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Mercruiser Running Hot/Overheating - Help and Ideas Needed

E

enginesilo

Contributing Member
I just got the boat uncovered and all tweaked up to get ready for the season and on the hose it was running around 190+ degrees. My engine has always seemed to run hot on the gauge (175 F degrees) with no other symptoms or issues, but this year it has me worried as it seems to have climbed higher.


It is a 2005 Mercruiser 3.0L with the alpha 1 gen 2. I wet slip it in salt water and have for the past 4 years or so.

I changed the impeller last season, and the thermostat today but the temp is still super hot.

Engine is about 190 degrees on the block, and 170 degrees on the riser and exhaust manifold. The thermostat housing only seems to read around 135 degrees but it is a 160 degree thermostat. It starts right up and idles no problem, but is super hot. I noticed that it looks like not a ton of water comes out of the back while running, but some does come out and more when I rev it.


What are the next steps to troubleshoot?
 
Due to many different models your serial number is needed to identify what thermostat your motor should have.

some use a 140 degree and others use a 160 degree.........just because you have a 160 does not mean it is the correct one unless you know for sure that is what your motor serial number requires.

assuming the thermostat is good (could be stuck, corroded or bad or debris in it), the raw water cooling system is fairly simple.

Running a 160 degree thermostat the normal running temp will be around 170. and running hard at wide open throttle I would expect to see 180 -190. assuming everything is in good shape.

over heat conditions are either lack of water coming in or lack of water getting out.

Also assuming the impeller is good as you changed it last season that would say water is coming in is ok. BUT when the impeller was changed was the small black boot installed on top of the impeller housing? this small black boot helps in keeping exhaust gasses from entering the impeller housing. If exhaust gasses do get in it will create bubbles in the incoming water stream and cause over heating.

Just because you see water coming out the back of the transom does not mean it is the correct amount.

If it is very hot and steamy that may indicate lack of water allowed to exit the exhaust elbow.

The fact that you keep it in salt water vs fresh water is the first key in this. The elbow I believe is cast iron and a one piece half round design???? (not 100% sure of that) but I know the exhaust manifold is cast iron and extremely susceptible to corrosion. there are several types of manifold configurations, some have end caps and some don't. If yours have end caps you could remove one and take a look at what you see. If it is clogged or extremely rusted and full of scale then that may be your issue.
If the elbow or manifold has corroded and heavy scale is there it is most likely choking off the water exiting the motor.

So that being said without being there I would guess the exhaust manifold or elbow is clogged up with rust and scale......

Also you could check if it is present, the power steering cooler to see if there is a obstruction slowing down the water coming in. If any large debris made it past the impeller it could be stuck in the power steering cooler.
 
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Also check the water inlet on the foot and make sure you don't have some growth/buildup there.......make sure some sea creature hasnt callled that area home........
 
kghost said:
Due to many different models your serial number is needed to identify what thermostat your motor should have.

some use a 140 degree and others use a 160 degree.........just because you have a 160 does not mean it is the correct one unless you know for sure that is what your motor serial number requires.

assuming the thermostat is good (could be stuck, corroded or bad or debris in it), the raw water cooling system is fairly simple.

Running a 160 degree thermostat the normal running temp will be around 170. and running hard at wide open throttle I would expect to see 180 -190. assuming everything is in good shape.

over heat conditions are either lack of water coming in or lack of water getting out.

Also assuming the impeller is good as you changed it last season that would say water is coming in is ok. BUT when the impeller was changed was the small black boot installed on top of the impeller housing? this small black boot helps in keeping exhaust gasses from entering the impeller housing. If exhaust gasses do get in it will create bubbles in the incoming water stream and cause over heating.

Just because you see water coming out the back of the transom does not mean it is the correct amount.

If it is very hot and steamy that may indicate lack of water allowed to exit the exhaust elbow.

The fact that you keep it in salt water vs fresh water is the first key in this. The elbow I believe is cast iron and a one piece half round design???? (not 100% sure of that) but I know the exhaust manifold is cast iron and extremely susceptible to corrosion. there are several types of manifold configurations, some have end caps and some don't. If yours have end caps you could remove one and take a look at what you see. If it is clogged or extremely rusted and full of scale then that may be your issue.
If the elbow or manifold has corroded and heavy scale is there it is most likely choking off the water exiting the motor.

So that being said without being there I would guess the exhaust manifold or elbow is clogged up with rust and scale......

Also you could check if it is present, the power steering cooler to see if there is a obstruction slowing down the water coming in. If any large debris made it past the impeller it could be stuck in the power steering cooler.
Click to expand...

midcarolina said:
Also check the water inlet on the foot and make sure you don't have some growth/buildup there.......make sure some sea creature hasnt callled that area home........
Click to expand...

Good info guys. Here is a link to my exact engine, and the water components (2005 and Serial starting with 0W0). Mine is raw water cooled, that link has both, this is what mine has:
http://www.marinepartsplus.com/catal...62+99/4893-150
http://www.marinepartsplus.com/catal...62+99/4893-200

Mine does call for the 160 thermostat, its shape is different from the older 140 and I had my parts guy verify in their parts catalog. I changed it yesterday and it seemed to have been better but after a minute or two of running it was back climbing to the 190 range.

I changed the impeller last season because I thought it was running hot, but no change occurred. I checked the intake and cleaned out a few barnacles, but other than that it looks clear from what I can tell.

So 170 at idle and 190 degrees while running is normal? Sounds hot. One main concern is that my riser is to hot to keep my hand on.

Water coming out IS steaming and minimal. Could that also mean a bad circulatory pump? I am considering pulling some of the hoses to see what type of flow is entering the thermostat housing, and then entering the exhaust manifold, and run for a few seconds to see if there is an obvious restriction of flow at any of these points. Next step I guess would be to pull the riser elbow, and then the exhaust manifold.

Anyone know the type of flow that should be entering the thermostat, and entering the exhaust manifold? Garden hose on high a good comparison?
 
I hadd similiar problem, took advice suggestion from one post
I disconnected hoses n chechecked for debris n found some at the drain valve
then also removed risers (for the price of a gasket , was worth peace of mind) another
post suggested n finally checked my circulating water pump on
engine n found it was burnt burnt out, replaced it n my problem solved.

Lots of great advice n direction on this forum
 
One main concern is that my riser is to hot to keep my hand on. Water coming out IS steaming and minimal. Could that also mean a bad circulatory pump?

Bad pump is possible but the most obvious (to me) key to your problem is that the riser is too hot to keep your hand on. Salt water engine, 9 year old manifold and riser. They've reached the end of their life expectancy. It's time to change them before they fail completely and ruin the motor.

My .02
 
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The next step is to put it in the water and test it. You cannot diagnose engine temp problems on a garden hose.

The hose will introduce air into the cooling system and cause false readings. If it runs hot when sitting in the water.............
 
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