My Mercruiser 5.7 MPI is still overheating...GRRR !!!!

[newbienewb]

Hi Guys,

This is a follow on from my recent thread where I discussed my overheating issue, and my lack of power issue. I will close out that thread with a link to this.......as my power issue seems to be resolved.

However, I still have an extremely annoying overheating issue, and im looking to you guys for help.

The package in question is a Mercruiser 5.7 Horizon MPI with a bravo 3 leg. It has what I believe is a half closed cooling system. I have small taps at the bottom of each exhaust manifold.

Here is the story.

I bought the boat about 3 months ago, and while doing the water test it overheated. On idle, the temp would sit at about half, but as soon as I was on the plane (about 3500 rpm) for any more than about 60 seconds, the temp would skyrocket.

I have been gradually changing and replacing parts (that I thought would have been causing the issues), but with no real effect........and this weekend I actually went backwards.

First thing I changed was the exhaust manifolds and elbows. They were badly clogged. I thought this would fix the issue.....but made no difference. At this stage I also found broken flapper parts in my exhaust y pipe. I removed them (I found all pieces), and replaced with new ones. I also took the leg off to confirm no other exhaust blockages.

Second thing I did was took a look at the raw water pump. Whilst it is obviously used, it showed only minimal signs of wear. The impellor is not brittle at all, and has no chunks missing. There are some signs of wear on both the impellor and the inside of the housing where I suspect the previous owner ran it without water. I don't think the wear is enough to cause my ongoing problems.

Third thing I checked was the inlet of the heat exchanger. In here I found a significant amount of old broken impellor (leading me to believe the impellor in it now is relatively new). I removed the heat exchanger completely and took both end caps off. I cleaned out all on impellor pieces and ensured no blockages in any of the small tubes running through it.

I took it back out on the water with no change. Still overheated (was mildly better though - I could at least hold it at about 3000 with temp maintaining about three quarters)

Still not happy I brought it back home and continued looking. I had previously noticed a blue check valve that I didn't pay a lot of attention to, so decided to put it out and look inside it. In here I found a whole lot more broken impellor. So again I cleaned it out, and put it back in. It has arrows on it that seem to indicate the flow of water, so reinstalled it with the arrows pointing in the flow direction.

Whilst I was at it I pulled the hoses off the power steering cooler, which was clear.

Now - from what I can tell, the water gets sucked up through the bravo 3 leg, and into the pump. It then gets pumped out of the pump, through the power steering cooler, then through the check valve, then into the heat exchanger. Where it goes after that is a bit cloudy, but I expect it must come out through the cool fuel system, and into the exhaust manifolds.

Anyway, this weekend I took it out again, and it seems to be even worse. At idle the temp is no longer on half, but about three quarters, and im back to overheating within about 60 seconds if acceleration.

Whilst on the water, I took the end caps off the heat exchanger again to ensure no more blockages........and it was clean. I also took the check valve out altogether, and wjilst It was out I started the engine momentarily. I confirmed that water was pumped strongly out the pipe running into it.

A few other things to note.

1) At idle, the starboard (right side) manifold is cool to the touch, whilst the port side (left) is HOT.
2) If I run the engine, and take off one of the plugs on the bottom of the exhaust manifolds, the water seems to "Spit" out, like there is a bit of air in the system?. Not sure if this is normal.
3) My boat has a fresh water tap connector at the top of the starboard manifold for flushing. When the boat is running in the water, if I push the one way valve, I get a small pocket of air come out before water. Again not sure if this is normal.

I have not at this stage inspected anything to do with the fresh water system. I assume there will be no impellor chunks in there, but obviously need to consider the thermostat, and the fresh water circulation pump.

I have read about a test that can be conducted by putting a hose from the out side of the raw water pump into a bucket, and measuring how much water I get at 1000rpm for 15 seconds. However given I have confirmed good water at idle, have checked the raw water impellor, and am getting quite hot at idle, I am not overly suspicious of the raw water pump.

So, what do you guys think?. fresh water pump (I don't think so), thermostat?, system blockage?, Inlet blockage?

