Marine Exhaust Manifolds. How Do They Work? 

Excellent job explaining this. Not knowing what a mechanic means when they say this ir that is bad, your explanation makes it crystal clear

Cool deal man I work night security at a marina so I see the stuff but no clue what it is and this definitely answered 101 questions.

Thanks for taking the time to make this video. Very informative.

Thank you so much!!! I am learning too work on my boat. Couldn’t figure out my water in oil until you helped me!!! Thankyou so much

Great informative video mate. Just purchased a Maxum with a 4.3 and your video has been extremely helpful as there is a build up of rust on the outside around manifold and riser connection which based on your video I’m thinking an external leak.. time to replace gaskets 👍 thanks for your video.

A most excellent description. Thank You you cause I just learned some stuff.

Great explanation on exhaust manifolds and risers. Thank you.

You are a life saver man thank you

Thanks. Now I have an idea of what I am up against with changing my Mercruiser manifolds.

Thanks very much. Learned a lot. Time to change riser gasket. This vid maybe saved me hours and hundreds.

Thank you for the video. Getting ready to replace on my boat.

Nice job, never seen the inside till now! thanks!

Very clearly information. Thanks .Leszek

Awesome explanation - great info! You just got a new subscriber!

great video man perfect for getting an idea of how they work ! keep it up

Excellent explanation for an amateur like me. I appreciate your clear, instructive video.

Great video young fellow 👏

Great video... Very educational...

Great info here. I never saw one apart. Interesting.

Thank you ,very well addressed...

Great Job Brother!!

Super informative. Thank you for the video!

Very informative video. Nice job in putting this together. Direct and to the point.

Dude, you're great! Thanks for the explanation on this.

Brilliant...really informative thank you 😊

Fantastic video!! Thank you for doing this!

Thank you- great presentation! I learned a bunch!

This is great! thank you

Thanks mate, very good!

awesome video. thanks for the great tips.

Thank you for the explanation, great to know

Great video, thanks 👍🏻

Thanks, this video was very helpful !

very very nice video.no stupid music and everything is nicely filmed and explained. thank you dude

very well said and explained...thank you so much.

Nicely done. Very nicely done. I was looking for 8.2L Mercruiser cooling system info and I came across this. I'm essentially looking for info in regard to how the entire cooling system functions; I do have a basic understanding of it but I want to know the intricacies of it. Knowledgeable and trustworthy marine mechanics can be difficult to find. I want to understand the system top to bottom/front to rear so that I don't have to wonder if someone knows what they're doing. This does help somewhat. At least now I know that the exhaust manifolds are water-jacketed using sea water, very similar to an engine block being water-jacketed and using engine coolant. Thank you for doing this. Subscribed!

Great video .. proplems are also inside the riser is a liner type wall that separates exhaust from water .. they tend to rust through .. easy to inspect when u pull it off .. .. be safe

1987 Bayliner 2450 Ciera with an omc GM 5.7 with a similar open cooling system. Thank you for giving me a new maintenance project, like I don’t have enough. 😆 You saved me $$$ by giving a diy amateur a bit of good know-how. Thank you. 🍻

Just had a Volvo Penta GL 4.3 where an old impeller failure sent fragments into the thermostat, clogging the fitting to the manifold feed on one side. Absolutely roasted the two rubber hoses on the discharge. Even melted the alarm switch. Always check for failed impeller blades.

Great explanation , my kind of videos.

Vary helpful video, thank you!

Good lesson, thanks

Great video. Thanks.

Thank you. Learned alot

Good info bud! Thanks for sharing your knowledge. I’m getting some moisture in a oil but nothing significant. Looks like mayonnaise on top of my valve cover and droplets of water on my oil fill cap. Had mechanic wanting to replace the block thinking it was a blown head gasket 12k to do the job!!! I will replace the exhaust manifold and risers and see if that will solve my issue with water getting into the crankcase. Thanks again!

Great Video!

The tiny holes are for water to escape. The manifolds have a built in heat exchanger where water stays in the the jacket around the manifolds to help keep them cool. The water escapes slowly out those holes along with the engine hot water. On the older log style a hose goes to the front and back and has one tiny hole in the restriction plate. A separate hose goes on top of the riser from the water pump. On these the water goes in from ends to end and water pump water comes out thru the bottom thru the big holes out the riser.

That was so good I subscribed

Thanks for the explanation. I just replaced my '97 model 4.3 Mercruiser V-6 engine in my boat due to the thermostat not opening during winterization and not getting any antifreeze into the block. Lesson learned. I have the 1-piece manifold/risers. Fortunately they seem to be ok. I have taken the boat out a couple of times with no issues thus far.

