Engine Overheating + Cooling System Issues with Maxum 2300sc with Merc 7.4 Bravo 3

[Timboat]

My buddy bought a 23ft Maxum 2300sc fiberglass powerboat that is having problems with the cooling system. The engine and sterndrive are the Mercruiser 7.4 V8 MPI with Bravo 3 (twin prop) all from the year 2000. This model has the freshwater cooling system (heat exhanger on top othe engine) and water pump impeller on the inside of the boat attached to the serpentine belt getting power from the engine. The engine runs and sounds great, and the drive train shifts and turns great as well.

[FONT=Helvetica, Arial, Verdana, sans-serif]The problem we are having is that at high speeds (higher than 32mph) the engine temp goes up past 200F and sets off the beeping alarm warning that the motor is overheating. This has occurred on our last few tests regularly after about 5 minutes of running at planing speeds. The problem does not occur at low RPM speeds like at 8mph. And we have not been able to replicate the problem at the dock with the shifter in neutral revving the engine. It only seems to overheat while the boat is in motion and going fast. On the water once the alarm sounds at about 190F we back the boat to idle speed and in few minutes the temp drops back to about 175F or less.[/FONT]

[FONT=Helvetica, Arial, Verdana, sans-serif]We topped off the heat exchanger with antifreeze, and were a bit concerned that it was about a liter low when we did. But we thought that might have been due to our first overheating event. Not sure if that would be the cause or affect.[/FONT] We have verified that nothing is clogging or blocking the intake on the lower unit pick holes.
We have ran the motor at the dock at 4000rpms for 10 minutes and did not see the engine temp on the dash climb past 175F (see pic)

Why is this boat having problems effectively cooling while moving fast and not while parked? It also seems the faster we go and higher rpm like at 45mph the engine gets hotter and overheats even faster.

Perhaps rubber debris from the an old impeller is clogging up on of the exhaust lines for coolant such as near the oil cooler?
[FONT=Helvetica, Arial, Verdana, sans-serif]OR[/FONT]
[FONT=Helvetica, Arial, Verdana, sans-serif]Perhaps one of both of the risers are bad and leaking coolant slowly or clogged up and not letting coolant flow fast enough.[/FONT]

Any advice would be helpful.


Thanks!

 

[Bondo]

We have ran the motor at the dock at 4000rpms for 10 minutes

Ayuh,.... Donno, Why you would do that,.... it serves No purpose,..... in neutral a motor don't build heat, as there's No load,....

Have ya checked, 'n changed the Impeller,..??

That's step #1 in any overheat situation,.....

 

[tpenfield]

As Bondo said, it is a myth that revving an engine in neutral replicates the same situation as the engine in gear. Everything works great in neutral, just remember that.

Impeller is usually the first thing to check & replace. . . but while you got the box box open and the hoses disconnected, back flush the oil cooler. Best way to do that would be to disconnect the sea water intake that leads to the heat exchanger and back flush from there.

 

[45Auto]

Funny, I haven’t been on here in years and this is the first post I see when I log on!

I'’m currently dealing with the EXACT same problem on a 2003 Crownline 225 with a freshwater cooled 496HO and a Bravo 3.

The boat was not in the water for the last year and a half, when I finally got a chance to go out again 2 weeks ago it had the same problem as your buddy’s boat. Replaced the impeller, belts, etc, before I took it out. First thing I did was all the standard back flushing, pulled and cleaned heat exchanger and oil cooler, etc, when it started overheating. Runs fine at low speeds, temp gradually rises till the alarm goes off at 180 (typically runs 160) on plane at 30 MPH.

Your buddy’s boat, like mine, has the classic symptoms of “Bravoitis”. It has restricted water flow through the sea water inlet hose. It works fine at idle, but It can't flow enough water to carry the heat away under load.

It’'s apparently a pretty common problem. Watching my water pressure gauge, it never goes above 3 PSI. I believe that it used to be about 3 PSI at idle and 12 PSI on plane. The Bravo raw water hose is attached to the aluminum gimbal housing with a tapered hollow insert. Corrosion in the gimbal housing gradually squeezes the hose and insert shut and restricts the raw water flow. A couple of companies make kits to fix it.