Why is one manifold so much hotter than the other?
How come the temp at idle is now hotter after I cleared the check valve?.
Do I need the check valve?. What does it do?

I have a bunch more ideas of things to check, but I'd like to prioritise what I look at so I don't just go changing things willy nilly.....

Thanks guys - I could really use some input and guidance here.

Cheers
Dave

[myusernam]

What fixed the power issue?

[Dan5]

Have you considered doing an acid bath for the heat exchangers?

We used to do that annually with the Volvo ones.......they could be clogged with scale etc...


Dan

[newbienewb]

hi guys - I believe the power issue was either plugs or leads. I changed both and it seems to be behaving well.

I have thought of acid bath for the exchanger. That's certainly on my list of thins to do. Not sure where to do that in Melbourne Australia though.

Dave

[Dan5]

Possibly a radiator place could do it for you.......they can do an ultrasonic clean also.

Dan

[newbienewb]

just rang a few radiator places in my area - and apparently acid bath's are a thing of the past.

[The Woo]

I'm feeling your frustrations.......
The raw water pump. Sounds like, given the amount of broken blades you have found, that the pump has been well abused by the past owner. I know you've said the impeller is in good condition, but what of the pump housing itself? Is it badly scored therefore not allowing it to pump good pressure, or more likely, has the cam within the pump housing worn down from abuse, which will also massively reduce the pressure it can pump.

They're a very simple system, and methodically working through it will solve your issue.

Also, your raw water bump is defineately fed via the leg? Many are fed by a hull mounted skin fitting. If its via the leg, I've seen several issues with the hose that feeds from the leg, up through the gimbal housing, and through the transom shield. Obviously if it is kinked, collapsed, or swollen, it will have significant effects on the water available to the pump.

[newbienewb]

hi The Woo - Thanks for the reply!.

I took the raw water pump apart, and whilst the impellor is still plyable and soft, indeed it does have scoring on both the pump and the inside of the housing. Looks like the previous owner may have run this one without water for a bit?.

Im quite sure that the water is being fed by the leg. I can see now other fittings on the hull. If I want to check this pickup, do I need to take the bottom part of the leg apart?. Is this a big job?.

One other thing (and im definitely showing my ignorance here) - someone else suggested to me the problem could be the "bellows". Looking at a few online diagrams, im not seeing how?. Any comments on that?

Cheers!

Dave

[Aussie123]

I think you need to fit a whole new water pump kit which includes the housing,wear plate and seals ect along with a new impeller.
I believe that will solve your issues.
Wear marks inside the housing is a very good indication the housing and wear plate is stuffed.

[terryc]

i wonder how the water circulating water pump on the front of the motor is looking ?.
I have the Alpha leg and was able to drop the gear housing part of the leg only, to replace the water pump in the leg.
I read somewhere that sometimes an additional water input is required from a pickup mounted to a skin fitting in the hull, to get adequate water flow, (you would need expert opinion on this).

[newbienewb]

hi guys.

Its a good point about the circulating pump. In fact just today I ordered some circulating pump gaskets, with the intent of removing the pump and checking it.

Ive also ordered a new raw water impellor and housing.

This evening I also pulled out the thermostat to have a bit of a look at it. I put it in a pot of water with a thermometer and slowly boiled the water. It opened right on queue at 160 degrees. I've ordered a new one all the same, but I am suspecting this will be wasted money.

Can anyone tell me what the easiest way would be, and the things I need to look for to inspect the intake side?. If I pull the leg off, will I be able to adequately inspect the bellows / intake pipe?....or will I need to pull off the transom?.......

Any other ideas?

Thanks guys.

Dave

[captain rednut]

Hi Dave
Sounds like the Water inlet hose on the transom gimbal housing may be your problem?? where it goes through the transom, unbolt the two small bolts on the inlet water hose tail on the inside of the transom shield and check the nylon hose sleeve and gasket for swelling.
Ive had plenty do this before and go one apart now if you want to see one.
Cheers Jim

[newbienewb]

hi jim....

thanks for the reply. If you have any photos or diagrams of the ones you have seen like this, id be very greatful to see them.

