2007 mercruiser 5.0 MPI overheat

01SeaRayBR

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Hello everyone, long time reader, I am in need of some expertise. I have a 2007 mercruiser 5.0 mpi (serial #0w656595) with an alpha gen 2
(don't know the serial#) that is raw water cooled with the single drain system.

It overheats at idle while on the muffs and in the water. I have replaced the circulating pump on the front, replaced the thermostat, tested the thermostat, backflushed then entire system. I removed the manifolds and risers,cleaned them out, I just purchased the boat so I am unsure how old the manifolds are. I removed the outdrive and checked the hoses and checked to make sure the exhaust didn't have any restrictions. I bypassed the outdrive, power steering cooler, and fuel cooler and hooked the water hose directly the cooling system( the water distribution bulb at the bottom of the engine that has the drain valve), still overheats. Very slowly. It creeps up, gets in the 160s and 170s,lingers, fights the good fight, then eventually loses and overheats. My water hose has plenty of pressure, I even checked the ball check valves on the bottom of the engine to make sure they weren't stuck. I am out of ideas, this single drain system confuses the heck out of me. Please help.


Thank you,
Nic
 

Fun Times

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You are connecting a garden hose where hose item number 8 is and it's still over heating? http://www.mercruiserparts.com/bam/s...1751/12840/200

If so then there could be a blockage inside the Water Distribution Housing So as difficult as it can be to go around the housing as a test, you'll want to give it a try as that housing is known to be able to clog-up. Try either hose number 11 or 12.

Also if item # 20 gets clogged, it could over heat as well.

Just to be sure, the engine started overheating before replacing all the cooling parts including/especially the new circulating water pump?

Though it's expensive, if your water hose between the circulating pump and thermostat housing has a drain with a blue plug then you may be interested in this flushing kit for testing....https://www.perfprotech.com/flush-ki...product/186808

Even though the title reads 350 MAG MPI, here is your correct parts catalog as your engine model is part number 90-884717003 too... http://www.mercruiserparts.com/350-m...-0w650000-thru
 
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01SeaRayBR

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Hey thanks for the response. Yes, the motor was overheating before I tried all of these tests. First thing I did was pull the foot and replace the impeller, reinstalled foot, still overheating. Yes, number 8 is where I have the water hose connected to. I checked the water distribution housing whenever I replaced the thermostat, its clear no blockage. Also, the risers are cold to the touch which makes me believe that #20 isn't clogged, good thought though. Any other ideas, I am fresh out of them. I would think timing is ruled out as well, the manual says the computer does that automatically and cannot be adjusted.
 
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01SeaRayBR

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Anyone else have any advise? I live in Florida and it is soo nice outside haha ready to get her on the water.
 

Fun Times

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How does the engine seem to run before it get's to hot?
Has the overheat alarm sounded yet? What temp has it reached thus far?
Have you tried raising the RPM to see if it will cool back down do to more circulating water?
No signs of excessive sand/debris inside the engine block sometime found when draining the engine block for the winter?
What spark plug part numbers are you running?
You may want to consider trying a cylinder compression test along with an leak down test while the engine is near as hot as you can stand to try and help establish if the head and intake gaskets don't seem to be failing.
Personally I would still consider trying to bypass the drain housing knowing it's potential failures....Just in case.
 

01SeaRayBR

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Very good questions, purrs like a kitten all the way to the overheat alarm when it gets to 195 degrees. I have tried raising the RPMs, doesn't help. As far as I can tell, no debris. When I had the circulating pump off, I sprayed water into the two ports on the front of the engine with the garden hose, good flow. Im not sure what plugs I am running, have to check when I get home. I had the heads redone when I purchased the boat, low compression in a few cylinders, had a few burnt valves. I did compression and leak down tests before I installed the motor back into the boat, all the tests passed. The drain housing actually came apart on me in two pieces, so I know for a fact it isn't clogged, glued it back together, no leaks. This issue was happening before I touched the housing and it came apart. I also built a temporary one out of PVC just to make sure, still overheated.
 

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Hum, this seems to be a challenging one then as it seems you've been through a lot of thorough testing.

Since you have 1.7 inch exhaust risers, did you use the full open or restrictor gaskets and what order?
Try removing the thermostat if you haven't already.
What brand head and intake gaskets?
No signs of water in the engine oil?

The spark plugs are AC delco 41-993 19256067
That's the correct plugs form new so you're good there. Just for other reference though, the latest pug numbers are now Merc part number 33-8M2018369 which is NGK ITR4A15.
 
