7.4 MPI overheating at WOT
 

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.

Re: 7.4 MPI overheating at WOT
 

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.

Re: 7.4 MPI overheating at WOT
 

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.

Re: 7.4 MPI overheating at WOT
 

"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

Re: 7.4 MPI overheating at WOT
 

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

Re: 7.4 MPI overheating at WOT
 

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.

Re: 7.4 MPI overheating at WOT
 

:rolleyes: :rolleyes: 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 :evilb:

Re: 7.4 MPI overheating at WOT
 

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?

Re: 7.4 MPI overheating at WOT
 

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.

Re: 7.4 MPI overheating at WOT
 

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)

Re: 7.4 MPI overheating at WOT
 

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.

Re: 7.4 MPI overheating at WOT
 

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.

Re: 7.4 MPI overheating at WOT
 

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.

Re: 7.4 MPI overheating at WOT
 

"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

Re: 7.4 MPI overheating at WOT
 

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

Re: 7.4 MPI overheating at WOT
 

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.

Re: 7.4 MPI overheating at WOT
 

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.

Re: 7.4 MPI overheating at WOT
 

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

Re: 7.4 MPI overheating at WOT
 

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.

Re: 7.4 MPI overheating at WOT
 

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

Re: 7.4 MPI overheating at WOT
 

Things to look for collapsed inlet or kink in hose from drive to transom plate air leak in hose transom plate to water pump to find air leak put a piece of clear hose inline and look for air bubbles have seen salt water boats rust the water ports closed in the risers last resort blown head gasket

Re: 7.4 MPI overheating at WOT
 

Sounds like you wrapped it up FSTboater. I am going to try the backflushing again in the morning. Last time I only did the oiler cooler, not the hose to the cooler. Then I will go after the inlet side of things. The boat has only been in freshwater.

I will post as soon I get another chance to get to work on the boat.

Keith

Re: 7.4 MPI overheating at WOT
 

Ok guys, just wanted to say thanks to everyone, I finally got it figured out. I talked to many people, and they all told me that the power steering cooler was a straight shot through and could not be clogged. Well turns out that is not true, it has some funny shapes in it that will hold an impellor fin really well.
I figured this out by doing my backflushing and just getting a trinkle out of the other end so I started removing all the coolers to run water through them and finnaly found the clog in the power steering cooler. I did also have a riser gasket that was trying to leak a litte so I replaced that also.

Thanks everyone

Tub Of Fun

Re: 7.4 MPI overheating at WOT
 

Glad you found the problem, thanks for the update.