Im not quite following what you mean by inside of the transom shield?. Do you mean on the inside of the actual boat, or the outside?. I think I have a few merc service manuals here. I might see if I can find something........

If you have any more info you can give me, or photos, that would be wonderful. Thanks very much.

Dave

[Dignity]

I am reading this with interest as my mechanical knowledge can be written on the ball point of a pen so forgive me if my conclusions are out of wacky but how would the water inlet or the raw pump cause the heat build up on the port side manifold only, just seems that these could be contributing issues but is it distracting from the real cause.

[The Woo]

Whoever told you bellows could be a cause of the problem is leading you up the garden path.
Hopefully Jim can come up with an image to show you where the issue probably lies.
Don't worry about the engine circulation pump. Unless it's leaking, it's fine. They don't corrode or fail otherwise, so more wasted money and poor info given to you there.

[newbienewb]

Hi Guys,

Dignity - its a questions that is plaguing me also. I cant seem to find an accurate document as to how the water flow in my setup is meant to be, so its making it very hard to pinpoint why just the one side is getting hot. For all I know the previous owner may have put hoses back incorrectly, causing water to flow incorrectly.

The Woo - Thanks for that. I did a bit of reading on the purpose of the bellows, and was not seeing how they could contribute to the overheating. I also agree with you about the circ pump......

At least I seem to have ruled out the thermostat. Yay.

Dave

[Fed]

Is yours in this lot?
http://www.mercruiserparts.com/selectModels3.asp?type_id=8

At the end of the day you still will have salt water going in, through the exchanger etc and then back out through the exhaust manifolds.
I'd be giving it a short run with the hoses off the exhaust manifolds to see if they had equal flow, maybe run that idea past achris at iboats first.
The plumbing looks pretty straight forward to me, surely not that hard to find why one side is running hot.

[scottar]

I am thinking along the lines of Dignity. You know the impellor has disintegrated at least once previously so a blockage on the side that is running hot is the first thing I would be having a serious look at before you spend money on parts. I'm no mechanical whiz but is it possible to back flush the cooling passages in the engine to clear any crap out?

[newbienewb]

hi guys.

fed - mine is - http://www.mercruiserparts.com/selectSerailRange.asp?doc_nbr=350+MAG+MPI+ALPHA%2F BRAVO

Oddly enough, I thought mine would be one of the "horizon" models given that this is the engine cover it has on it. And the serial number of the engine marries the serial on the block just above the starter.

I certainly cant see any check valve in that diagram. Which now has me questioning whether I need it or not.

I cant imagine how impellor fragments would have been able to get as far as the exhaust manifolds. It would mean the pieces needed to somehow make it through all the tiny rods inside the heat exchanger.

Dave

[The Woo]

Double Post

[The Woo]

Exactly, impeller bit can't get past the heat exchanger.
Your water flow OUT is ok now you've replaced the risers. There is no harm removing the couple of hoses leading to the risers that have salt water running through them and feel them for stiffness/scale. Abused boats can have abused hoses, and it's possible for scale to build up within them. However, it would have to be a really extreme case to affect the temp as badly as yours.

Now, back to the more likely cause of the stuffed pumps and hot running. To find the spot where Jim is talking about, simply trace the inlet hose for the raw water pump back to the INSIDE of the transom shield. We call the transom shield the part that the rear of the motor is bolted to. It is attached to the transom of the boat. You'll see the hose appear from there where is comes from the leg pickup via the hose that runs through the gimbal housing.

Edit: I almost forgot, have a REALLY good look in the inlet hose PRIOR to the raw water pump. When an impeller disintegrates blades can and sometimes do go into the inlet hose, causing a restriction to the flow to assist in stuffing yet another impeller.

Edit 2: I've never seen one with any sort of "check valve". Got a pic?

[newbienewb]

afternoon guys :)

Before i go on - just want to throw a sincere thanks out to all involved here. I truly appreciate the help and guidance you are all giving me.

