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

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WalleyeBrian

Member
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 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 said:
Need a higher temperature rating I think.
Click to expand...
Thanks racerone, concur, need a higher temperature rating. I'm not an expert at these sorts of things, so was hoping someone would be able tell me the specific thermostat to purchase. Any thoughts on how to make sure I order the correct parts?
 
According to my shop manual there were 4 tstat temp ratings available when these first came out...

Hi temp ....158° to 176°f...
...lift...3-4mm

Medium....140° to 158°f...
...lift....2-3mm

Original or
Standard....140° to 158°f...
...lift...3-4mm

Low temp...122° to 140°f...
...lift...2-3mm

The problem is, the stat you are probably looking for....the high temp....was discontinued fairly early. The manual I am using lists the hi temp as "no longer available" and it was published in 1984.

After looking up the stat on the site boats.net I found only two part numbers:
19300-881-741
And
19300-921-003

Looking up on the yumbo.jp parts site I found:
19300-881-741
And
19300-881-761

So, there are apparently 3 options but no info about what temperature range.

The yumbo.jp prices are about 1/3 of what's listed on boats.net but shipping might offset that. I don't know.

While I do agree that a wrong temperature stat operated in cold water could be the cause for "making oil", it might not be the sole reason.

Things you might try before buying anything would be to:

Of course, test the stat. The prescribed method would be to suspend the stat with some wire in a pot of water on the stove and monitor the water temperature. Note when the stat begins to open but, just as important, measure the maximum "lift" of the valve after it stops moving.
The "exercise", in this case, isn't as useful as it would be if diagnosing an overheating condition but it will give you baseline data to compare the performance of any replacement that you purchase.

Check valve adjustment and timing. Just a small difference from specifications could have the engine running rich.

With valve lash and timing correct, check ignition:

Make sure plugs aren't fouled. If blackened they can be gently cleaned with a stiff toothbrush but it's better to replace them. Especially if you haven't done that for a while. Make sure the plugs are properly gapped.

If it's a CDI fired outboard, the ignition pulsar coil can be seen through an opening in the camshaft sprocket. Spray it with contact cleaner if it looks dirty and let dry.

Rotate the flywheel clockwise and align the "F" on the flywheel with the timing mark line stamped next to the recoil starter hold down bolt threads.

Check that the line on the pulsar housing aligns with the line stamped in the block and/or the hub of the camshaft sprocket and that the pulsar is held firmly in place.

You might also want to adjust the carburetor.
The specification for idle speed is 1200 rpm +/- 100 rpm in neutral. But I know trollers like to turn that down sometimes. If the engine is idled too low, that can cause a "lugging" condition and that's when the plugs start fouling and carbon starts building on piston crowns and valves.
My opinion is the engine will run better if the idle is not set too low.

Lastly, if you suspect that carbon build-up could be causing a rich condition that could contribute to the making of oil, there are several ways of addressing that. I prefer the plain old water-spritz-steam method but running fuel additive concentrations using Marvel Mystery Oil, Sea Foam or many others has been shown to work too.

If you want to find one of the higher temperature thermostats, I don't know. You will probably need to find some new-old-stock hoard someplace. And, the starting place is ebay I would think.

Sorry for the book....I just get into this stuff.

Good luck.
 
Water temp here runs in the 40’s to 70’sF, never heard of a thermostat causing your problem, I think mine is a 160. None of the boat shops around here sell different temperature thermostats, I’d say your issue is something else. My old carbed cars run a 180, same if it’s real cold, no different thermostat.

What temperature water do you run in?

Sounds like you are running rich for some reason, trolling at real low rpm for long periods of time, these Hondas are not creating much heat, does you pee stream feel warm?
 
Thanks everyone for all the 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. I'm frustrated if Honda doesn't offer a more appropriate thermostat for colder climates.

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!
 
Sounds like you have a 160 thermostat so it’s closed until the operating temperature of the water circulating reaches 160. It opens at 160 when you are running at higher rpm’s. Putting a 180 in means it opens at 180, it doesn’t change the operating temperature of your outboard at idle/trolling, since it’s running well below 160 and your thermostat is closed?

My old 130 runs a 160, a very common thermostat in numerous Hondas. I don’t troll and my outboard doesn’t make gas.
 
Greetings Everyone,

Thanks again for all the great insights regarding my Honda BF75 and the thermostat situation. I'm ready to tackle the thermostat replacement myself to see if that fixes the problem.

Unfortunately, I can't seem to find a Seloc marine manual specific to my 2019 model year. Here are my questions:

  • Has anyone used an older Seloc manual for this engine with success? Are there are any specific years that would be the most relevant?
  • Would it be worth investing in an official Honda service manual for the most accurate info? Are there good places to find these besides a Honda dealer?
Any guidance would be greatly appreciated. Thanks!
 
Well, my contribution to you was completely inappropriate because I misread your model and was thinking old 7.5hp model instead of newer 75hp model. My apologies

So, I hope that you can believe me when I tell you that, in my opinion, Seloc, nor Haynes nor Clymer manuals are not a good way to go. They typically try to cover too many models and years ending up not covering any of them very thoroughly. And, they are famous for providing incorrect information, such as inaccurate specifications, and that could cost you dearly in the long run.

While I also think Honda shop manuals could offer more for the prices they ask, they are still the best, most accurate source of information you can have by your side when working on a Honda.
In that sense, they are a comparative bargain next to the others even though they cost more initially.

If you want to order one, here's the link:

Honda Marine | Shop Manuals Publications | Honda Power Products Support Publications

publications.powerequipment.honda.com publications.powerequipment.honda.com

I see that there is a 75A model manual as well as two different manuals for 75D models.
Make sure that you select the one most appropriate for what you have.

There are also some free downloads for the 75hp models, both A and D including some trim/tilt wiring diagrams and tiller handle wiring. So take advantage of what matches up with your outboard while your on the website.

Good luck.
 
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