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01SeaRayBR

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I did try removing the thermostat, still overheated. I have the gaskets with the restrictor hole in it, restrictor hole pointing to front of motor on top of riser, full open gasket on bottom of riser, I know I did those right, I verified at least 10 times before I put them together haha. I honestly don't remember what brand gaskets, but knowing me, because of how important they are, I went with OEM. No signs of water at all in oil. I have read that usually the manifolds and risers will cause an overheat when you raise the RPMS. Is it possible for them to cause an overheat at idle as well?
 

01SeaRayBR

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Does anyone know if it is possible that the manifolds and risers are causing the overheat at idle?
 

alldodge

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IMO the single point drain systems are problems waiting to happen, just making something simple into something complex.

I'm thinking you don't have enough restriction somewhere. Instead of water going in the motor it's going out the exhaust. What is the part number used for the circulating water pump replacement?
 

01SeaRayBR

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circulating pump # 8503991. It is possible, the very first point of exit is before the thermostat, and before the circulating pump, a tee straight to the risers.
 

alldodge

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circulating pump # 8503991. It is possible, the very first point of exit is before the thermostat, and before the circulating pump, a tee straight to the risers.

Well that's the correct part number, its easy to put the wrong one on. Some have vanes which rotate clockwise others counter clock, all depends on how the serpentine belt goes on.

The hoses from the tee going to the riser is for draining, the water should come up thru the manifold. Take two flat pieces of flat stock or something and use it to clamp off the line going from the distribution hosing to the tee. Doesn't have to seal it completely but ok if it does. Then see if it overheats and also check other hoses to see if there is pressure building up, there should be anymore then normal.

Since this has always overheated and has had top end work, does it have the original intake manifold on it, and is the thermostat housing the correct one?
 

01SeaRayBR

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No Title

This has the dry joint exhaust manifolds that have water feeds to the risers(the tee which receives constant water from the top of the water distribution valve), the hoses that feed the bottom of the manifolds are fed from the thermostat housing. as well as feeds to the bottom of the manifolds. The intake and thermostat manifold are the originals.
 

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alldodge

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This has the dry joint exhaust manifolds that have water feeds to the risers(the tee which receives constant water from the top of the water distribution valve), the hoses that feed the bottom of the manifolds are fed from the thermostat housing. as well as feeds to the bottom of the manifolds. The intake and thermostat manifold are the originals.

Ah yes and NO. Yes they have dry joint exhaust manifolds and elbows but that just means that the water does not pass from around the exhaust holes around the exhaust hole, in pass thru a side port opening and then into the elbow.

The following is for open cooling, not closed

You have this, dry joint
Slide1.JPG


Wet manifolds have this
Slide2.JPG



So if you installed gaskets which do not have a pass thru hole, then there is your problem
 
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maicoman

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Here you go ! I had the same problem with the same boat and motor essentially.Get rid of the single point drain system.It is not hard to do and all you need are a few new hoses and a new thermostat housing.The flow on the single points do not know where to go when you simplify the system the boat runs cool.6^ hoses will come off your therm housing and there will be no cross flow on that bulb distributor thingee.Toss that plastic crap in garbage
 

maicoman

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If you look up your motor in the manuals most of these motors came with Single Point and 7 Point drain systems.Just buy the parts for the 7 point Drain System and stop testing something that was broken the first day it was designed.
 

01SeaRayBR

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I was just about to do that maicoman, I ended up replacing the elbows, no change. replaced the manifolds, boom, problem fixed. I still don't trust this single drain though, I may remove it anyway. Those check valves where in pretty bad shape when I removed them from the old manifolds.
 

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I was just about to do that maicoman, I ended up replacing the elbows, no change. replaced the manifolds, boom, problem fixed. I still don't trust this single drain though, I may remove it anyway. Those check valves where in pretty bad shape when I removed them from the old manifolds.
Thanks for posting an update on this one.

So are you at all able to locate anything that may be obviously wrong with the old exhaust manifolds? Did you continue using the restricted gaskets or go to full open this time around?
 

01SeaRayBR

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I apologize, i know i am way late at responding this, i have been gone a while. I could not find a single thing wrong with the manifolds, just took a leap of faith, changed them out, and it was them. i honestly think if i would have changed the drain system back to the seven point, i could have gotten a few more years out of them. they were right on line. Even with the new manifolds, the temp will sometimes creep, and i will have to raise the idle just a tiny bit to bring the temp down. I followed the specs with the gaskets, restricted gaskets.
 
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