another Mercruiser overheat investigation

[Neutron68]

What are the water paths though the Mercruiser 120 engine?

What is the water path for the water that comes out the right and left sides of the transom shield? 
What obstructions could be in that path?

I get the sense that the water coming out the center of the propeller goes through the exhaust manifold and exhaust elbow, so we won't see any water out the propeller until the thermostat opens.  Also, the water through the exhaust elbow goes through 3 holes towards the back end of the elbow.  Those 3 holes, in their best case, sure look like a restriction to water flow, so I'm not ever going to expect a gusher out the center of the propeller.

Eric

[OleRed]

all the water through the motor will go out the prop hub when you are under way, the only reason the little "relief holes" are in the transom shield is to relieve back pressure when the boat is just
running in place idling, or putting around, when the prop is not drawing the exhaust and water thru the prop, they are relief holes, wither it be a 120 or a 260. 

[buckz6319]

it will be hard to tell how much water is flowing through once that foot is submerged in the lake, unless you have a water flow meter installed.

[Neutron68]

I guess I'm looking for a flow diagram, so I can see where water is supposed to flow, and where flow obstructions might occur.  From my point of view, the engine is a black box, where I can't see what's going on inside.  I see water go in and I see it come out, but I don't know where it flows inside, and how it finds its way out the various exit points (like the relief holes in the transom shield).

Ideally, I'd like a version of this for a Mercruiser 120 (2.5L, 4 cylinder engine):


from
http://www.perfprotech.com/blog/boat-and-engine-technical-tips/marine-cooling-systems/mercruiser-closed-cooling-system-flow-diagram

Eric

[Neutron68]

all the water through the motor will go out the prop hub when you are under way, the only reason the little "relief holes" are in the transom shield is to relieve back pressure when the boat is just
running in place idling, or putting around, when the prop is not drawing the exhaust and water thru the prop, they are relief holes, wither it be a 120 or a 260.

Are you saying that if water is freely flowing (or being drawn) out the propeller, it won't be gushing out the relief holes?
Those holes will only have heavy flow if water isn't freely flowing out the propeller center?
Since we saw a heavy flow out of the propeller, there isn't going to be a heavy flow out the relief holes?
The water will go one way or the other, not both places at the same time?

Eric

[Hyperacme]

http://www.boatfix.com/merc/servmanl/13/13a6r2.pdf

[Neutron68]

http://www.boatfix.com/merc/servmanl/13/13a6r2.pdf

Thanks!

I see the lake water inlet on the side of the thermostat housing.  That fitting was cold the whole time we were running the engine - makes sense.
It looks like there is a bypass section in the thermostat housing, so there is always a little fresh water going through the exhaust manifold, no matter if the thermostat is open or closed.
And, it looks like the only way out of the engine is the exhaust elbow.

So, now the question is, how does the water get from the exhaust elbow to the relief holes in the transom shield?  Can that path get obstructed?

[Rich_V174SS]

Where the exhaust pipe joins into the transom assembly the water/exhaust passage splits into 3, the center going through the exhaust bellows which goes out through the prop, and right & left channels that branch off to the outlets on the sides of the transom shield. If you have a pre-alpha transom assembly the split occurs between the exhaust pipe joint and the exhaust bellows so most of your water will exit out straight through the prop but larger amounts of water flow will also seek exit out through the side ports. On an Alpha assembly the split occurs right at the exhaust pipe joint and depending on how much water or how fast the water is flowing it will direct to the most convenient port which in the case of the in-line engines will usually be the starboard-side port when flow is optimal.

[Neutron68]

Where the exhaust pipe joins into the transom assembly the water/exhaust passage splits into 3, the center going through the exhaust bellows which goes out through the prop, and right & left channels that branch off to the outlets on the sides of the transom shield. If you have a pre-alpha transom assembly the split occurs between the exhaust pipe joint and the exhaust bellows so most of your water will exit out straight through the prop but larger amounts of water flow will also seek exit out through the side ports. On an Alpha assembly the split occurs right at the exhaust pipe joint and depending on how much water or how fast the water is flowing it will direct to the most convenient port which in the case of the in-line engines will usually be the starboard-side port when flow is optimal.

This 1978 stern drive is SN 4906558, which is pre-Alpha.  Based on what you just said, most of the water should exit out the prop, which is what we see in my video.

[OleRed]

when the drive is submerged in the lake, the relief holes give an excape passage for the exhaust and water flow to prevent back pressure in the exhaust system, which could cause a water reversion into the exhaust manifold, that could cause a hydro-lock, water in the combustion chambers, that would caust substantional engine damage.
The relief holes are just a way of helping prevent that, but the holes and the exhaust thru the drive are in the same passage, no diversion or anything like that, just closer to the water surface when the boat is in the water, any pressure will take the least path of resistance.
When the boat is under way, the prop will draw the exhaust and water flow from the motor thru the prop, that is the least path of resistance, the prop creates a vacumn for the exhaust and water flo from the motor to pass through, greater than the exhaust and water flow pressure from the motor .

