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BF75A Obstruction in Cooling System?

D

Dtersch

New member
Hi All,
I have a 1998 Honda BF75a and it overheated the other day after a long fast run.
I found some debris blocking the port leading to the tell tale, but nothing too crazy.
Replaced thermostat, still overheats right away and looks like good flow out the prop / tell tale.
Remove the thermostat, no overheating, and looks like good flow out the prop / tell tale.
So I guess I have some flow reducing obstruction somewhere below the bypass valve on the way down n out.
Anyone have an idea to get this cleared? I tried vacuum and flushing with hose, but can't seem to get it.
PS boat is in the water and I do not have a trailer, so all work shall be done in the water. FUN!
Thanks all. I got some videos too. If anyone ever ran their honda w/o thermostat - what happened in the end? lol
 
Hi Dterch,

I would advise testing or replacing the tstat before assuming a blockage is causing the problem. It could be as simple as the stat is stuck closed.

A pot of water heated on the stove is all you would need in order to determine if the stat opens or is stuck closed.

If you have a thermometer you can place in the pot, that would be a help in that you would minimize the amount of time spent heating the water and make the test accurate.

I don't have the specifications or service information for that outboard but I would guess that the stat should be fully open at 140° to 160° degrees Fahrenheit. Again, that's a guess on my part.

If the water temp gets higher than 160° to 180° with no opening of the stat, I would definitely replace it and then try running the outboard again.

Good luck and let us know what you find.
 
Thanks sir! I tried the pot of water method and found both old and new opened at right temp. So the thermostat doesn’t seem to be my issue . I suspect a piece of old impeller from a previous owner or something else maybe obstructing the water flow.
 
The interesting thing is, I ran this motor for maybe 20 hours at least before the problem developed. As it is a 1998 motor I’m sure it has many hours on it.
 
Yes, probably many hours and mineral buildup too. Especially if it did most of it's work in salt water.

I don't want to be the bearer of bad tidings but you may be in for some unpleasant disassembly to look for that blockage. A task that you will  not want to be doing while in or over the water.

You may want to try a vinegar flush in a trashcan prior to any disassembly to help dissolve and break up any internal deposits.

This issue may require that you remove the gearcase with pump and try pushing whatever might be in there out the bottom by backflushing through the thermostat housing with as much pressure as you can produce. . Creating a seal with a rag helps to contain and direct water or air pressure for increasing velocity

If it is a piece of impeller, there's a chance it might come out that way. You'll want to be where anything that does come out, you'll be able to see what it is.

If nothing comes out, you might need to go into the water jacket or even remove the cylinder head to find the issue.

With I had better news.

Good luck.
 
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