OK - So i just popped outside (despite actually being home from work crook) - and removed both hoses from the bottom of each exhaust manifold. I then connected up my hose and turned it on. The water was most definitely not coming out evenly. The water favored the starboard side considerably. However, when i blocked the starboard hose, the water certainly started flowing freely out the port side (indicating no blockage?). I repeated this same test 4 times, with the same results every time. Water initially favors starboard, but will happily flow out the port side if i block starboard.

Just to clarify, when i initially plug the hose in, the water out the port side is MINIMAL.

Also to add, this test was conducted without the engine running. I was simply using mains pressure.

Attached picture of check valve. Still cant work out why it would have been in the system - or if i really need it. Strange thing is, before i took a bunch of impellor chunks OUT of the check valve (that were actually forcing it to stay open anyway) - the temp was lower.

107915107916

[The Woo]

Pics aren't working for me mate. Where did you attach the hose? Raw water pump outlet hose?

The water flow from each port/stbd hose will never be identical, it will always favour one side, especially at low flow and pressure rates. There'll always be a fraction more restriction to one side than the other.
What it relies on is shed loads of water flowing through the system, more than one just one circuit can handle, forcing it through the other circuit. It's still pointing to a pump/flow problem. A garden hose is ok for flushing, but typically won't supply the volume the raw water pump can flow.

[newbienewb]

Hi The Woo,

Not sure why the pics didnt work - ive uploaded them to photobucket and attached them here a second time.

107917 http://i1284.photobucket.com/albums/a571/davus123/Whittley/IMG_2013_zpscohoqu5n.jpg

I attached the hose to the small connector that sits up the top of the starboard exhaust manifold. The one i always use when flushing the motor after a run in the bay.

http://i1284.photobucket.com/albums/a571/davus123/Whittley/IMG_1489_zpsy7afl1c0.jpg

I was told by the previous owner to NEVER run it with earmuffs - and only ever run it off the hose connector. If im not going to cause any issues, id rather run it off earmuffs.......at least then im using the whole water path.

[StevenM]

Strange

Seems funny to me that it overheats as soon as you accelerate .

Would tink with all the work ou have done on the sea water side and your systems that the fresh water side is the issue.

I would be looking to see if you have block pressure. Or just remove the fresh water pump and ensure the impeller on it is not slipping or broken up. May be a Bakelite impeller

[newbienewb]

hey Steven,

I believe i have both block pressure, and circulation.

At operating temp - the fresh water cooling system has good pressure. Additionally, i have a hot water service in my boat (which is about 6 feet away from the engine), that uses hot water from the engine to heat. This get nice and hot - so im assuming i have circulation.

:(

Dave

[The Woo]

I'll be honest and tell you I'm not familiar with the engine flush kit on your engine. It is factory, but an option I believe. Not sure how it is plumbed, but obviously, if you're running the engine with it attached, it should be wetting and therefore lubricating the raw water pump somehow.....

That check valve is a bit "special". Looks like a nice restrictor and prone to breaking and failing, cooking your engine. I'd pif that over my shoulder asap.

[newbienewb]

thanks The Woo.

I took the valve out and pulled it apart. Luckily the guts of it simply unscrew, leaving a straight through pipe. This is how i have reinstalled it.

Im starting to think i am going to need to pull the leg off, and unfortunately the bell housing so i can try and get to the inlet hose. I googled something called "bravoitis".....and am starting to think this might be the next thing i need to pay serious attention to.

Im sure others call it something different, but it seems to be very common to bravos.

I tried to get the hose connector off on the inside of the boat (the other side of the transom) but it looks like someone has tried to get it off before me and stripped the head off one of the bolts. There's no way i can get it off. which is a real prick because with the endoscope camera it would have given me a pretty quick answer.

Dave

[snatchy]

Attached picture of check valve. Still cant work out why it would have been in the system - or if i really need it.


