Mercruiser MAG 350 with Alpha drive running hot when pushed hard

[twgroom]

I have a Mercruiser MAG 350 with Alpha drive. Engine is running well overall. Temperature will climb up to 180 if you run at full throttle for a while.... might go higher but I usually back it down before letting it. Slow it back down into the 3500+ rpm range and it will run in the normal cool range. Impeller on outdrive has been replaced and at this time thinking I have some level of partial blockage somewhere that is restricting flow. Before I start pulling hoses or parts, I wanted to get some opinions on the best place to look.

 

[alldodge]

What year is the motor and better yet maybe the serial number?

 

[twgroom]

The original is a 2004... block replaced 5 years ago... Cooling setup is standard seawater cooled MPI Alpha model oM300000 and above.

 

[alldodge]

The 350 MPI came with a bravo drive and belt driven seawater pump. The Alpha drive does not provide enough water to keep the motor cool

Take that back, has come with an alpha but also has seawater pump
http://www.mercruiserparts.com/350-mag-mpi-alpha-bravo-0m300000-thru

 

[twgroom]

Other notes: water circulation pump, thermostat and distribution housing are most likely original. No obvious kinks or leaks in any hoses. I have not done any kind of backflush yet. Did not have any obvious event that occurred before this issue.(ie suck up stuff off the bottom, or other failures)

 

[alldodge]

Do you have a belt driven pump on the starboard side of the motor

 

[twgroom]

Yes I do. With an easy drain on the front on the other side. I could upload the water flow diagram for it but it says I can upload attachments

 

[twgroom]

Attachment went through that time... see diagram for what I have.

 

[alldodge]

I asked if you had a belt driven sea water pump on starboard side and you said

Yes I do.

Your pic shows an arrangement which does NOT have a belt driven sea water pump

The belt driven one looks like this



So if you have the pump you should not have an impeller in the drive

 

[twgroom]

Answered too quick... no it does not have a belt driven seawater pump .... just the water circulating pump per the diagram

 

[alldodge]

OK, still is strange that a 350 mag mpi would be using an Alpha. In any case, back to your problem.

Need an IR temp gun to verify temp at the thermostat housing
When the impeller was replaced was the pump housing also done?
Check the Tee fitting where water comes from the drain up to go to each exhaust riser. The T restricts water flow to help keep the block full
In your diagram there is item "C" which is a check valve. Remove and inspect it for debris

 

[twgroom]

It was a whole pump kit replacement 3 weeks ago as it was due from a maintenance perspective and figure that was the first place to start. The old impeller had no damage /missing pieces. Buying IR temp gun tomorrow so I can get better readings around the engine but given the touch test my bet is the gauge is working pretty well... Thanks for the suggestions. The check valve makes sense as I can see something getting caught there. Is there anything special about that T fitting besides just being a T fitting ie no one way check or something?

 

[alldodge]

The T is either a one piece which reduces the ID of the ports going to the risers, or a 3 piece which has reducers placed inside the T.

 

[QBhoy]

All dodge...
Although it?s unusual, some boats with a 350mpi came with Alpha spec.
On a similar note, my own 5.0 mpi with its cam also runs a little hot if you trim her up and run over 60 for more than a few minutes. Hence the reason I don?t often.
I think the only way to solve it on boats like mine would be to get a low water pick up bottom end...it?s matbe not worth the expense though. I just avoid going too quick for sustained periods.

 

[twgroom]

What is the life on the risers?
Life on manifold?
Would you automatically change both manifold and risers or evaluate based on condition?

 

[alldodge]

If your in salt, 5 to 7 years. If your in fresh water, very long time

 

[QBhoy]

Speaking from experience, trying to mate new risers to old manifolds...I?d defo change both at the same time. Not just for good practice, but also because it?s a pain in the arse trying to get them to seal properly otherwise.

 

[twgroom]

I pulled one riser and it looks so good that I did not pull the other. See pics of riser. I did not pull the manifold. I did pull the thermostat housing.. there was scale but not sure if it was enough to cause flow restrictions and the issue I am having with full throttle overheating. Any ideas on where to go next?

 

[alldodge]

What does the gasket side of the riser look like? While your thermostat appears to have some rust bubbles inside, it should be OK so long as the exhaust doesn't have any more

 

[Fun Times]

I have a Mercruiser MAG 350 with Alpha drive. The engine block was replaced 5 years ago.

Was the new engine block from a Marine distribution warehouse/factory, etc.? or GM/automotive?

How was the new to you block listed; 'New or remanufactured, short or long block (meaning what new parts came with it such as heads, intake manifold, "new" to you front water circulating pump, etc.)...Have your receipt that may indicate new items?

The reason I'm asking is did your new engine happen right around the 2012 timeframe? If so then "maybe" you received a new to you water circulating pump of 2012 already mounted to your new engine which GM ended up having an design flaw to their circulating water pump blade impeller which caused different levels of overheating issues.

http://www.themalibucrew.com/index.php?/forums/topic/48335-indmar-water-pump-service-bulletin/

From PCM:

In the event you are diagnosing an overheat or running hot condition, the engine circulation pump should be tested and inspected prior to extensive diagnostics.

PCM is issuing Service Update SUP2012-03 to address a problem with 5.7 engines.
General Motors vendor for the circulation pump has supplied some pumps with impellers that do not meet the "hardness" spec on the metals. The impeller metals in some cases are too hard. This condition results in the impeller cracking and is no longer able to circulate water through the engine and exhaust system. The crack can be anything from a hair-line crack to completely broken as shown below.

^ If not then one other area you'll want to check would be the main water feed hose on the outside of the boats gimbal housing area (portside) for things like kinks, twists, cracks, etc., plus remove the stern drive and check for debris within the water flow passages as you may have something right at the hose opening when you first pull the drive off of the bell-housing....Sort of common....Ask Ted..

As for the T fitting mentioned if it's specific, yes there's one fitted/sized for an Alpha drive design where the impeller is inside the stern drive area and one for the bravo drive which uses an engine mounted, belt driven sea water pump impeller....See the "notes" under item number 20 if needed, http://www.mercruiserparts.com/bam/subassembly/31747/7585/200