Vintage Mercury Cooling Issue

[tpenfield]

Greetings,

I've been trying to figure out a cooling/overheat issue with my 1969 Merc 7.5 HP. When I was prepping the engine at the beginning of the season, it was not peeing properly despite a brand new water pump. I set it aside and grabbed the next in line for the dinghy (Yammy 3 HP)


Today I figured it out ( I think) and wanted to share for those who may have same/similar Mercury OB.

Anyway, today I spent some time going through the cooling flow of the Merc. I pulled the lower unit and ran the pump via the drive shaft with a drill . . . good flow. So, on to the next thing. I figured that the insides had some salt build-up, so I ran some Barnacle Buster through the cooling system. It improved things, but still not great. At least the engine was peeing again, but it was not a consistent flow.

It seemed like the water jackets around the cylinders were not getting enough water flow. I then noticed a plug on top of the engine block where the upper water jacket was located. I removed the plug and ran the engine, hoping to see water pumping out of the plug hole. What I got was exhaust instead of water .

OK, at least I found the issue . . . the gaskets and baffle along the exhaust side of the engine had perforated and exhaust pressure was getting in the cooling system, preventing the water from flowing properly and surrounding the cylinders. See the culprit parts below . . .
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So, I'll be seeing about getting a new set of gaskets and baffle for the engine. It will be nice to have the engine working again.
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Here are the culprit parts . . .

 

[Scott Danforth]

Great writeup Ted

 

[tpenfield]

Great write-up Ted

Thank you!

It is similar to blowing a head gasket on a 4-stroke engine, where the exhaust gases are pressurizing the cooling system. I wish that I had thought of it sooner, but usually with cooling issue the focus is on the impeller.

I was getting close to throwing in the towel on this motor, given its age (53 years old), but perhaps it will have a new lease on life . . . after all it has 10+ years to go until it reaches retirement age

I may fabricate the baffle plate from a sheet of stainless steel that I have, since the OEM plate is aluminum.

 

[Scott Danforth]

I would not mix aluminum ans stainless in a salt water environment.

 

[tpenfield]

I would not mix aluminum and stainless in a salt water environment.

True, but the baffle plate is isolated by the 2 gaskets, so I am wondering if it will be OK

 

[racerone]

Johnson / Evinrude used stainless for that plate for many years.

 

[tpenfield]

I'll give the stainless a try . . .

If things don't work out with this engine of maintenance reasons, I have visions of converting it to electric with a donor motor from an electric lawn mower (or something like that)

 

[tpenfield]

I made the stainless plate out of some stainless steel that I had lying around. I did order the OEM plate (Aluminum), so I'll see which one I want to use.
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[Scott Danforth]

iff you can keep the stainless from touching the aluminum, you may be OK. however that is a lot of stainless (cathodic) bolted into the aluminum (anodic)

 

[dingbat]

True, but the baffle plate is isolated by the 2 gaskets, so I am wondering if it will be OK

Depends on the alloys in question, but the last time I checked, saltwater was conductive

Good info on metals in saltwater

https://darchive.mblwhoilibrary.org/bitstream/handle/1912/3194/WHOI-04-72.pdf?sequence=1&isAllowed=y

 

[tpenfield]

I do like the aspect of a SS baffle plate, just got to think about maybe using a stud & anode combination for the fasteners to give the engine block some galvanic relief

 

[racerone]

Just install the stainless plate and go fishing !!

 

[tpenfield]

I got the parts . . .



Of course I won't be able to re-assemble the engine for a few weeks. It would be nice to have it running in 'tip-top' shape again.

 

[tpenfield]

Quick update . . .

I put the new baffle in (used the OEM baffle as my handy work with the stainless steel plate was not 100%). Anyway, the engine still does not pee very well . . . better than it did, but the tell tale stream dies out after a minute or two.

I removed the inspection plug on the top of the powerhead's water jacket and I'm still getting exhaust in the cooling system. Not quite as much, but no where near enough water.

With the IR gun the lower cylinder is at 170˚F and the upper cylinder is 205˚F . . . this is of course with no load on the engine.

I'm not sure where else to look ??? Perhaps the salt water has taken its toll and the exhaust side cover no longer makes a seal ? Not sure where else the exhaust pressure could be getting into the cooling system to the extent that it is.

Any help might save the motor . . . Any ideas? @racerone . . . others?

I'm thinking this motor may be going up on CL as a 'parts motor' and I'll be an outboard shopper soon.

 

[racerone]

Did you inspect plastic washer at the top of the water tube ?

 

[tpenfield]

Did you inspect plastic washer at the top of the water tube ?

I did not, but I believe I checked at the top of the tube for leaks . . . although it was difficult to see.

 

[tpenfield]

Did you inspect plastic washer at the top of the water tube ?

Also, do these engines rely on enough water pressure to counteract the back-pressure from the exhaust? If so, a leak in the cooling water supply would make sense.

 

[racerone]

Take the powerhead off.-----Inspect washer at top of the water tube.

 

[tpenfield]

Take the powerhead off.-----Inspect washer at top of the water tube.

OK, will do. Just have to remember how I took the thing apart a few years ago

 

[tpenfield]

Looks like I re-built the motor in 2016. Here is a picture from my thread back then. I assume the 'washer' is where the arrow points?

From 2016 . . .


Looks to be a rubber grommet maybe and a pressure fit once the tube is inserted?