Thank you for this video. My boat started is blowing white smoke and running bad. Probably the risers.

thanks for information!

Nice video. Thanks.

Good article very informative. its time to change my gaskets! Gonna use a camera scope to look first though.

Great tips brother, I’m having water in the oil so I’ll check that out

Well done-

Nice job

THANK YOU!!!

I've been told the gasket with the 2 small holes are used on the most common open systems where sea water comes in thru the system and goes out the exhaust. If you have a closed system the gasket is different and the holes are open.

I just did this job on my boat today. My boat is raw water cooled, and only used in salt water. Mine rotted internally causing a pathway from the water jacket into the exhaust path. Water was then leaking into my exhaust ports. I find that I need to replace these every 6 years or so. Mercruiser now makes these with ceramic coating on the inside to keep them from rotting internally.

Good job. Of course my system looks completely different but i really appreciate the info.

Thank you for the video!! I am getting ready to repair my risers on my 1997 ski Nautique…has the gt-40 engine in it!!! Definitely wish there were more RU-vid tutorials on the step by step of it!! Thanks for this video tho as it helps to guide a bit!!

Been saving this video for a couple months so that I could reference it with an upcoming riser maintenance job on my 2006 Volvo Penta 4.3gl (I bought it used a few months ago). Unfortunately, I'm now being forced to do this job sooner than I expected because last weekend while in the Gulf, we were cruising FOT and I stood up to readjust my seat but forgot my safety strap was clipped to my shorts. Ignition cut out as expected but then I couldn't restart the engine no matter what I tried. Thought maybe it was the battery died suddenly or starter even. Had to get a tow boat. He tried jumping but the battery was fine. Starter just clicked with no engine turn over so he towed us back in. When I got home I took out the plugs and turned the starter and 2 cylinders shot water out like a cannon. Talked to some boat mechanic friends and they claim the water must have came up backwards through the exhaust system and fed down through whichever cylinder valves were open at that time. Anyone else experience this issue?? Is it common? I should mention that I have raw water cooling. Thankfully, after a few days of turning it over using the starter plus some fogging oil and ensuring there's no water in the engine oil, I was able to get it dry and get it running again. Obviously, I'm nervous about this happening again! I don't think I have any cracks or water in the manifolds but this weekend I'm going to take the risers off to check that and probably replace them since they are original and exposed to saltwater.

Thank you! I had 2 mechanics tell me that I had a blown head gasket (water in oil) pulled my 3.0 all apart and found the head gasket wasn’t bad (tested with good compression on all 4). My riser/ manifold were the base of all my problems.

While idling out in the water tap your hand on the risers make sure one is not hotter or too hot, one time I had a boat where the water jackets inside the risers were so full of rust NO water was getting through. After checking the price for another one I just got a long drill bit and cleaned all the rust out.

Cool

On a Merc 3.0 thermostat, the hose on the right (as you face it) goes to the exhaust manifold correct? So if I fill that with antifreeze should that not run out the back of the engine? Winterizing my boat myself this year and nothing runs out the back. But the hose does fill up?

A wet joint exhaust is shown in this video. Older style. The fuel injected engines found in boats from at least 2000+ use a dry joint which don't have this design flaw where it can leak into the engine and seize the engine. You could still lose an engine with a dry joint exhaust but it not as common. Replace them every 5 years in salt water and you'll never have to worry.

Great overview, I was clear on the process. Could you do a similar video on marine engines vs regular auto engines? I have an old 87’ chris craft that has a seized motor and was thinking about picking up a used motor from junkyard and swapping it. Pros/Cons? Boat needs sone serious TLC from needing floors, interior and motor. Trying to salvage it vs taking it to a landfill.

Quick question How much pressure to tighten the screw

very interesting Seems to me if they made the exhaust manifold with a flange that fits up into the riser that would eliminate the possibility of ever having an internal leak. As you said water cannot flow up hill. Just my $.02, I am a car guy. I know nothing about boats. This is actually the first time I have ever seen one of these.

On the older ones (mine anyway) the water just doesn't go in and out , the water flows thur constant and back thru system before being released.

Great video. Just the basics- perfect. I already have a job, no desire to be a marine mechanic. Thanks!

I would think that with the invention of cerakote, the manufacturers should be lining the interior of the water jackets to prevent rust through. I'm no engineer but I would think they could do the math on cooling temps and heat transfer/absorption or discharge of heat and figure it out. I would pay the extra cost in the manufacture process to ensure not having a rust through and engine failure.

Perhaps it would be beneficial to point out just why marine exhausts are so different from car exhausts. One could think that car exhausts are essentially air-cooled & marine exhausts are water-cooled. Why is this? The marine engine bay is a fully enclosed space, the automotive spaces are open to airflow. The car uses air thru the radiator AND along the exhaust manifolds to extract heat, the marine engine only uses a body of water as the source to extract heat.

What about the risers flaps right below. I just took my risers off and don't notice any rust in the exhaust chamber. The issue im having is after a few minutes running, the engine gets so tight to turn, and doesn't. The battery cable terminals melt off, and the engine is to tight to turn by the crank. Happens at the lake. I noticed one of my risers flaps open fully and one closes when I took it apart.

Thanks well done you should get a plug for your shop , and sugestion on repairs , welding possible?

What are all of the bolts for and how do you remove them? I understand the small 10mm is just drainage plugs but what about the big plugs?

Hey Josh, love the info. Do you do one on thru hull exhaust? Thanks

I have 5.0 Merc IO one water jacket is cool and one hot. That normal or both should be cool?? Thanks

Should I make my own or just by a new one?

Great video, learned how they work, just bought a boat with volvo penta 4.3 engine that has 2 of these, are they different from mercruisers ? should I go ahead and replace the gaskets? thanks for the help

The small hole is to create restriction so the pressure can build up .

Like the video. Very helpful. Question. Can a bady/ leaky exhaust manifold gasket cause an engine to hydrolock?

I have a 4.3 liter Yamaha. I’ve been experiencing some heat issues. Last year I replaced the impeller and the water circulating pump (I did use one from an auto shop, vehicle application). It apparently did not have a thermostat in it since I’ve had the boat for several years. I left it out yet then. Seemed to work well last year with a few moments where it went up to like 175. This year I have an issue again. I can stay at low RPM’s all day long, but 98% of the time, if I go mid to high RPM’s the temperature starts to rise. If I back it off, it generally cools back down at about the same pace. I’m wondering about my risers. There are some water stains on the exterior of the exhaust. I haven’t known until recently that they should be inspected occasionally. I called on gaskets before I tear it open, but they said they’re discontinued. What are your thoughts? Nick

Plus boats tend to run a cooler thermostat that an automotive engine, I can actually touch the engine block right after running all day and it’s just warm.

Can you reuse the bolts on the risers?

If you have a known issue with hydrolock would you replace only the gasket to see if that does the trick or would you have the manifold tested. If you replace the manifolds, Would you replace the risers also?

What do you think caused milky oil to come out of my air intake?

Why do they do it this way can they just make plastic guides for this manifold? This happened to my jet ski

What about compression gas in the water. Has misfire and when reved the top water hose swells up. Any ideas.

If you roll with Crusader inboards you’ll likely not have to worry about risers or manifolds. Roll with Mercruiser and figure every 2-5 years depending on use and salinity Cheers

Hello, I got a problem with my boat "milky oil" I replace the head gasket, thermostat, and all gaskets, and everything looked good but the milky oil is there I check the exhaust manifold, and had a big crack on it if I run the engine without the manifold nothing happen, just inspected the intake and exhausts on the block and looks like water gets into there because seems some oxidation, Now I understand why.. i will check the manifold with some pressure just to make sure there are no leaks on it, thanks, man.

Great information. Calling from New Zealand. How do you feel about these engines running in seawater?

This fail has happened to a boat I just bought, they said it was a head gasket, but couldn’t find the issue till this video.

Did this come off a boat that was in fresh water ? Or salt ?

What does it mean when one side is ice cold and the other side is warm to the touch after running for 10 min? 1995 5.0 cobra

HI, Thanks for this informative video. I have a mercruiser with alpha one Gen 2 , that was overheating, i replaced the impeller and that solved the problem, but my concern is that the water is coming out of the outboard is cold on one side and hot on the other side with weaker water flow. Does any one know why ? should i be concerned? Thanks in advance for any replies. Sam.

Water come out under lower unit right?

So I have a question please, I’m looking to purchase a used boat where the owner or seller ran into sand bar with his inboard a Mercruiser 4.3 not realizing the motor started to heat up till it stopped and won’t start. He didn’t flush the motor and didn’t winterize it because he wasn’t able to start the motor. What is the possible scenario in this situation about what could have happened?

Is there supposed to be a hose on the bottom? My manifold just has a hole and water leaks out when running on a water line. Like to find out before I sink it in the lake lol😅