One manifold hot (5.0)

[ryno1234]

Just one of my manifolds becomes very hot to the touch. You'll definitely burn yourself if you left your hand there for 1-2 seconds. The other manifold is very warm to the touch, but it doesn't seem like it would burn you. The engine is NOT overheating.

I did find what I believe was an exhaust shutter in the outdrive when I removed the outdrive a month ago - just half of one shutter. I removed it. That said, I'm not sure how an exhaust shutter would affect a manifold being warm vs. hot, so I'm thinking this may not be related.

Any ideas come to mind about what would cause this with one manifold?

The boat seems to run fine and honestly I forgot about this issue and we've taken it out several times since I discovered this originally.

EDIT: What are the concerns of the manifold running hot? What are my risks?

Specs:
1998 Maxum 2400
5.0 Mercruiser Alpha Gen II
Raw water open cooling system.
Engine serial number: 0L027045

 

[QBhoy]

I’d say it’s almost certainly related. You may need to make sure the other one is where it should be too.

 

[achris]

Need a LOT more information on the engine to even start to think about a reason... Let's start with the year, then add if it's open (raw water) or closed cooling, then let's throw in an engine serial number, whether the engine is plumbed for cold or warm manifold and finally how you're doing this diagnoses (on the trailer running on muffs or in the water under load?)

Exhaust shutters in or out will make no difference to manifold temperature.

Chris....

 

[QBhoy]

Need a LOT more information on the engine to even start to think about a reason... Let's start with the year, then add if it's open (raw water) or closed cooling, then let's throw in an engine serial number, whether the engine is plumbed for cold or warm manifold and finally how you're doing this diagnoses (on the trailer running on muffs or in the water under load?)

Exhaust shutters in or out will make no difference to manifold temperature.

Chris....

Was thinking more if one of them was blocking the flow perhaps ?

 

[ryno1234]

Need a LOT more information on the engine to even start to think about a reason... Let's start with the year, then add if it's open (raw water) or closed cooling, then let's throw in an engine serial number, whether the engine is plumbed for cold or warm manifold and finally how you're doing this diagnoses (on the trailer running on muffs or in the water under load?)

Exhaust shutters in or out will make no difference to manifold temperature.

Chris....

I updated my question to provide these details

 

[Lou C]

Well perhaps the elbow started clogging first, then water flow was reduced on that side and the hot exhaust gas with not enough water flow melted the flapper resulting in what you found. I leave an IR temp gun on the boat so I can take temp readings at idle, just after coming off plane, to keep a eye on things. If that's the case then an exhaust system inspection is in order, perhaps replace both elbows if the manifolds are in good shape.

 

[ryno1234]

Here is a diagram of my system. There doesn't seem to be much between the "T" on the top of the engine and the left (from this perspective) manifold / riser. Given this, it seems likely that there is a blockage in this left side tubing or manifold / riser and hopefully that should be easy to troubleshoot by taking one thing off at a time and seeing if water can flow through it.

My only concern would be pushing material through the manifold / riser / Y-pipe that would require more work to fish it out.

 

[achris]

Here is a diagram of my system. There doesn't seem to be much between the "T" on the top of the engine and the left (from this perspective) manifold / riser. Given this, it seems likely that there is a blockage in this left side tubing or manifold / riser and hopefully that should be easy to troubleshoot by taking one thing off at a time and seeing if water can flow through it.

If you're running the boat on flushers, then yes, one side will get hotter than the other just because of gravity. Water will prefer to flow through the side that is lower....

My only concern would be pushing material through the manifold / riser / Y-pipe that would require more work to fish it out.

If you're concerned, just pull the manifold and elbow and examine then...

Chris....

