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2004 Mercruiser MAG 350 with Alpha drive running hot when pushed

twgroom

New member
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. In fact, if you just do a normal cruise around the lake you would never see any issue. Only when full out for a while or pulling tubes and skiers for a while do you even see an issue. Impeller on outdrive has been replaced. The engine block was replaced 5 years ago... Cooling setup is standard seawater cooled MPI Alpha model oM300000 and above. I have attached a picture of the water flow diagram. 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) 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.
 

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Whole pump kit just did it 3 weeks ago as I was due to replace anyway from a maintenance perspective and figured that was the first place to look for an issue.
 
everything in the lower unit seemed in good shape with no signs of overheating. I think I need to pull the risers this weekend. Seems like given the feedback on the symptoms, age of risers I probably just need to inspect them.


Or maybe plug in a CDS diagnostic tool and see if your water pressure is elevated or low at higher RPM.
Make sure your drive belt is routed correctly........

What brand water pump did you install? Any pump besides OEM is crap and will cause unexplained over heat issues. Sierra is the worst. The liner in the housing will rotate and restrict water flow.
 
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everything in the lower unit seemed in good shape with no signs of overheating. I think I need to pull the risers this weekend. Seems like given the feedback on the symptoms, age of risers I probably just need to inspect them.

Your seawater cooling system schematic shows elbows only (item "j" in the schematic) ...... (i.e., no risers/spacers)
None-the-less, you would be on the right track if you were to pull the elbows and inspect the "seawater transfer ports".
When these transfer ports become restricted by rust scale, they will cause over-heating at higher RPM.

This will cost you a set of gaskets and your time only.





.
 
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?

Rider from back 1.jpgriser from back 2.jpgRiser inside 1.jpgriser inside 2.jpgThermostat.jpgt
 
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?

View attachment 16770View attachment 16771View attachment 16772View attachment 16773View attachment 16774t

Or maybe plug in a CDS diagnostic tool and see if your water pressure is elevated or low at higher RPM.
Make sure your drive belt is routed correctly........

What brand water pump did you install? Any pump besides OEM is crap and will cause unexplained over heat issues. Sierra is the worst. The liner in the housing will rotate and restrict water flow.
 
Used OEM pump kit. Pulled the other riser and it looked as good as the other one. Exhaust flappers good. No restrictions or issues with lines on all sides of the T that splits to both risers. No real aha moment yet.
 
If everything is clean like you say...Including the casted water dam, the plastic water dam and the filler block in the outdrive...Find someone with a diagnostic tool that tell you what the water pressure is at idle and when running in the upper RPM range...Unless you have a smartcraft gauge.

Make sure the divertor valves are clear on the bottom of the exhaust manifolds. If you pull them off and the opening is restricted with black crap...it will be time to replace the exhaust.
 
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Listen to Chris. He knows the Mercruiser very well.

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....
Please understand that WOT RPM is a test RPM ONLY......... it is not an RPM to be sustained for any duration.


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.
In fact, if you just do a normal cruise around the lake you would never see any issue.
Yes...... that would be more in line with the boat's intended usage!

Only when full out for a while or pulling tubes and skiers for a while do you even see an issue.
Understood!

....................

Your seawater cooling system schematic shows elbows only (item "j" in the schematic) ...... (i.e., no risers/spacers)
None-the-less, you would be on the right track if you were to pull the elbows and inspect the "seawater transfer ports".
When these transfer ports become restricted by rust scale, they will cause over-heating at higher RPM.

This will cost you a set of gaskets and your time only.

Your photos show the mixing chamber ports/passages, but they do not show the seawater transfer ports between the "elbows" and the manifolds.
Are these transfer ports clean and non-restricted?



Or maybe plug in a CDS diagnostic tool and see if your water pressure is elevated or low at higher RPM.
Make sure your drive belt is routed correctly........
I would do what Chris has suggested.


Manifold .... Spacer/Riser .... Elbow
(example photos only)
 

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Further: Full Throttle for awhile... Per Ricardo... "test rpm only". My engine spends less than 30 SECONDS a year @ WOT. For decent life, with the CORRECT PROP*, limit continuous RPM to 80% of the actual WOT RPM you see. 75" for better life, and 70% for even better life.

*CORRECT PROP... That prop which allows the engine to rev up to within the published ( typically on the decal of the flame arrestor for MERCs) MAX WOT range. This is with the boat with its "normal load", clean bottom, good state of tune and with a full tank of gas. If engine won't get to the lower published number, it is over propped which is a life shortening situation for the engine. If it overrevs slightly, this is liveable in so long as you take care to never run over the maximum published WOT RPM.
 
I run them at wide open throttle all the time for extended periods of time.... If everything is right, this will do no harm.

True in the short term, however. My comments about continuous % RPM vs. engine life is not my opinion. It is the data I got from product engineers from three different OEM engine manufacturers over a period of over 30 years. Was true for engines built in the 70s, true for engines made in the 90s, and still true when last I talked to a product engineer at Mercruiser several years ago. Improvements have been made, true, but it still reduces engine life running continuously @ MAX WOT RPMs. You pays your money, you takes your chances.
 
Bob, you are absolutely correct!


....
I run them at wide open throttle all the time for extended periods of time.... If everything is right, this will do no harm.

Chris, I am surprised to hear you say this...... and I'm even more surprised that you would suggest that this is OK to do.... let alone post this on the forum.

Running any Gasoline Marine Cruiser Engine @ WOT RPM for long duration will certainly shorten that engine's life span.
This is NOT even debatable!


