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BF90A overheating alarm

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b4j2co

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hello. 1999, BF90A carb motor. 280 hours.

changed impeller, changed thermostat, check poppet. started engine with thermostat out, water flows good. Engine will idle just fine, overheats at higher rpm's with the thermostat in (works fine in heat test). take thermostat out and engine will run good. Noticed water in oil to slight degree but seems to be a result of condensation that could've taken place, plus i found the pcv valve laying limp to which i hooked it back up.

anyways, i'm at a loss. Not sure how it overheats with a working thermostat, but doesn't overheat without one. Water flow seems fine
 
Does this motor run in saltwater or freshwater? If saltwater, was there much build up in the thermostat when you removed it?
 
Fresh. Thermostat looked almost new when I pulled it out. I'm envisioning the water rerouting when thermostat isn't in, almost like the lack of pressure makes it take a different path.
 
You must use a thermostat, it's not good for these engines to run cold. I suggest you find an infra red thermometer and see what's happening to the temp at the sensor base, It could be the sensor activating prematurely
 
Agreed, ran without thermostat once. It is overheating though. If I pull out thermostat an the alarm, there is steam coming off from thermostat housing. Blockage somewhere? If so, where
 
There is a possibility you don't have sufficient pressure, what is the tell tail like, are you getting a really strong stream? There is a possibility that the grommet the water tube goes into
 
Sorry,didn't finish. Where the water tube goes onto the oil pan may be perished and leaking badly.Has this motor always run in fresh water because if so there is not a lot that can go wrong where blockages are concerned?
 
Water pressure out of tell tail is very strong with the thermostat in, less strong w thermostat out (as I would expect). Even w thermostat out and housing off, start up the engine and water flows good right out the thermostat housing within seconds
 
I still think you should try a infra red thermometer, if you have the correct thermostat it operates at 72 deg C, there will be steam when you open it.The only other thing you can do is remove the cylinder head, they do have a tendency to get a lot of silt build up round the bottom cylinder which gives the symptom you have.Apart from that there is not much that goes wrong with these cooling systems.
 
So let's pretend the block temp is fine in various spots, but the alarm is still going off. That would indicate a bad temp sensor, yes?
 
Didnt even get that far. Pulled oil plug and its straight chocolate milk. Looks like Brad gasket is gone. @ 280 hours, I am absolutely shocked
 
Actually I got one more question; I mentioned I found the pcv valve tube laying limp in cowling. Now, as I unplugged oil plug, about 5 drops of water came out, following by milky brown oil indicating alot of water in oil. My question, therefore, is what are the side effects of that pcv valve not being in place? Could enough pressure build in crankcase to cause said issues? Would it lead to water in the oil?

Thanks again, appreciated
 
Not sure what you are calling a PCV valve, they don't have one? You have obviously got internal issues, the oil should not be milky, even with the absence of a thermostat. I think you are going to have to pull this motor apart to try and find the cause of water ingress. The quickest and easiest is to remove the cylinder head and carefully inspect for cracks, failed head gasket etc. The other area that is prone to failure is the mount case to oil pan around the exhaust where it can erode, this will necessitate complete power head removal. Have you inspected the spark plugs for sign of water intrusion?
 
Hmm. Pcv valve: found a piece of plastic tubing, about 9" long, flared on one end about a shade smaller than a pencil in diameter. Laying in bottom right of casing below carb. It had oil like substance which matched side of carb. There is a nipple on bottom of card, and another about 5" away on bottom of block. If not pcv then what is it

As for spark plugs, on my list this evening along w compression test.

How does this happen to a motor w 280 hrs!
 
20 years old, you still get corrosion in freshwater, just not as aggressive as saltwater. There could also be a fatigue crack somewhere. Can you post a pic of what you think is a PCV?
 
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