Mercruiser 5.7 MPI Horzin still overheating ..... ARGH!

[davus]

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

 

[Bt Doctur]

In most cases the water flow is up from the drive, thru the cool fuel module, thru the power steering cooler, to the water inlet of the heat exchanger, outlet from the heat exchanger T`s into the manifolds and water goes out the riders.

 

[alldodge]

What is your engine serial number which would be best, but at least what is the year?

 

[davus]

thanks for the flow diagram Bt Doctor - that's helpful. I could very well be wrong about the order of the flow.

Engine serial is 0M093415 - not sure about the year....can anyone tell me based on serial number?

 

[Bt Doctur]

350 Mag, poss 1998 manufacture Service Literature for MERCRUISER 350 BRAVO HORIZON 350 MAG BRAVO MPI
PARTS CATALOGS

• 809488 99 - 350 MAGNUM MPI ALPHA/BRAVO (GEN+) ENGINE

 

[wrench 3]

I can't help but wonder if the previous owner overheated the engine when the impeller went out. Thus causing a leaking head gasket or something. Can you install a piece of clear hose in the line from the thermostat housing to the heat exchanger, and check for bubbles.

 

[Bt Doctur]

Another possibility of cooling failure is in the water outlet fitting at the transom housing
the other place is the water inlet at the gimbal housing. It can corrode closed the same way..

 

[alldodge]

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?

One manifold can be a bit hotter then the other but not that much. Pulling the hose off and seeing the water spit out with air is not correct.

What check valve are you referring to? I don't see a check valve in your system. Look at BtDoc pic and this one and see if you can identify what your looking at. Since you have found so many pieces of an impeller in different locations, I would remove each hose one at a time and flush them out. In most cases the pieces get caught in the steering cooler and thermostat housing. It can get caught in the cool fuel but chances are less. Either way things need to be flushed.

Also remove hose going to seawater pump and back flush to the outdrive




EDIT: You did use gaskets between the manifolds and risers with holes in them

 

[agallant80]

Have you changed the temp sensor?

 

[davus]

No Title

Hi Guys,

OK - Ill try to answer all questions above.

Wrench 3 - I thought about head gasket as well, however I have performed two tests which I believe rule that out.
1) I did a radiator chemical test on the closed cooling system. Effectively what this does is shows if there are exhaust gases getting into the cooling system, which I have confirmed there are not.
2) I did a radiator pressure test, and confirmed that the closed cooling system is holding pressure - IE there are no leaks into exhaust / compression chambers.

BT Doc - A blockage at the water outlet at the transom is something I have not checked. I believe I would have to take the leg off to do this inspection?. Do I need to take off any more than the leg? Im fearing you will tell me the bell housing has to come off?
Im not sure about the water inlet being the problem, as when I hook up my hose to it (which connects at the motor - right on top of the starboard exhaust manifold) - the temp is still slightly high. (Remembering for some strange reason after I took all the pieces out of the check valve - the temp went up for some reason - even though I confirmed the correct orientation of the valve on reinstallation). Wouldn't this effectively bypass the inlet being the problem? All the same I think I should check it but still waiting to find out the best way to do this.

AllDodge - I have a check valve in the raw water system between the fuel cooler and the inlet to the heat exchanger. It is a blue valve. I don't see it anywhere in Docs image, however I can see it in the cooling system diagram in the Number 31 Merc Service Manual (screenshot attached showing valve at point "i".
I did use gasket when I installed the new elbows - but they didn't look like the ones you show. They were similar but only had two of the larger holes, and then two very small round holes. (picture attached.)
Im not sure how impellor pieces can get caught in the thermostat given it is a closed cooling system. Arent all the raw water components separate to the fresh water system?

agallant80 - I haven't changed the temp sensor yet. I hadn't really suspected it.

Last night as another test I checked the thermostat operation. I removed it, and put it in a pot of water. I put a thermometer in the water and slowly boiled it. The thermostat opened right on queue at 160 degrees. So I suspect it is operating fine.

ARGH!

Dave

 

[Bt Doctur]

 

[Bt Doctur]

 

[Fun Times]

Hi there, your engine serial 0M093415 and looking at a few items on the parts catalog puts your engine year between 1999 2001 range. What year is the boat and is the engine original to the boat?

The image in post 12 is over your engine serial number range so yours shouldn't look like that.

The check valve may not have been original to your engine model but it could have been added later usually to help with fuel vapor locking issues to the cool fuel system located under the portside engine mount area. It keeps the hot water from back flowing down to the fuel pump area.

I'm onboard with you'll want to check the water inlet fitting to ensure there is no blockage. First back flush the hose from the pump to the stern drive to get a feel of how much water is flowing to the water inlet holes at the drive. Then do your best to remove the black hose plastic fitting on the inside of the boat at the transom assembly to see if you see any corrosion starting to buildup. It a common problem with bravos that play in saltwater, etc. and since you had bad exhaust, you'll want to know the condition of that area anyways.

It's really not recommended to run the engine on the added inline flush attachment on land because you are going to get a false reading because you are still going to suck air down at the stern drive do to all the inlet holes being open. "Any" air at the impeller = air bubbles in the cooling system which = overheat.

