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Honda BF90D

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CrabbyTug

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Hello,

Currently I have a newer Honda BF90D with only 60 running hours. We have ran into the issue of it going into limp mode after 5-10 minutes of running due to an overheating issue. We through a code reader on it to confirm that it was an overheating issue. With that being said, we have swapped all thermostats, ensured water pump works well, checked relief valve and made sure cooling passage were clear and flushed. What we did was pop the thermostat covers off and ran the engine in the water. We noticed exhaust gas coming out from where these covers out, meaning there is some sort of exhaust leak that is getting into our cooling passages which is causing it to overheat. The oil is not emulsified meaning no water in the oil, and it runs perfect and smooth which would mean no water getting into our cylinders. This thing runs perfect, but definitely overheats after running for over 5 minutes. Has anyone had this issue, or any insight on what it may be, before we start ripping into the thing?
 
You ran it with the thermostat covers off? I’ve never seen that but I’m thinking that’s not a good thing. You may have been seeing steam, not exhaust gases.

Is this a salt water outboard?

Were the old thermostats filled with salt?
 
Is the telltale pumping freely and how long did you flush it?

If the old thermostats were full of salt you may not have flushed it long enough and there are some chunks of salt still in there. I’d flush it in a barrel for 20 minutes at 1500-2000 rpm, thermostats removed, covers on.
 
NSDON said:
Is the telltale pumping freely and how long did you flush it?

If the old thermostats were full of salt you may not have flushed it long enough and there are some chunks of salt still in there. I’d flush it in a barrel for 20 minutes at 1500-2000 rpm, thermostats removed, covers on.
Click to expand...
We flushed it for a long time, but why would there be exhaust gas in the cooling passages? We may try and flush it again, should we use redlyme?
 
CrabbyTug said:
What we did was pop the thermostat covers off and ran the engine in the water. We noticed exhaust gas coming out from where these covers out, meaning there is some sort of exhaust leak that is getting into our cooling passages which is causing it to overheat. The
Click to expand...
Didn’t know what this meant. Where was the exhaust gas coming from?
 
The exhaust was coming from where the thermostat covers are, we had the thermostat and the cover both removed, started it and had exhuast gasses coming from them.
 
Hi,
I don't know this outboard nor have ever worked on one of them before.

But....based on your description of symptoms and salt encrusted Tstats, I would bet $$$ to donuts that there is a blockage somewhere in the water jacket. Remove the jacket cover (item 19 in link below) and take a close look.

Be prepared to replace the sacrificial anodes and mounting screws while in there...items 1 and 42....as routine maintenance items.



I don't know how the exhaust system is routed on this engine and I can't find a depiction of the exhaust manifold. But, most marine engines cool the exhaust with engine coolant so that might explain why you see exhaust gasses and it could have something to do with the blockage I suspect is in the water jacket. Although, NSDON's suggestion that it could simply be steam seems reasonable too.

The exhaust gas oxygen sensor is adjacent to the water jacket. Item 25 in the link. You might want to look in that cavity and make sure that all looks good in there.

If you do decide to disturb the O2 sensor, use care taking it out and handle it cautiously because those sensors are somewhat delicate. (and EXPENSIVE!)

When putting it back, you need to torque it properly so as not to distort it. Using the proper tools is often essential so as not to cause damage and introduce an additional problem you don't need.

Good luck.
 
With 60 hours on this, it would shock me if this has anything other than a clog in the cooling system somewhere. Run it in a barrel, covers on, thermostats out, add 2 or 3 jugs of white vinegar and keep the barrel full and the rpms at 1500-2000.

I would replace the water pump too, do the full kit from Honda. Hopefully the impeller is intact. If this thing was ever run on muffs or run dry, your water pump may be toast. They will last a long time but not if run dry. Sometimes they will pump good out of the telltale even though they are not pumping enough water to cool the outboard.
 
Got a flushing bag with some Redlyme, flushed it at 1800 RPM's for around 20-30 minutes. Ended up overheating which I think was because of the constantly recirculated water. Going to keep flushing it over the next few days and give it a test really soon, will keep everyone updated.

Thank you guys for the responses!
 
I use an IR temperature gun on the water in the barrel, it gets up over 160f when the outboard is run long enough and will trigger the shutdown.
 
Hello everyone, just flushed for about 20-30 minutes multiple times. Seems cleaned out, but we took it apart a little more, water pump is in pristine condition, checked all O-rings and gaskets. We noticed that we have exhaust coming up through the water bypass along with the pipe above the water bypass also has exhaust gasses running through it, has anyone seen this? So odd to me with only 62 running hours.
 
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