Ideas on overheating??

[Stroh]

2006, 240 DA 5.0 MPI (ser# OW 610149) Bravo III drive 700 hours. Raw water cooled three-point drain system. Completely original and stock. Used in fresh water about first 9 years of life, lift or trailer kept and regularly serviced. I have owned for 2 years. Up until about 2 months ago consistently cruised at 160 degrees & maybe 162 at lower speeds, now it overheats, especially at idle but inconsistently at any rpm. I can lower temperature by revving engine above 1500rpm in neutral, but as soon as I throttle back to idle temp begins to rise back up.

First I pulled thermostat and checked operation in hot water (checked good). Then, I r&r’d the raw water pump. Previous owner had receipt raw water pump impeller replaced at 500 hours. I went ahead & rebuilt raw water pump (had bad bearing & seal but impeller was good, I replaced it anyway, very minor grooves in brass end housing plate and I smoothed), r&r’d circulating pump checked operation (good), removed port side exhaust manifold, riser (6”) and elbow and inspected. All looked good especially for 11 years old, not much rest scale, etc., I have always flushed after use with “Salt Away. Plus, I live on canal that is semi-salty. Next, I pulled the transom water hose fitting (inside of transom) and found it crushed to about half closed (Bravoitis). Removed the plastic internal clamp that was crushed and cleaned inside and outside of the hose coming from outdrive, to full hose opening. As a test, I put it all back together expecting the temperature to stay below 195 degrees but it doesn’t at idle, in the water, the temperature rises and doesn’t stop at 160 degrees (thermostat), it keeps on right up until the alarm goes off.

I have replaced the old but working thermostat with a new 160 degree one. I have removed & inspected virtually all of the cooling hoses, “Y-fitting”, checked the power steering pump cooler, the engine oil cooler (they both are totally open, nothing to impede flow, no heat exchanger fins, etc., just open tube. I have checked the water distribution housing, again just open mixing ball (not air operated), nothing in it restricting.

I did a water flow test of the raw water output. Started engine & gauged water flow into 5 gallon bucket for 15 seconds at 1000 rpm. This was done downstream of the power steering oil cooler, just to get an idea. Result was nearly full 5 gallon bucket. I believe minimum flow according to service manual is like 3 gallons.
I have scheduled an appointment with a reputable mechanic to remove the outdrive and permanently (I hope) cure the Bravoitis by replacing the hose with the aftermarket kit. I realize the hose end on the outdrive side may also be crushed but with the raw water flow as good as it is I feel I have another issue causing the temp to rise at idle. I have also checked operation and accuracy of temp ending unit, both heating it and a digital thermostat and pointing an infrared thermometer at thermostat housing and it appears to be right on.
I have redone virtually all of the above twice and am no closer to the answer. My question involves the fittings with check balls attached to the bottom of the exhaust manifolds. Has anyone had a problem with them? I checked the one on the port manifold, when I had it off and it seemed to work properly. The only other question I have is, in researching this, I have read some applications have check valves (balls) in the two smaller hoses running off of the thermostat housing to the exhaust risers to limit cooling water flow to the risers and divert it through the engine. There are no indications my setup has or had anything like that. Anyone know anything about those check valves?

Logic would have it that something changed fairly abruptly to go from no problem at all to such a dramatic change in operating temperature, but I’ll be danged if I can figure it out. I understand people with this problem have just installed a raw water pickup in the hull and capped off the transom hose. I would prefer not to go to that extreme and fix what has gone wrong. After all it has worked properly for 10 years and I am not big on “red-neck” type engineering. I prefer to keep it stock.

Any other ideas on this most perplexing problem? Thanks in advance, for this great forum.

 

[scoflaw]

So the outpipe for your flow test was before the manifolds, and you were getting plenty of volume ? If so, obstruction points to exhaust system...no ? I'd look those over closer, pull them for a good close look. Maybe an exhaust flapper broke loose and is causing an obstruction. Doesn't sound like an intake issue with the flow you are getting.

A thru hull pick up is not a redneck fix ...proper term would be an " upgrade"

 

[Molson]

Sand in the block possible?

 

[CliffA]

Maybe an exhaust flapper broke loose and is causing an obstruction

+1....this is my first guess at the problem.......

cliff

 

[CliffA]

these symptoms sound exactly like what happed to the engine on a previous boat of mine.....i know you said you checked the operation of the water circulation pump but it may be worth replacing it even though it looked good.....this is what cured the problem on my boat....the pump visually looked good and the bearing seemed tight....but the vanes evidently had worn down enough over time to cause a reduced water flow at idle speeds.....before i bought it the boat was used in Florida on shallow inland lakes so i suspect sand sucked up by the intake was the cause of the wear of the vanes in the pump....like in your situation i could rev the engine up and the temp would go down but at idle it would overheat.....everything else checked out OK....

like you i went through the process of elimination and decided it had to be an issue with the circulation pump....as soon as i replaced the pump the problem went away.....the pump was not very expensive at around $100 and was very easy to change....

cliff

 

[romanservices]

My first instinct would be the raw water pump however you said you rebuilt it all. I would pull it again to double check the seals, perhaps replace rather than rebuild. I replace impellers every year, housings when needed. Check the weep hole on the circulation pump for any evidence of water / rust. FYI, I also replace manifolds and risers every 3- 5 years regardless of condition. My first boat had a manifold failure about 100 miles from home port in the middle of the Puget Sound. That was the younger dumber version of me that also did not have towing insurance.

 

[Stroh]

To answer all of the good ideas you all have come up with so far:

I suppose it is possible sand in the block could cause anissue with cooling, but I see no evidence of that. After pulling the thermostat housing andwater pump, I ran water into the two water holes where the water pump pusheswater into the block and it came out the thermostat housing hole clear and whenI reversed the water I introduced into the block, the same thing.When I run the engine, the block itself doesn’tseem to overheat, just the thermostat housing where the temperature sensorresides.I used my infrared thermometeron various parts of the engine, hoses etc, the thermostat housing seems the hottest.

I had inspected the exhaust butterfly (shutter) in the portside when I removed the exhaust manifold, riser and elbow.It was free, functional and not damaged ordistorted.This afternoon I pulled thestarboard elbow and inspected that shutter, it was free, functional and notdamaged or distorted, either.I may pullthe starboard side riser and elbow and inspect that side just to be sure.I am still not convinced the drain & checkvalve on the bottom of the exhaust manifolds are properly working.I will pull those off tomorrow and inspect.

When I rebuilt the raw water pump, because a bearing was badand seal leaking water into the bilge, the shaft & bearings come as a unit andso I did replace the seals at that time.

My gut feeling is it is the circulating pump not circulatingthe water enough.I pulled it back offand took the plate off the back.Whilerusty inside, it seems to be fine.The impelleris not slipping and what I could see, the fins on the impeller look normal, theseals are not weeping at all.I alsohooked it up to a drill and dunked it in my pool, it seemed to move water justfine.If all else fails, as much as Ihate just replacing parts, that will be my next experiment.

Thank you for all the ideas, so far.

 

[romanservices]

Did you replace the wear plate & check the housing for cracks? I've seen plastic housings have "invisible" cracks that only show up when you pull around the housing spreading it out.

 

[1956olds]

any possibility the last mechanic to change the thermostat installed it backwards?

 

[KevinC]

Verify your serpentine belt routing - this might not be the problem but it is worth checking. My first year of having a 5.0MPI I did a bunch of work including alternator, water pump and thermostat. I put it all back together and it seem to work fine. Took it out and it overheated. Turned out I did not properly route the serpentine belt and it was slipping on the seawater pump pulley.

-Kevin

 

[SCORPIO]

If you are in the water in FL, do you have a build up of marine growth on the water inlets? If you have a thru hull, is the strainer clear of marine growth?