.
 
Bob, you are absolutely correct!


Chris, I am surprised to hear you say this...... and I'm even more surprised that you would suggest that this is OK to do.... let alone post this on the forum.

Running any Gasoline Marine Cruiser Engine @ WOT RPM for long duration will certainly shorten that engine's life span.
This is NOT even debatable!


.


It is not 1970.

Any newer Mercruiser Built marine engine is built to run at wot for extended periods of time. I will put a new 350 mag MPI in a boat and run it 12 miles down the bay at WOT and not even think about it.

it is not going to come apart or even show any wear.

I just ran my own 7.4 MPI Lingenfelder(that i built) at 5000 rpm for an hour. No worries. If it comes apart, there was something wrong with it.
 
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It is not 1970.

Any newer Mercruiser Built marine engine is built to run at wot for extended periods of time. I will put a new 350 mag MPI in a boat and run it 12 miles down the bay at WOT and not even think about it.

it is not going to come apart or even show any wear.

I just ran my own 7.4 MPI Lingenfelder(that i built) at 5000 rpm for an hour. No worries. If it comes apart, there was something wrong with it.

Do I recommend that an engine should be run at WOT all the time or that the average boat owner run their engine this way? No, I don't....But, if you don't push the throttle wide open during you boat ride how do you know that the engine is performing the way it should?

When I scan Mercruiser computers, the amount of hours logged over 4000 is amazing......still running strong, some are 2002 engines.

Lack of maintenance, sub standard parts and sea water will kill your engine before higher rpm will.

I would not run an aftermarket rebuilt engine like this either.....
 
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.....................
Do I recommend that an engine should be run at WOT all the time or that the average boat owner run their engine this way? No, I don't....But, if you don't push the throttle wide open during you boat ride how do you know that the engine is performing the way it should?
Precisely...... and this is why we refer to doing this as a WOT RPM TEST!
 
Ayuh,.... While I run my barge up to Wot a few times a season,....
I don't advocate long, Long runs at Wot,....

There's a reason that off-shore hot rod boats need their motors rebuilt after 500 or so hours,....
Where as a cruiser/ fishin' barge motor can easily run several thousand hours before needin' a rebuild,....
 
I'd lean to Chris's point of view. My previous boat (1991 Sea ray 170/3.0 engine) spent most of its life at or close to WOT. Every ski or tubing trip was close to a WOT test run, because it was under powered and had a ski prop on. Tubing with the kids it would be between 4000-5000 rpm for basically an hour straight. Sure it's going to shorten the life but we got 25 years and it was still running strong, only lost about 10 psi compression pressure from when it was new.

Not a ton of hrs, probably around 900-1000, but the engine outlasted the floor. Maybe I'm in the lucky column, and I did the maintence but very few repairs. The fresh water helped, but The outdrive always impressed me only an input shaft seal, a couple bellows, two gimbal bearings, three impellers, and u joints. Engine was a couple carb cleanings, never had to open it.
 
I sure as hell don't, and that's probably why my 1986, original hardware Mercruiser (230 hp Alpha One) still runs perfectly. The previous owner took good care of it--damn good care!--and believed in taking it easy. So far, other than when I first bought the boat, the tach needle has rarely been over 3,200 rpm.

Even oval track racers have learned to "breath the motor" anywhere on the track where possible to keep them running.

Jeff
 
Your boat do what you want but I wouldn't be afraid of WOT a just make sure you're propped correctly and not above RPM limit for your engine.
 
"Back to the topic" Why is the OP's engine running hot on the top end......
Yeah but he'd have to run WOT to know it was over heating...

Ok seriously what in this design cooling system restricts the raw water flow to the elbows? I have a different raw water cooling tgat has the T with check balls on top of the thermostat housing to restrict or cut off the raw water to the elbows once thermostat opens and feeds the eblows through the manifolds. I could see this getting stuck open and sending raw water overboard vs through the block especially at high rpms.

also mentioned block was replaced, to Chris comment regarding belt routing, is the block circulation water pump a marine unit that is bi directional? You mention thermostat is likely original - for the cost of this and the sleeve (broken everyone I've tried to get out) I'd rule it out.
 
Yeah but he'd have to run WOT to know it was over heating...

Ok seriously what in this design cooling system restricts the raw water flow to the elbows? I have a different raw water cooling tgat has the T with check balls on top of the thermostat housing to restrict or cut off the raw water to the elbows once thermostat opens and feeds the eblows through the manifolds. I could see this getting stuck open and sending raw water overboard vs through the block especially at high rpms.

also mentioned block was replaced, to Chris comment regarding belt routing, is the block circulation water pump a marine unit that is bi directional? You mention thermostat is likely original - for the cost of this and the sleeve (broken everyone I've tried to get out) I'd rule it out.

If everything in the drive is 100% he has verified that there is no air in the water stream coming in from the drive and the thermostat is working.....Then he has a restriction, Either blocked divertors or blocked divertors and restricted manifolds. He has posted pictures of the elbows that showed the exhaust passage, not the water passages.

This is why I suggested he have it hooked to a scan tool and run it down the lake....He will see high or low water pressure.....
 
Problem solved. Thanks for all the suggestions. First of all it is nice to know that my 5 year old risers are in very good shape. Problem ended up just being a faulty thermostat and most likely the scale in the thermostat housing was probably a contributor to the problem. Instead of the fluctuations in temperature that I had before, the engine now warms up to 160 and even running full out for extended periods never gets more than 165.
 
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