Every time an overheat is suspected it's best to install a new impeller and in your case a new impeller housing as well. Yours should be the one piece plastic housing which you can buy in a kit for around $100.00.
Did you see "any" grooves in the impeller housing when you had it apart? If so, you need a new one to continue to diagnose this properly even if it's not the problem. You can save the one you have now for an on the water emergency when needed. < I'm not trying to spend your money, just being helpful/truthful.

Member agallant80 hit's a good point, you need to ensure the engine is really getting hot and the gauge is not giving a false reading. If the warning horn system is working correctly, the horn should sound once the temp get's to around 196+ degrees.

An infrared temperature gun is your friend here. http://www.harborfreight.com/catalogsearch/result?q=Temp+gun

Does your exhaust system have the 4 inch risers between the manifold and elbow or just manifold/elbow?

Using clear water hoses to see air bubbles may be in your future.

Back flushing all points from the heat exchanger to the impeller housing then to stern drive is important since you have/had a lot of old impeller parts. As you know your thermostat would not be affected by old impeller parts but it's a good thing you still checked it because you just never know.

Also be sure you can see light between the two heat exchanges end caps when removed.

If your antifreeze is green in color, it's recommended to completely change it out every two years as a preventive maintenance... Orange is 5 years.

Good luck.

 

[davus]

Thanks Fun Times!.

I love reading long posts like this with loads of info. Puts a smile on my dial.

To answer your questions - i was told by the previous owner that the boat had a new engine put in it, however the boat is a 2001 model, which coincidentally falls smack bang in the middle of the years you mentioned. Perhaps he spun my a porky pie.

I was told by the prvious owner to never run the motor on land with earmuffs, and that i HAD to use the water attachment. Is this true?. In some regards id rather use earmuffs. At least then im using the "real" pickup path.

I sure did see grooves in the on the inside of the impellor housing. No doubt about that. I have ordered a new pump and will install it tomorrow.

My exhaust is just the manifold and elbows. No risers. I am using the gaskets in the picture I attached above with two small holes and two large holes. Are they the right gaskets? I would have thought the ones with 4 larger holes would be better = more water flow?

I can definitely see light through all tubes inside the heat exchanger. I have also run brushes through to clean all tubes.

Thanks guys. Really appreciate all your help.

Dave

 

[davus]

hi guys - just wanted to report back with a bit of an update.

i think i may have (read MAY) found a smoking gun. Going from advice in these threads and others, 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.

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

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

Whats the easiest way to go about fixing it?

Dave

 

[Bt Doctur]

Gee, looks familiar.(post 11) get the hose and the 2 inserts, and carefully make the tool from a piece of steel or buy the tool to screw the inserts in. Corrosion builds up and crushes the retainer and the water hose.

 

[alldodge]

Looks like you found your issue as BtDoc mentioned.

The exhaust gasket I showed was not the one for your engine it was just a pic I had handy. The one you installed is correct.

Your correct no pieces would wind up in your thermostat housing

 

[Fun Times]

]Looks like you found your problem... Yes the plastic has collapsed due to corrosion buildup. Either you'll have to try and clean it out the best you can and maybe coat the area with something to try and slow it from happening again or you could try this kit. http://www.amarket.com/IM654.HTM Click on the links inside this link for more info.

 

[Bt Doctur]

Now that looks a bit funky, I would do the thru hull using a thru hull and hose to the pump and somehow block the transom side.

 

[Fun Times]

Thanks Fun Times!.I love reading long posts like this with loads of info. Puts a smile on my dial.

With your long post above, I had a feeling you were hoping for a long response.

To answer your questions - i was told by the previous owner that the boat had a new engine put in it, however the boat is a 2001 model, which coincidentally falls smack bang in the middle of the years you mentioned. Perhaps he spun my a porky pie.9

While he may have put a new engine in, the question would become is it a new block of the same type or a complete engine with all new accessories. There are usually two areas to look for the serial number...One is on top of the engine around the flame arrestor area and the other (if it's from Merc) is towards the rear starboard side mounted just above the engine starter on a metal tag riveted to the engine block. Compare the numbers someday.

I was told by the prvious owner to never run the motor on land with earmuffs, and that i HAD to use the water attachment. Is this true?. In some regards id rather use earmuffs. At least then im using the "real" pickup path.

The stock Mercruiser flushing systems is more designed to flush the engine if the boat is sitting in the water whether the engine is running or not. If the boat is out of the water, it's best to use the flusher on the stern drive....But you need to be sure the flushing kit inside the boat is sealed/closed/capped up tight so it won't suck air either.

I sure did see grooves in the on the inside of the impellor housing. No doubt about that. I have ordered a new pump and will install it tomorrow.

With the low water flow you found due to corrosions, it's good that you bought a new impeller kit.
Normal use develops the grooves around the 100/150 hour mark on basic average....sometimes less. Add any sand, debris, etc. etc. to it reduces the time frame.

My exhaust is just the manifold and elbows. No risers. I am using the gaskets in the picture I attached above with two small holes and two large holes. Are they the right gaskets? I would have thought the ones with 4 larger holes would be better = more water flow?

You are using the correct gasket recommended by Merc. The full flow 4 hole gaskets are designed for the risers had you had them.

I can definitely see light through all tubes inside the heat exchanger. I have also run brushes through to clean all tubes.

:thumb: