Exhaust Manifold Problem

k9th · Post by **k9th** » Wed Jul 01, 2009 6:26 pm

On our recent trip to bring SeaDog to her new home our starboard side raw water pump failed and we had to have it replaced as I have reported here on the forum before. I told the mechanic that replaced the pump that I was also having a problem with the port side water temperature gauge not working. I knew the gauge and the sending unit were fine and couldn't figure out what was wrong. When he tracked down the problem he told me that my port engine had an after market exhaust manifold and that a blocking plate was left out. He said that the water was therefore not circulating properly which did not allow the engine to warm up properly thus the gauge was reading correctly, the engine just wasn't warm.

He ordered the parts for me and would have installed them except they did not arrive before we had to leave to complete our trip. They have now arrived and I am going to have them shipped so that I can install them.

Does anyone have experience with this type of manifold? If so, what can you tell me about it?

chucka · Post by **chucka** » Wed Jul 01, 2009 8:05 pm

K9th,

To get a real answer, you should add info about what kind of engine you have, fresh water cooled (FWC) or raw water cooled (which is much more common in fresh water boats).

Here are some sample exhaust gaskets from a Chrysler 318.

The upper gasket allows raw cooling water to flow from the riser into the elbow though the hole above the exhaust passage. There is another set of cooling passages in the riser and elbow that wrap around the center exhaust passage, however the gasket blocks that flow requiring the riser to fill almost completely with cooling water before it spills through the round hole in the top from the riser to the elbow.

The lower gasket is used on my engine between the FWC manifold and the riser. The antifreeze in the block circulates through the manifold and back to the heat exchanger. The gasket with the top hole blocked prevents flow from the manifold into the riser which is cooled with raw water.

Installing the manifold, you are best to use studs with nuts rather than bolts. Stainless mounting kits are available. Bolts tend to rust and you have trouble when it's time to remove them. I've heard of people who grind off the bolt heads if they have trouble removing them, rather than risk snapping them off in the block, then sliding the manifold off over the headless bolts, then removing the bolts using a vise grip when you can grab the bolt down near its base.

I would also search the discussions at marine engines where you will find separate forums for each engine type. http://www.marineengine.com/cgi-bin/dis ... ?pg=topics

k9th · Post by **k9th** » Wed Jul 01, 2009 8:44 pm

Sorry for the confusion. Here is more info.

The engines are Mercruiser 255's that bring water in from the lake and expel it back out. From your description, that sounds like raw water cooled and I guess since I had to replace a raw water pump that makes sense.

Hope that helps.

Your description and photos made a lot of sense. When I get the parts I hope to learn the brand of the after market exhaust manifold and search the web for more information and installation instructions.

I was curious if this was a common scenario where there are plates that can be left out. This is my first inboard and I am still learning everyday.

Thanks for your help.

chucka · Post by **chucka** » Wed Jul 01, 2009 9:38 pm

FWC and raw water cooled both use a raw water pump to draw cooling water into the boat. The difference is the FWC system has a heat exchanger filled with antifreeze that forms a closed loop. The raw water pump pushes water from the lake or ocean through the heat exchanger to cool the antifreeze. The antifreeze circulates in the block and the raw water is mixed with the exhaust in the elbow and is discharged out the back of the boat. FWC is much more common on ocean boats to keep salt water out of the engine block.

You can do some reading here.
http://www.oscomotors.com/catalog/catalog12.asp

k9th · Post by **k9th** » Wed Jul 01, 2009 10:14 pm

Mine has no heat exchangers, of that I am sure.

I remember the mechanic mentioning the word Osco and I searched the web and found that they make after market exhaust manifolds for many marine engines so I am confident that is what it is. The mechanic said it was an easy fix so I am going to get the parts down here and try to do the install next weekend.

rossjo · Post by **rossjo** » Thu Jul 02, 2009 7:40 am

Do you have a water tank and heat exchanger mounted on/near the engine. This would indicate its FWC, not RWC ...

Do you have pics?

k9th · Post by **k9th** » Thu Jul 02, 2009 12:24 pm

There is no water tank or heat exchanger mounted anywhere near the engines.

I am going to be on the boat for the next four days starting tonight and I will get some photos of the manifold.

David Kane · Post by **David Kane** » Fri Jul 03, 2009 8:23 am

I'm interested in knowing if your exhaust system has spacer blocks just above the manifolds. Mine do. I believe we have the same engines, Merc 255's, GM blocks. I have all the parts/pieces to replace the manifolds on both banks of my starboard engine except for these blocks.

As you can see from this picture, water hoses attach at 2 sides. I believe 1 hose is a supply, the other a return. I'm raw water cooled. I'm hoping to locate these blocks thru John at Marine Tech, who is searching for me now.

If anyone else out there knows where I can get these, please let me know.

k9th · Post by **k9th** » Fri Jul 03, 2009 9:22 am

I am now at the marina and will be checking soon. I will take photos to post.