Honda BF75 "Making Oil" – Need Help with Thermostat in Cold Climate

[Walleyehunter4]

Hey everyone,

I own a 2019 Honda BF75 outboard and I'm having the all too common problem of it "making oil." I live in Minnesota, and I suspect the problem is that the engine isn't reaching and maintaining a high enough operating temperature due to our colder water.

Here's my understanding of what's happening:

• The engine's thermostat probably has a higher temperature rating, designed for warmer waters.
• With the cold water, the engine doesn't fully warm up, leading to incomplete fuel combustion.
• Unburned fuel mixes with the engine oil, giving the appearance of more oil.
• Cold weather also may lead to increased condensation within the engine, further diluting the oil.

I do a lot of low-speed trolling with this motor, which worsens the situation. I believe the proper solution is to find a thermostat with a lower temperature rating that's better suited for cold climates.

Has anyone else dealt with this issue on a Honda outboard? Did changing the thermostat help? I'd love to hear about the following:

• Any recommendations for thermostat options for colder water (including specific part numbers).
• Experiences other owners have had with this problem in similar climates.
• Advice on whether it's better to go with a genuine Honda thermostat or an aftermarket option.

I'm planning to consult with a mechanic, but I'd like to gather some info beforehand. Any insights or suggestions would be greatly appreciated!

Thanks!

 

[racerone]

??----And I think you need a thermostat with a higher rating.----But be careful as it is not a cooling system with a radiator pressure cap.

 

[Lou C]

Many OBs use a 140 stat you might need a 160 but not sure if Honda offers that.

 

[Alumarine]

Have you checked the thermostat to see if it's stuck open or opening at the marked temperature?
There's many honda motors running in northern climates without issue.
Someone may have put the wrong thermostat in.
Not sure what you mean by a "higher temperature rating"

 

[dingbat]

Guessing you’re running a 140 thermostat.

Thermostat should not open until engine temp reaches 140 then regulate at that temp.

I’m run stock 140 with by-pass hole, but had situations where we had to drill a hole in the non-vented stat to prevent the motor from overheating at idle. Had to goose motor to get stat to open, temp would drop, kick back to idle and start the process over again. The by-pass hole stop that nonsense

How long do you run before you start trolling?
Making oil is typically associated with not running the motor long enough on throttle to “burn” the moisture out of the oil.
I

 

[Walleyehunter4]

Thanks for thoughtful responses! I really appreciate the in-depth advice and information.

You guys made me consider something that might be relevant — I have my outboard connected to my sonar/mapping units via a NEMA 2000 network, which includes temperature readings.

Here's what I observe:

• Warm-up: If I warm up the motor with a long, high-speed run, it does reach around 160 degrees F.
• Quick Cooling: After the initial warm-up, the temperature drops quickly to approximately 120 degrees and seems to remain there regardless of how long the motor is running. It's worth noting that this rapid cooling begins almost immediately as soon as I reduce speed for trolling.
• Restart Temp Drop: Even a brief shutdown and restart causes another temperature drop, sometimes going well below 120 (down to 108).

Based on this observation, does this provide any further clues?

Also, my local marina (primarily a Yamaha dealer) has mentioned that the 'making oil' issue is pretty common in our area due to the cold water. They attribute it to the thermostats not being well-suited for the environment.

My current plan is to pick up the boat this weekend, test the existing thermostat, and potentially replace it. Any additional guidance in light of this new information would be greatly appreciated!

 

[dingbat]

There is little correlation between an open loop cooling water circuit and oil temps.

You need to get the oil up to operating temperature and keep it there long enough to evaporate the moisture out of the oil.
What do you consider a "long run"?

The cooling water temp is going to naturally drop when you reduce engine speed (less heat generated) but would not expect the temp to drop much, if at all below the rating of the thermostat.

Sounds like your stat(s) are not closing

 

[MattFL]

Even if the thermostat keeps the block up to temp, I think your main challenge is the oil sits in a metal pan below the block where it's cooled by the environment. At idle or low speeds there just isn't enough heat being put into the oil to keep it all hot when so much heat is lost through the metal and water. More high speeds run would likely help steam off the built up fluids, but maybe it's just best to change the oil more often.

 

[Scott06]

There is little correlation between an open loop cooling water circuit and oil temps.

You need to get the oil up to operating temperature and keep it there long enough to evaporate the moisture out of the oil.
What do you consider a "long run"?

The cooling water temp is going to naturally drop when you reduce engine speed (less heat generated) but would not expect the temp to drop much, if at all below the rating of the thermostat.