 

[Thornton69]

No experience with raw water cooling systems so take this with a gran of salt. Just throwing out an idea. Sounds like you have covered most of the obvious. Something that stands out to me though is that it sounds like lower rpm or under load (high combustion pressure), is when you are having your problems. Wonder if you are getting combustion gasses introduced to the cooling system? A quick compression test might help to identify that among other things. Good luck!

 

[boatrboy]

any possibility the last mechanic to change the thermostat installed it backwards?

please don't be offended but I was going to say the same thing. My friend ran all year like this and all of a sudden he started overheating. He pulled thermostat housing and sent me a pic and sure enough he installed it upside down. I have no idea why it took so long to become a problem. The single thin cross bar should be "up" when installed.

 

[Jeremygavin]

I just pulled my bravo III lower and upper units apart because I could see barnacles through the water intake holes on the lower unit. Sure enough the chamber that the intake holes feed was FULL of barnacles. There no barnacles in the upper areas that the water flows through to get to the transome assemble. I am not sure how I didn't have a cooling problem last season.

I also went to Hardin Marine raw water pumps seasons ago because my pump housings were scored. you will lose a lot of pump force do to the scored housings. This spring I changed the impellers in the Hardin pumps after 2 years and 200hrs and the wear plates where scored a little bit the kit comes with new wear plates so you change them out with the new impeller and good as new again.

 

[scoflaw]

I am not sure how I didn't have a cooling problem last season.

Your boat has thru hulls doesn't it ?

 

[Lazy Daze]

double post deleted

 

[Lazy Daze]

Stroh, does your t-stat housing have a "T" coming off the top of the housing? Essentially, 6 hoses at the housing?

You got almost 5 gallons in 15 seconds on a hose? Wow. That's way above spec. So you definitely have good flow. I'll usually do it by removing the intake hose at the t-stat housing, but from what you stated, it sounds like you were just at the other end (upstream) of this hose. So that should be fine.

 

[Jeremygavin]

Your boat has thru hulls doesn't it ?

No, raw water comes through the drives.

 

[Stroh]

Ok, to addressthe latest ideas:

My raw water pump is the brass one. No wear plate, while the brass end plate is slightly scored, I feel it should be more than fine and no cracks of any kind. Based on the output, just under 5 gallons in 15 seconds per Mercruiser test for the raw water pump, I feel that pretty much eliminates intake water through the outdrive into and out of the raw water pump.

That last mechanic to install the t-stat triple checked the darn thing. It is orientated as it came out the first time, which is temperature sensing"pellet" down. Also that thermostat "sleeve" is indexed so it can only fit one way and is also the way it came out, originally.

The serpentine belt is exactly as it came off and exactly as it is shown on the thermostat housing decal. I also verified proper rotation ofthe raw water pump. It is easy to turn in the correct direction but impossible to rotate by hand in the opposite (wrong) direction.

Build up or growth is not a problem as the boat has always been kept either on a trailer or on a lift, never sits in water.

I will do a compression test, but I have installed clear hoses on the two smaller hoses coming out of the thermostat housing going to the bottom of the exhaust manifolds and when running I am not seeing any bubbles, which should be present if exhaust gas is working its way into the cooling system water.

As was stated, the only through hull I have is for the air conditioner. Raw water for cooling comes in through the Bravo 3 drive via suction of the engine driven raw water pump.

No, my t-stat housing only has three hoses. One large one that tees off from hose going to water pump inlet and two smaller hoses going to the bottom of the exhaust manifolds.

Once again, thank you all for bouncing ideas this way. Typically, this kind of thing ends up, for me at least, as some dumb simple thing I have overlooked and I eventually stumble upon it after throwing a ton of time and dollars at it. So, I will keep at it. Also, I meant to apologize for the "red-neck engineering" comment, if I offended either red necks or engineers, I didn't mean to.

 

[Raeoflight]

Raw water is not pulled through the outdrive by the engine mounted water pump. There is a lower unit water pump that is basically an impeller. This pump pushes lake water up through the outdrive and the engine mounted pump simply circulates it throughout the block. They are supposed to be replaced every 2 years.