BF 175 Overheating

I have a Honda BF175 engine 2008, which is the same blockas the 200hp and 225hp sisters, V6 engine.

The engine was bought with the boat when it only had 20h of salt wateroperation, I am not sure of the previous level of maintenance by since I hadthe boat all the schedule maintenance and more has been carried out. The boatis used every summer for around 30h each time. Now the engine has accumulated80h and had every year changed, the engine and gearbox oil, oil filters andfuel filter.
Up to now it run like a dream, this year I noticed a weakpee stream form the tail-tale pipe but I did not overly concern me as I thoughtcould be from the thermostats opening and closing. By the way the thermostatswere changed at 20h, the old ones were ok with a bit of salt debris around thembut nothing alarming.
Anyway after a long run on the plain holding the enginearound 3000-3500rpm I noticed that the pee stream was getting very week andslowed down to idle, after a few second the buzzer came on, and immediately thoughtI had caught something and was not sucking water properly, but the lower legwas clear. I started again the engine and the buzzer came one and the red lighton the console, I did not notice at the time if the buzzer was intermitted or continuous.
Anyway to cut a long story short, I pulled the boat out,removed the lower leg inspected the impeller (condition OK), change the impeller,changed the thermostats, blowed with air the cooling system from the top(thermostats) and from the bottom, with nothing substantial coming out. Run theengine in a barrel with Salt Terminator solution. The engine run fine for 20minand the pee stream was very strong.
Boat back in the water, did a test run, everything wasfine until I pulled in the harbor and at idle the buzzer came on again(intermitted) and the pee stream was reduced again. I plugged the engine on aHonda diagnostic tool to identify any register faults and it showed a problem withthe O2 sensor. Played around with trying to test the O2 sensor, used a blowtorch and measure voltage, it was oscillating between 0.1 Volts to 0.9 Volts,so I assumed it was OK. Run the engine again in the harbor at idle and now theengine was overheating, or at least that I was reading from the heat sensor,97C and the engine was cutting off.
Took the boat again out from the water, took the bottomleg off, checked the impeller (again OK), run the engine without thermostats,again the engine overheated very quickly and shut down. The pee stream was alsovery week. Some engineers suggested that the water cooling system was cloggedup with salt and suggested various remedies, one of them was to run a vinegarsolution through to clean the system. This was done but no real change, the continuousbuzzer still came on.
Use the flushing tube and flushed the engine for ½ hour,without engine running, the pee stream was ok although not very strong.
I am totally perplexed to what is happening, I haven’tchecked the engine with an infrared thermometer but touching the manifold itwas very hot, so I tend to believe the overheat alarm.
So can anyone suggest what could be wrong, as I havegiven up right now. The last thing I want to do is dismantle the engine toclear a possible clogged up cooling system, which I tend to believe is not thecase for at least an 80 hour motor.

Is the O2 sensor playing up a result of overheating? Do Ihave both problems ie Overheating and an O2 sensor issue?
Any help or previous experience is appreciated.

A faulty HO2 sensor should not overheat the engine. It will just make it run less efficiently.

It sounds like you probably have a clog somewhere in your cooling system. However, you need to eliminate a possible blown head gasket. Pull and inspect the plugs for any obvious fowling. They all should be milk chocolate brown. Then do a compression test - fresh battery, all plugs out, throttle full open, and kill switch activated. Crank at least 6 seconds for each test. On an engine that few hours, each cylinder should be about 200 psi and there should not be more than 10% difference between the highest and lowest compression reading.

If that all checks out, pull the bottom leg again, pull the thermostats (check down in the seats for any corrosion of clogs there), then flush the cooling passages by wedging a hose in where the t-stat sits with as must water pressure as you can get. Put a bucket under the engine base to see if anything comes out. While you have the leg off, look at the impeller again. See it every vane is making really good contact with the metal casing. Those casings can get worn and distorted, reducing the effectiveness of the pump, but not likely with the low hours on your engine. However, check it. You never can be too sure. You might want to consider replacing the whole mechanism, which is not a a whole lot more expensive than the impeller kit alone.

If none of that works, then the two pressure relief valves could be seriously corroded. See item 9 at:
http://www.boats.net/parts/search/H...7 and Later/BF175AK1 XA/MOUNT CASE/parts.html

Those are a real bear to get to, so your bast bet would be to try to break up the corrosion again. Vinegar water is likely too mild. So do some research on what works best. Most people use concentrated Sat Away, but there is other stuff out there.

chawk_man

Thanks for taking the time to reply with really usefulsuggestions.
Coming back to the head blown gasket, that certainly didnot cross my mind but if that was the case, at least in my experience from theautomotive field there should be some water contamination in the engine oil,last time I have checked it was crystal clear, no bubbles or frothing toindicate a blown gasket. Worth checking anyway.
Now regarding a warped water pump body, some engineersuggested due to the heat generated by the exhaust outlet in the leg, it canhappen sometimes. When I dismantled the water pump for the second time I putall the components (ie the stainless shroud were the impeller fits and theplastic water pump body) on a true flat surface and there we OK, but I wouldchange them anyway just to completely eliminate this possibility.
The interesting suggestion is the “pressure relief valves”that you have mentioned, what is their function and what would happen if thestuck either open or close? Also you mention they are hard to get at, are theyburied in the engine?
What I was thinking is to take the thermostats out andfill from the top with a salt breaking solution, block most of the water outletpoints as best as possible, and leave it there for as long it takes and keepfilling up, in the hope I would remove the obstruction.
The question is are those engines so unreliable? or Ijust been unlucky.
Everyone is praising the Yams and I was close to swappingthe engine out if I did not have to drill new holes on the transom and do a lotof work on the console to replace the Honda instruments with the Yams.
Up to now I have been very happy, lots of torque, quietand low fuel consumption, but in the Sea what counts is reliability!

These engines are exceptionally reliable if maintained properly. I have over 2000 exclusively saltwater hours on my 2007 BF 225 and no serious problems whatsoever. But I'm a stickler about maintenance. Mainly because I'm a tightwad, and not inclined to shell out another $16 grand for a new engine. I would suggest that these engines are probably more reliable than the equivalent yammi's. All of these big outboards have their quirks and weak points.

It's my understanding that the pressure relief valve is there to bleed off excessive pressure to the cooling system caused by high revolutions on the engine. As engine rpm's increase, water pump rpm's increase and increases water pressure. At some (unknown) pressure, the relief valve activates and bleeds off some of that pressure. I've never had to get into them on my engine, but from the parts diagram, it looks like a major job. There are likely other folks on this forum who have done it. Hint, hint!

Your idea of soaking the water passages in a corrosion killer is probably what I would do, after a high pressure flushing, then another high pressure flush after the soak.

Thans for your help and suggestions anyway, I will firstly replace water pump, check the pressure relied valves and the soak it in some salt busting cleanner.

l will report in couple of months as the boat is in storage now.