Sounds like your stat(s) are not closing

Agreed- sounds like he needs to verify the stat is opening should stay near the stat temp, 120 is too cold in my opinion.

As others have suggested making sure you get some higher speed runs it to burn off condensation is a common issue. Does the oil get milky ? IS the level rising

 

[Walleyehunter4]

Agreed- sounds like he needs to verify the stat is opening should stay near the stat temp, 120 is too cold in my opinion.

As others have suggested making sure you get some higher speed runs it to burn off condensation is a common issue. Does the oil get milky ? IS the level rising

Yes, looking forward to changing the thermostat as based on everyone's help here, it certainly seems like it's not operating properly. And, I always thought 120 seemed too cold, but didn't know for sure. Ordered parts, should be here in a week.

Regarding lengths of runs, there isn't a relationship between run length and how the motor operates temperature wise, currently. That is, it behaves the same (returns to 118-122) whether running at WOT (5900 rpms) for 40 min or 10 min. And, oil is not milky whatsoever. So, seems unlikely condensation is driver of oil-making. Should long runs also burn off (evaporate?) fuel in the gas, at least once I get the thermostat fixed? I'm assuming there must be a little bit of fuel getting in the gas as the motor warms up, but that as long as everything else is running the way it should, that the amount of gas is rounding error?

 

[Lou C]

I like to test thermostats even new ones, to make sure they open at the rated temp, but also that they close 100%. I have found that in salt water, with a raw water cooled engine, sticky thermostats are common. When that happens the engine will never fully warm up unless you keep it on plane for a few minutes. Normally my engine runs at 160* with a 160 stat, when it starts dropping to 120* at idle, I know that something is stuck in the stat. Usually a flake of rust, or grain of sand. What I have done a few times is run it on plane to get the stat fully open, then bring it back to idle and tap on the stat housing with a small hammer. Believe it or not the vibrations often loosen whatever is causing it to stick open. I've done this a few times. Just gently tap on it, could use a plastic hammer.....

 

[MattFL]

Yes, looking forward to changing the thermostat as based on everyone's help here, it certainly seems like it's not operating properly. And, I always thought 120 seemed too cold, but didn't know for sure. Ordered parts, should be here in a week.

Regarding lengths of runs, there isn't a relationship between run length and how the motor operates temperature wise, currently. That is, it behaves the same (returns to 118-122) whether running at WOT (5900 rpms) for 40 min or 10 min. And, oil is not milky whatsoever. So, seems unlikely condensation is driver of oil-making. Should long runs also burn off (evaporate?) fuel in the gas, at least once I get the thermostat fixed? I'm assuming there must be a little bit of fuel getting in the gas as the motor warms up, but that as long as everything else is running the way it should, that the amount of gas is rounding error?

The oil isn't milky because the "newly made oil" is fuel. Not all fuel burns, some unburnt fuel gets by the rings. In a hot motor it steams off, the PCV system routs it back to the intake where it eventually gets burned. In a cool motor it condenses in the oil. You can send a sample for oil analysis and they'll tell you exactly what's in it, how much fuel. You've got to get it hot for some time to steam it off.

 

[Lou C]

If you do an oil analysis you may be able to tell if it’s fuel dilution, condensation or both. I wonder if Honda makes an optional thermostat for cold climates? I have used a 160 in my Chevy/OMC 4.3. This was the standard one used in many OMCs and Volvos. People say that in salt water you need to use a 140 stat to reduce calcium build up but when I had the cyl heads off about 7 years ago I didn’t see much in the way of calcium build up.
If you’re in fresh water & they make an optional higher temp stat for sure I’d try it.

 

[dingbat]

Regarding lengths of runs, there isn't a relationship between run length and how the motor operates temperature wise, currently. That is, it behaves the same (returns to 118-122) whether running at WOT (5900 rpms) for 40 min or 10 min.

It’s not the temp of the cooling water causing the problem. Its simply a heat transfer mechanism used to transfer “excess” heat away from the motor.

The real problem is not getting the engine (block and oil) up to operating temperature (~ 230 degree) and keeping it there long enough to evaporate any fuel or moisture that has accumulated in the oiling system.

And, oil is not milky whatsoever. So, seems unlikely condensation is driver of oil-making. Should long runs also burn off (evaporate?) fuel in the gas, at least once I get the thermostat fixed?

The boiling point of gasoline in atmospheric conditions is 181 degrees. Boiling point of water is 212

 

[Lou C]

Because of the so called deposit build up in the cooling passages I actually tried a 140 stat one year & didn’t like it. Engine took forever to warm up and I had condensation in the exhaust system so went back to the 160*.