Tub of Fun

Ok, guys here is the story. I bought boat with a stock 7.4 mpi with "through hub" exhaust bravo III. The boat runs and drives great up to around 4000 rpms, then is starts getting hot. 225* to 250*. I checked the impeller and housing, took the cooling system apart and back-flushed it to make sure there was no rubber in there. I put the thermostat in hot water to make sure it was opening
I think the exhaust risers might be corroded, or the gasket is bad. How do I check them. They are hot to the touch after running. If I remove the riser to check do I have to put a new gasket back on or just bolt it back together.

Thanks for any possible help, this is really starting to get to me. I paid to much money to be having these problems.

Pismo10

Change the impeller, even if it looks fine it can be set and not work well under hard throttle. It will work fine slow and heat up when you're on the gas hard. It will then cool off again ok when you slow down. Any big steam coming out the back? If so there is a leak into the exhaust at the manifold. Old impeller blades won't get thru the oil cooler so thats where they plug things up, upstream of the oil cooler just after the raw water pump. It's a boat, you are going to have a steady flow of problems.

hillbilly24

I agree with everything pismo said, There is one other thing to look at though. Do you have full exhuast bellows or just a tube that connects to the transom plate but not the bellhousing? you can check by raising the drive and looking to see if it seperates from the bellhousing or not, it should. With a B-III they are suppost to use the tube but some boat buiulders still use the full bellows and this will cuase a high speed overheat due to extremley high exhuast back pressure.

Tub of Fun

"Any big steam coming out the back?"

Yes, it steams alot, a whole lot. I did not know if it was because of the swim step collecting the steam. What seal or gasket needs to be changed if it steaming, and can that cause my heat problems.

I looked at the drive, and I see what appears to be an exhaust bellows. Also when running on the hose I don't have any water running out the prop like my alpha 1, should I have water coming out? The previous owner said he never had a problem with it over heating and that the impellor was just changed in Nov. The boat is a 1998 I like the idea of the wrong exhaust hose, but I would think that would have been an issue by now. Course the original owner was an older guy and maybe never ran it hard enough to find out.

You guys are leading me the right direction. Thanks for your help lets see if we can get this under control.

Keith

hillbilly24

your steam may also be cuased by low water volume wich is due to a worn impeller or an exhuast obstruction.

Tub of Fun

Hillbilly please read edited msg above.

To make sure I am looking at the right thing, I see the bellows where the shaft goes though, then I see another bellows below it that is about the same size that I assume is the exhaust bellows. I don't see where that "tube" would be.

Also how can I test, or look for an exhaust obstruction. I used to be an auto mechanic, so I have a good knowledge of motors, just starting to get lost when it gets to the marine parts and what effects what. If this were a car I would say to change out the radiator, but we both know that can't happen.

50boat

as the outhers stated , but take annother look at the oil cooler, 1 remove hose from pump and at cooler 2 -- 7/16bolt, retains the cover, 3light twisting to remove, has ruber gasket. 4 look in to the cooler it a grate in it. it will stop up here. if any thing is small enght to pass through dont wory, if you dont find any thing reed moor posting

Tub of Fun

I removed the hoses on both sides of the oil cooler, and blasted water straight through it. I ran a piece of wire through several of the "grates" and got nothing. What is the other cooler on the back of the motor?

hillbilly24

that the Poweer steering cooler also a common blockage location, one other thing i din't think of earlier, check the water inlet hose between the bellhousing and the transom plate to see if it kinks when you lower the drive.

Pismo10

The rubber piece are usually in the hoses say between the raw pump and the oil coolers. Then when you startup they press against the oil cooler blocking flow. I have also found rubber pieces in the raw pump housing and in the hose from the transom assem to the raw pump. Steam is from low water flow or water leaking into the exhaust manifolds often from the joint between the manifold and riser. Are you running in salt water, if so, check those manifolds. BUT leaky manifolds don't usually cause over heating, just lots of steam so you are back to low water flow cuased by obstruction(since the impeller is new)