 

[Lou C]

Might want to start with the thermostat housing make sure you get equal flow out of both sides of it, then if that's good move on to the manifold/elbow on the side that was hot. Just pick up spare gaskets before hand. These raw water cooled systems can pick up a fair amount of crud depending on the waters you boat in and also raw cast iron produces a lot of flaking rust depending on how corrosive the water you boat in is.
In salt water this is a regular thing, I've replaced the exhaust 3 times on my engine in 20 years. The originals were the OMC one piece V6 manifolds like what Merc used for a few years (though I never had any leaking back into the engine problems with them), replaced those 2x then by the time #3 was needed those were long since NLA so I switched to the later center riser exhaust used by OMC & Volvo.
Here's an example of how the outlets can get clogged in just 5 seasons in salt...you can see the one water passage at the top is totally blocked with rust.


and what the 2 piece system looks like when new....notice how much bigger the water passages are vs the one piece units above. interestingly the one piece units only ran slightly hotter after coming off plane (145*F vs 125-130*F). Normal temps for this engine at idle is about 95-105*F, these are in effect a "cold manifold" system not the warm manifold system that Merc used. With the "warm manifold" system the manifolds do not get water flow till the thermostat opens so it would be normal for them to be a good bit hotter, like close to the engine temp, the idea being to prevent condensation in the exhaust. I think to offset the cool manifolds OMC & Volvo used a 160 stat even in salt water vs Merc using a 140 stat.

 

[ryno1234]

If I run the boat without fixing this a time or two more, am I risking any significant damage? My understanding is that it can affect the water shutters but I've apparently already lost one.

 

[Lou C]

If it burned up an exhaust shutter then it can burn up the rubber exhaust hoses that connect the elbow to the exhaust down pipe and Y pipe. If that happens water can leak or flood into the boat depending on how hot it gets. Fix it now, would be my recommendation. Those hoses can look OK on the outside but be burnt up on the inside.

 

[Lou C]

BTW I had this problem once when a cooling hose popped off the ps cooler on the back of the engine. Overheated the engine & filled the bilge with seawater up to the pulleys. When I pulled the boat both flappers were fried and those exhaust hoses looked OK on the outside but were burned up on the inside. So as you see maintaining your exhaust system is very important to the safety & reliability of your boat.

 

[ryno1234]

BTW I had this problem once when a cooling hose popped off the ps cooler on the back of the engine. Overheated the engine & filled the bilge with seawater up to the pulleys. When I pulled the boat both flappers were fried and those exhaust hoses looked OK on the outside but were burned up on the inside. So as you see maintaining your exhaust system is very important to the safety & reliability of your boat.

I appreciate the comments. I just purchased new exhaust flappers.

I'm not looking forward to this change out - my engine is halfway under the back deck of the boat... I have to remove a ton of stuff just to even get to the headers. I'm hoping it's possible without removing the engine, but thinking it is!

 

[Lou C]

Get all your tools together
If you don’t own a rechargeable 1/2 drive impact gun get one
Start early don’t do it on a hot day
Drink lots of water
I’m lucky that I can remove the rear seats in about 5 min then take apart the wood support panels in 5 min it’s wide open. The benefits of older simpler boat design.

 

[Lou C]

 

[ryno1234]

I pulled the manifold / riser. I put a hose up against the water inlet elbow on the manifold and water would only come out of one of the 4 water jackets on the top of the manifold and I obviously felt a ton of back pressure indicating some sort of blockage.

I tried removing the inlet elbow so I could get a better view, but it is plastic and showing signs that it might break before it unscrews.

Also, the manifolds look pretty rusty. When is it "too much"?

Any ideas on how to clear the blockage from the manifold?

Side note - this boat only has 290 hours on it (but is a 98). I'm surprised they're this rusty. Is that common?

 

[achris]

Especially with things like exhaust manifolds my old saying applies...

It's not using a boat that kills it, it's not using a boat that kills it...

Those manifold and elbows are 24 years old, they have done well, but now it's time to replace them...

Chris.

 

[flashback]

I'm dealing with much the same problem, I did cook the lower hose, the manifold is questionable but the elbow definitely scrap iron. I'm going to replace it with new hose and if it works I'll be lucky and when fall comes I will change all of the exhaust system. The other side can't be far behind..

 

[Lou C]

The problem with cleaning out rust from the passages is that the rust….USED TO BE CAST IRON….
I look at manifolds & elbows as consumable items due to salt water use….

 

[ryno1234]

The problem with cleaning out rust from the passages is that the rust….USED TO BE CAST IRON….
I look at manifolds & elbows as consumable items due to salt water use….

I'm wondering if the previous owner took this in saltwater... My last boat which spent time only in freshwater never looked even close to this bad.

It makes me concerned about what the condition of the internals of the engine are. If the headers and risers are this bad... wouldn't the engine follow suit?

Attached are pictures of the thermostat housing.