The check valve should stop the heated raw water from flowing backwards to the fuel cooler when the engine is hot after shutdown. When it does that you are more prone to Vapour lock as it heats the fuel. If the check valve has been installed then the previous owner may have experienced Vapour lock so I suggest you leave it there so you don't develop another problem before you solve the overheating issue;)
More info on vapour lock is in the Mercruiser Service bulletin 99-7. Here is a link;
http://www.boatfix.com/merc/Bullet/99/99_07.pdf

Good luck with sorting out the overheating. You seem to be going through it methodically and properly so you will fix it!

[shaungonemad]

Is the temp gauge accurate? is it getting hot and staying at that way it should come up fairly quick to about 80c and hang around that.

[newbienewb]

snatchy - thats a great post. Thanks for that. It makes sense as to why the check valve is there. The boat used to operated in queensland, so it may have been more of an issue up there.

I think ill run without it just for a period of time to see if it makes any difference.

On the up side.......i think i may have (read MAY) found a smoking gun. Going from advice in these threads, i started looking at the pickup from the leg, up through the transom and into the pump. Whilst i couldnt really be bothered pulling the leg off this afternoon, plus i dont have the required tools, i opted to limber up and dive in behind the engine to see if i could pull out the hose connector for the raw water on the inside. This would give me a direct look at the end of the hose where it meets the transom. I had a heck of a time getting the part off, but finally i manged. And in there i saw a significant blockage. It doesnt appear to be corrosion in full, but in actual fact a collapsed water tube insert???.............one of these.

http://i1284.photobucket.com/albums/a571/davus123/41674T_zpsyj8mdxuz.jpg

Here is a picture of what i found. I contorsionised myself for these photos.

http://i1284.photobucket.com/albums/a571/davus123/FullSizeRender_zpscr81ogtm.jpg (http://s1284.photobucket.com/user/davus123/media/FullSizeRender_zpscr81ogtm.jpg.html)

http://i1284.photobucket.com/albums/a571/davus123/FullSizeRender1_zpsrj3ly1ee.jpg

So - what do we think guys?..........confident?.

Whats the easiest way to go about fixing it?

Dave

[shaungonemad]

That can't be doing you any favours there can't be to many demons left in the system

[sharkymark2]

geez I hope that is it. You must have lost a fair bit of hair by now.

[StevenM]

Mate your onto it. That would be reducing water flow by more than half.

[The Woo]

Now ya talkin! Get better mate, then fix and enjoy.

[captain rednut]

Thats the water inlet fitting i was trying to explain, you done well and can now go boating with a cold engine.
Regards Jim

[captain rednut]

That is a very common problem and one solution is to block that failed fitting off and put a hull skin fitting in to feed the engine which is common but you will have to fit a sea strainer.
If you do this you will need to cut the inlet water hose off the transom shield on the outside to allow the leg to be cooled.
If you require any more info feel free to call me.
Jim 0418748023

[Moejoes]

Hi Dave,
I had a similar problem on a 32ft Searay with Volvo legs I bought.
It ended up being a corroded alloy fitting (similar to a Holden 186 thermostat housing) that was connected to the pick up hose half way up the leg that then ran up through the transom. Same thing, ran at normal temp then over heated straight away as soon as it planned because it was then sucking air.
The previous owners spent a motsa trying to fix it but couldn't diagnose the problem.
It cost me $40 for a new part & problem was solved.
Rob.

[snatchy]

That is a very common problem and one solution is to block that failed fitting off and put a hull skin fitting in to feed the engine which is common but you will have to fit a sea strainer.
If you do this you will need to cut the inlet water hose off the transom shield on the outside to allow the leg to be cooled.
If you require any more info feel free to call me.
Jim 0418748023

Hi Jim
If you do this as you suggest, does the leg still need its own impeller to keep it cooled? So the engine has its own through hull pickup and water pump, plus the leg needs its own to keep it cool? I have this setup already and was hoping to ditch the leg impeller so i only need one water hose to flush after use (Not the two hoses I currently need) but I am not sure of the cooling needs for the leg and I am starting to research this idea.
Apologies for the slight deviation off topic

[captain rednut]

Providing you have an engine sea water pump which im assuming you do?? yes we remove the leg pump totally and you can either cut the hose in behind the shield to allow cooling at speeds or fit a drive shower for $250 off Ebay.