Honda BF75L cooling water issues

[flashback]

I had a 1978 27ft oday sailboat with a BF75 and found the motor adequate to power the boat but it wasn't used often. I gave the boat to the boy scouts last month as a project. The motor still ran well but the boat needs love that I couldn't give it...

 

[presleymarkw]

I think my engine is slightly different than the one in the diagram. 1)

I'll definitely try and flush it...
I can't run it with the thermostat housing off. the housing is integrated into the intake manifold which also facilitates the fuel/ air mixture

Drop the lower unit with the mani fold off. Hook a hose to the water tube. I found a garden hose size end with a nipple so I could run a smaller tube off it. Water will flow out tell tale and thermostat housing then you know it not blocked.

 

[Cpotts]

update:
I was able to blow out the water jacket and there wasn't much debris coming out. I think I'm all clear there.
but what I'm noticing is no water or exhaust coming out of the two little exhaust holes on the back. I figure that's where the hot water should be flowing out...not the tell tale. Is that correct?
Also, when it overheats after a couple minutes, a lot of steam/smoke seems to be coming from the manifold area below the block.

 

[MattFL]

On Honda models I have seen, the above-water exhaust ports don't typically discharge any water. You should have hot water coming out the prop with the exhaust.

 

[km1125]

Those two little holes are "vents" that allow air BACK IN to the exhaust so that a cooling exhaust won't suck water up the exhaust and into the engine. You would not normally see water coming out of them. There is water coming out WITH the exhaust but that gets routed down and discharged at the prop. The telltale will be warm but shouldn't really be hot. It's mostly bypassed water that should indicate if your water pump has flow and is developing pressure.

Something is blocked in your passage in either the block or the head, which is where the bulk of the cooling water should be passing through and where the most "cooling" takes place. The thermostat could do that if it was not opening, but if you don't have a thermostat then something else is blocking the path.

 

[km1125]

Here... I've edited one of your pics that might help explain what's going on. You either have blockage from the BIG hole back into the block somewhere, or from the littler hole into the head somewhere. You said you put air into this littler hole and some came out from the tiny hole at the bottom (which it SHOULD) but if you block that tiny hole then there still should be air flowing through the head and out the exhaust. If there's NOT, then you need to troubleshoot THAT because that would indicate a blockage.

 

[km1125]

Oh, and just to clear up a couple earlier comments... that engine will cool JUST FINE even with NO thermostat. It might run COOLER than it's supposed to, and take longer to warm up, but it will not overheat just because it doesn't have a thermostat.

It is likely that a previous owner removed the thermostat because it was overheating but the problem was some other blockage and not likely the thermostat (unless it was defective, but I'm guessing not). They might have still had enough flow at the time that it took longer to overheat, but it would have eventually overheated because they weren't resolving the real issue. If you use the engine in salt water, then you probably should rinse the head cooling passages with rydlyme or something else (as Matt noted earlier) and it will help clear the problem.

And the diagram flashback posted DOES match your engine.

 

[Cpotts]

wow km1125, thanks for the insight.

I blew air in the bypass connection and it came out the thermostat hole. so there's definitely flow through the block

I'm thinking the water blockage is somewhere down to the exhaust . The only way out is through the telltale. I'm guessing that's why it's hot.
So if I hear you correct, I should block little hole at the bottom, push air through the bigger hole at the top, and see bubbles coming out at the exhaust down by the prop? (it's in a bucket)

 

[km1125]

Block the TINY hole at the bottom, put air in the smaller of the two at the top (the big one is the INPUT to the thermostat, the smaller is the OUTPUT from the thermostat).

 

[km1125]

The bypass connection DOES NOT go through the block... it BYPASSES it.

 

[km1125]

I'm thinking the water blockage is somewhere down to the exhaust . The only way out is through the telltale. I'm guessing that's why it's hot.

This is not correct. There is a path from the cooling jackets IN THE HEAD that goes out the exhaust. That's where the MAJORITY of the water should flow and where the MAJORITY of the cooling takes place. The telltale is a BYPASS around that process and just to let you know that the water pump is flowing and it develops pressure. The telltale does very little to actually COOL the motor. IN FACT, you could COMPLETELY BLOCK it and the motor would run fine (if the rest of the cooling system was operating properly).

EDIT-- well, the FIRST part of that is probably correct. There IS blockage but it's likely in the head cooling passages and that's why water is not getting down to the exhaust.

 

[presleymarkw]

wow km1125, thanks for the insight.

Yes from me as well following this and having been through it before I learned a couple things.

 

[km1125]

Here another edited pic that lines up some of the notes I've used (especially "big hole", littler hole" and "tiny hole") with where there are on the flow diagram.

 

[Cpotts]

The bypass connection DOES NOT go through the block... it BYPASSES it.

yes I get that. I blew air INTO the block at the nipple where the pump pushes water to the block . it's my understanding that the water tube from the pump Y's at that point to feed the block and the bypass. I could feel air exiting by the thermostat so figured the jacket through the block is clear

 

[km1125]

yes I get that. I blew air INTO the block at the nipple where the pump pushes water to the block . it's my understanding that the water tube from the pump Y's at that point to feed the block and the bypass. I could feel air exiting by the thermostat so figured the jacket through the block is clear

Yes, that makes more sense. It should have been coming out the "big hole" at the thermostat.

It's more likely the problem is in the head water passages because they get much hotter than the block passages, and they're typically smaller too. So any dissolved solids in the water (salts, calcium, etc) precipitate out of solution there and stick to the metal.

 

[Cpotts]

Block the TINY hole at the bottom, put air in the smaller of the two at the top (the big one is the INPUT to the thermostat, the smaller is the OUTPUT from the thermostat).

if I do as described, I should see air coming up as bubbles from the exhaust, right?

 

[km1125]

if I do as described, I should see air coming up as bubbles from the exhaust, right?

YES, or you'll hear air coming out those two small holes at the back of the housing

 

[Cpotts]

Yes, that makes more sense. It should have been coming out the "big hole" at the thermostat.

It's more likely the problem is in the head water passages because they get much hotter than the block passages, and they're typically smaller too. So any dissolved solids in the water (salts, calcium, etc) precipitate out of solution there and stick to the metal.

this has been a fresh water engine since I've owned it.

YES, or you'll hear air coming out those two small holes at the back of the housing

roger that....I'll update after work today

 

[MattFL]

Has it overheated ever since you owned it? If yes, I'm going to take a wild stab in the dark. How does the paint on the prop and lower unit look? If it's missing a bunch of paint, perhaps the previous owner ran it into a sand bar or similar and sucked up a bunch of sand and that could be the blockage.

When it's running, do you have any water coming out the center of the prop with the exhaust?

 

[Cpotts]

Has it overheated ever since you owned it? If yes, I'm going to take a wild stab in the dark. How does the paint on the prop and lower unit look? If it's missing a bunch of paint, perhaps the previous owner ran it into a sand bar or similar and sucked up a bunch of sand and that could be the blockage.

When it's running, do you have any water coming out the center of the prop with the exhaust?

It's never overheated before. I ran it for several months even without the thermostat and no problems. this is a recent occurrence when I tried to use it this season.
there's lots of chips and dings but the tell tale is strong so I don't think the blockage is on that end
this engine exhausts above the prop. I've only run it in a bucket so I can't tell if water is coming out the exhaust tip. but blowing air through the system isn't producing airflow through the exhaust