amarket1

I had this same problem on my 97 7.4 Merc.Previous owner had a impeller come apart and the mechanic never back flushed the system.Mine only ran hot at WOT.Took the power steering oil cooler which is on the side of the engine it was clear but when I took the oil cooler off the on on the front of the motor it was full of rubber and other trash.Cleaned it out and never had a problem again.

RMM

Not sure how similar design is to a 1990 model, but I had a similar problem where only overheated at 3400+ rpm and it ended up being the springs were shot on the ball/rod/spring mechanism in the thermostat. Replaced them and solved the overheating. This was after checking everything else, replacing impeller, hoses, etc.

Possibly someone can clarify better technically for me.

East Coast B

TOF.... The exhaust ports on your bravoIII are located on the bottom of your gimble assembly on the outside of your transom. If you follow your out drive up to where it meets the gimble, you will find two holes, approximately 1.5" diameter each, this is where your exhaust water should be running out. My most recent boat was same engine/outdrive as you have.

What other cooler are you referring to on the back of the motor?

If both manifolds are getting hot, not just one, then I say the problem is outside the manifolds. Odds of both getting clogged up at same time, minimal.

Tub of Fun

"ball/rod/spring "
HMM what are you refering to, just the thermastat in general? I am going to pull the hose on the back of the pump, and the top of the oil cooler and flush again, then check the line coming from the transom.

Will post more when I get that done. Thanks for all your help guys.

Keith

hillbilly24

if the lower bellows connects to both the drive and the transom plate this is incorrcert and many boatbuilders havent figured it out yet for some reason. The bravo 3 is too restrictive for exhuast and there should just be about a 4 inch long tube that comes off the transom plate and kinda seats over the bellhousing when the drive is lowered but does no tottaly seal too it this allows the exhuast a bypass. the big bellows on top is the u-joint bellows and if the lower one look s like it (ribbed) it needs to be removed and be replaced with a tube. its an easy job. when you install the new tube use some bellows adhesive to seal it and let it set up for 15 minuits or so before you tighten the clamp if you dont it will slide off when you ighten it cuase there is no lip. if you have any more questions give me call
805 732 8375.
Jake

Pismo10

If the exhaust bellows were installed incorrectly then this problem seems like it would have been occurring since the boat was new. This is a new problem I believe.?. The old alpha drive used to have this same tube setup for exhaust, the newer one have a full bellows. The old tube setup was nice and easy, no bellows to change and any back pressure just leaked out around the tube.

BadDog

I think Jake is on to something there. Bear in mind, you have a gosesinta and a goesouta for your water. If your exhaust is stopped up the proper amount of water will not goesouta. If your coolers are stopped up you do not have enough goesinta. The housing for the check balls can be a problem as stated above, that falls into not enough goesouta. Thermostats do fail and for the price you may as well change it. The raw water pump housing may have a some scars in it which can cause low water throughput. There is a water pick up tube on the transom assembly that can leak as well. Follow the hard suction line from the pump back to the transom and check to make sure the hose and fitting are snug. If these are loose you can suck a lot of air into the system.

JaayTeee

If the rest of the system is free from
obstruction,this is where I'd be looking.

hillbilly24

iv'e seen boats with the wrong bellows setup that didn't start overheating till 100 hours or so. The stock engine seems to be acle to handle it till it gets a little loose and other passages start blocking up its kinda like a chain reaction that takes a while.

Mudball

On my 1999 7.4mpi, I lost an impellor a few years ago.
While flushing, the only place the pieces will be is in the oil cooler. The other coolers are straight through with no baffles...
Look at the water hose between the PS cooler and oil cooler. Pieces can fall back down that tube only to push back up to the oil cooler when you run again...It's a good idea to flush that line.

Best of luck
Kent