thread that when turning it started to over heat. Another poster asked if you replaced any parts in the transom assembly, and your response was "will report back" but there was no response

https://forums.iboats.com/forum/eng...ives/10850196-automotive-vs-marine-thermostat

If your talking about replacing both mans, then I would say yes

Would also ask where do you boat that you can get black crud building up?

 

[woodguy81]

The black crud is metallic so I’m assuming deterioration due to poor maintainence. It’s spilt salt and freshwater but I’ve only owned the boat a couple seasons. The buildup in the engine was there when I got it so I’m guessing thus was close to being a problem as well. I had replaced the upper unit on the drive in the spring and possibly had something incorrect there. It has since been removed and reinstalled much more carefully in my effort to track the issue down. Not that I want to buy manifolds but it’s encouraging to actually find something that is significant. When I purchased the boat it had cracked heads no doubt from overheating due to absolutely no possibility of water flow through them.

 

[alldodge]

I would replace the mans, and then start saving for another set in 7 years

 

[QBhoy]

I’d do the above as suggested on last post...and start leaving the block full of antifreeze each winter

 

[woodguy81]

I was able to get them cleared out. Took some work and drilled one new hole to get the last bit cleared. Just gotta tap and plug that and repaint so I can try it out. Hope to get a season or 2 out of it before I replace them. Used a piece of a band saw blade to poke and cut through the crud and kept rinsing it through. Got a lot of rust chunks out too. Lots of flow through them now. I did fill with antifreeze the last 2 seasons for winter. No idea what the previous owner did but I’m still finding stuff that isn’t right.

 

[Searay205]

Soooo when the manifold rots through since all the rust you flushed out came from manifold wall thickness and you fill your engine cylinders with water and hydro lock it you will be ahead of the game. No need for manifolds when engine is shot. Manifolds need replaced ASAP.

 

[woodguy81]

Actually the majority of what was flushed out was the same debris that was found in the engine block last season. For sure some wall thickness was lost but it’s still solid. Expecting to finish the season on these and then look into new manifolds over winter.