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.