[dorelse]

This 1978 stern drive is SN 4906558, which is pre-Alpha.  Based on what you just said, most of the water should exit out the prop, which is what we see in my video.

Just as an FYI...there's no such outdrive model called a 'Pre-Alpha'.  You have an Model 1 drive (MC-1)...'Model 120'...I have an MC-1 as well, 'Model 898'...they liked to label the outdrive to match the engine back then.

After that comes the 'R' drive, then the 'MR' drive and then the Alpha. (There's some TR & TRS drives in there too.)

https://plus22.safe-order.net/mercstuff/drive_id.htm

http://bluewatermarinesvc.com/html/mercruiser_fyi.html

[Neutron68]

Just as an FYI...there's no such outdrive model called a 'Pre-Alpha'.  You have an Model 1 drive (MC-1)...'Model 120'...I have an MC-1 as well, 'Model 898'...they liked to label the outdrive to match the engine back then.

After that comes the 'R' drive, then the 'MR' drive and then the Alpha. (There's some TR & TRS drives in there too.)

https://plus22.safe-order.net/mercstuff/drive_id.htm

http://bluewatermarinesvc.com/html/mercruiser_fyi.html

Thats good info. 
My outdrive has a 120 sticker on it and ribs on the sides of the top section, so it's clearly not an Alpha (which are ribless).
So, I should refer to it as a model 120 outdrive?

[dorelse]

I'm not trying to be a stickler on the name...I used to call it a pre-alpha too until someone else educated me on it...probably Rich (maybe?).

Yeah, its a Model 120, which, except for the gear ratio, is the exact same unit as my Model 898 based on my understanding of them.  I usually refer to mine as an MC-1.  (Mercruiser 1) drive...as 99% of our drives are exactly the same.

Rich - Did I get anything wrong?

[Rich_V174SS]

Rich - Did I get anything wrong?

Nope, you're good. I too am guilty of referring to any drive before 1983 as a pre-Alpha because there were several variations whereas the Alpha became the standard.

[CamaroMan]

Hi all, first post here   - sorry to hijack but perhaps some of the members are still active -

Having overheating issues with a 1964 GM 120 im working on for a friend - here is a basic pic of the setup:




If we connect a hose to the yellow, motor runs perfectly cool (yes i know high pressure cold water vs pumped).. replacing the impeller today - even though checked everything last week (pumps good).

New bellows/water pipe / etc..

when we connect hose to the inlet it runs a bit hotter - there is not visible t-stat housing in the front. we removed both brackets and couldnt find a stat anywhere.. all square holes -

will be doing a water flow test today and will post back - since google found this as #1 for a gm 120 cooling search the thread is very valid!

thanks all!

[75starflight]

Hello, and Welcome. I have worked on a few of these I4 Mercruisers in the 70s generation.  everyone I have worked on has had a thermostat. On the front of your motor you may have something that looks like this.



This is the thermostat housing on the mid 70s I4 mercruiser engines. hope it helps

[75starflight]

I brought up a parts book for the 1964 120 and did find it does have a thermostat. In the following link you will see it is item #s 59 for the thermostats and 60 for the gasket.

I hope it helps.

http://www.mercruiserparts.com/bam/subassembly/30879/1559/50

[CamaroMan]

hi starflight - many thanks for responding! We took off both front brackets and all we found we square holes (see pic 1) (ironically the clymer manual I have has every model EXCEPT the raw cooled 120).

here are 2 pics:






I think there might be a stat housing missing in ours? on the images I saw theres a separate bracket on top of the one in our picture (that houses the stat) - it looks like ours just routes straight from the in/out - you can see the 2 new blue felpro gaskets on the 2 brackets in the 2nd pic.. both of those had square holes inside -


I will be replacing the impeller later today and will report back. It seems near impossible to get the exhaust elbow at the back off (i want to check the exhaust shutter)..

Running a hose as indicated keeps the motor running 100% fine on the hose..

Where the incoming hose to elbow comes in - another pipe splits of the same brass bracket and goes to the top of the elbow, i assume this introduces raw water into the exhaust system for cooling?

thanks again - hoping to get this motor running cool asap. Everything else is tuned so well, she idles at the touch of the key / cold -

[CamaroMan]

ok finally managed to find it - the ONLY model not listed in my clymer was the one I was working on - had to remove the whole manifold to get into the stat housing at the exhaust elbow -

motor seems to be running very well now thanks!

[75starflight]

ok finally managed to find it - the ONLY model not listed in my clymer was the one I was working on - had to remove the whole manifold to get into the stat housing at the exhaust elbow -

motor seems to be running very well now thanks!

Glad you found it!