See below:

http://www.amarket.com/BRAVOitis.htm

and

http://www.hardin-marine.com/p-74054...epair-kit.aspx

It appears that there are several solutions to the problem:

1. Put in a through hull for the raw water pick-up and just bypass the restricted Bravo inlet. Probably by far the easiest and cheapest if you have to pay someone to fix it.
2. Clean out the corrosion, reinstall the factory setup, and wait for the problem to reappear in a few years.
3. Install one of the aftermarket repair kits which pass a stainless steel tube through the gimbal housing.

I don'’t want a through-hull on my boat, so I'’m going with #3.

Problem with #2 and #3 is getting to the transom plate fitting inside the hull (since I'’m going to have to pull the outdrive and bellhousing anyway for either of those options, might as well replace all the bellows while I’m at it).

The fitting inside the hull is behind the engine, right below all the steering gear on the transom plate. It’s #20 in the diagram below. Hose that connects it to the bellhousing is #15, hollow threaded insert that clamps it in is #16:

Cross section sketch below, might help show where it corrodes and squeezes the hose:

Here’'s a pic of the fitting in my boat, taken with my phone at arm'’s reach. I can'’t see the fitting with my eyes. Have to do it by feel. If you’re lucky, you'’ll have more room than I do. Big engine in a small boat with switchable exhausts, etc, in the way here. Yellow arrow is fitting, red arrows are bolts that have to be removed.



I ordered the Hardin kit yesterday, it'’ll be here tomorrow, I'’m hoping to get the internal fitting off tonight if it’'s not raining. If I can'’t get to it with the engine in place I guess I'’ll be pulling the engine this weekend.

Hope this helps, good luck!

 

[Timboat]

45 auto! thanks for turning us onto the Bravo-its issue.
We actually looked at another Maxum with Bravo 3 engine that we didn't buy and it had the thru hull solution with seacock and then a special made clam shell scoop to increase water intake flow.
It though it overkill at the time and shy'd away from recommending that purchase.
I guess this is more common problem than we thought.

Regardless we will probably attempt your Fix 3 as well.
Any guess on how many hours to pay a marine mechanic for a job like this is we get the boat on the trailer for him?

thanks!

 

[Bt Doctur]

 

[45Auto]

Well, I'm now pretty sure that "Bravoitis" is the cause of my overheating problem.

Got the transom fitting off without pulling the engine.

Although hard to turn, the first bolt on the transom fitting came out relatively easily with a six point 7/16" socket and a 1/4" drive ratchet.

Second bolt was a pain. Not enough room to get a socket or ratcheting wrench on it because of the angle of the hose fitting, no way to see it, everything in the world in the way. Had to use a six-point box end wrench since it was so hard to turn that anything else would have stripped it. Could get a turn of one flat of the bolt then had to spend about 10 minutes trying to get the wrench back on it. Took almost 3 hours to get the thing off.

Luckily all the threads were fine.

Water fitting as it came off:

Water fitting cleaned up:

Pass-through hole in the transom. It's badly corroded and clogged:

Should be pretty straight forward now, it'll be just doing a bellows job and installing the Hardin Bravo Repair Kit as part of the bellows job.

I'll knock the corroded hose out from the rear once I remove the drive.

I'm going to make some custom bolts with taller heads so I can use a ratcheting wrench to re-install the transom fitting. Probably be this week-end before I get a chance to pull the Bravo and do the bellows job, I'll update this thread when I do.

Any guess on how many hours to pay a marine mechanic for a job like this is we get the boat on the trailer for him?

It should cost you the same thing as a bellows replacement plus the time to remove the gimbal housing fitting inside the boat.

A Bravo bellows replacement around here will cost you about $1000. If you can get to the water fitting on the transom without removing the engine and everything goes well, it'll probably add two or three additional hours to the time.

If you can't get to the fitting without removing the engine, or if the threads in the gimbal housing strip, I wouldn't be surprised if you're looking at a couple of DAYS of labor to fix it.