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BF 9.9 Dies soon after starting

I'm think I will try both suggestions starting with checking continuity of wiring from sensor to cdi plug, and if that checks out then do a coolant line flush light,
 
I would like to know if the sensor is a Positive Temperature Coefficient (PTC) type sensor. That means that the material it's made from increases in resistance as it gets hot.
At room temperature it would have very low resistance between the wire terminal stud and the threads on the sensor body.

Conversely, if it's a Negative Tempeeature Coefficient (NTC) type, the material it's made from decreases in resistance as it heats up. It should have high resistance between the body threads and the terminal stud at room temperature.
 
I think the sensor is probably an NTC that loses resistance as it heats up and provides a ground to "turn on" the engine protection feature causing the power reduction and engine stall. Disconnecting the wire takes ground away and disables engine protection.

But...I think it's possible that wire could be very slightly touching ground someplace where the insulation is chafed or damaged and is causing the engine protection to be activated.
Just a theory though.
 
jgmo said:
I think the sensor is probably an NTC that loses resistance as it heats up and provides a ground to "turn on" the engine protection feature causing the power reduction and engine stall. Disconnecting the wire takes ground away and disables engine protection.

But...I think it's possible that wire could be very slightly touching ground someplace where the insulation is chafed or damaged and is causing the engine protection to be activated.
Just a theory though.
Click to expand...
I just measured the original sensor ; at room temperature = 2.5k ohm after heating up sensor with heat gun reading is .4k ohm so yes resistance drops as sensor heats up
 
Because of the symptoms you're seeing, you might want to look carefully at that wire for any sign of it possibly finding unwanted ground someplace.

It's sort of a "hack job" approach but, just to get my outboard working so I can use it, I would be tempted to snip that wire near the CDI plug and the sensor and splice in a replacement. If it works, I might just leave it that way or choose to dig the original out of the harness and tidy it up during off season.

If a new wire doesn't cure the issue then there's the possibilty of something being wrong with the CDI.

HOWEVER...
...making sure the oil pressure portion of the Engine Protection system isn't causing the problem would be wise before making that call.
 
And , since it appears that your heating the original temp sensor may indicate that there's nothing wrong with it, there's still the possibility that there could be some sort of overheat condition taking place like the "localized hot spot" I mentioned previously. Possibly caused by mineral deposits. Your plan to try flushing could help with that.

As to flushing, NSDON has written about how effective his use of vinegar mixed in water has been. You might want to read those posts
 
Ran a few more checks. The wires running from the overheat sensor to the cdi plug check out good. With a warmed up engine, measuring across the pins of the sensor was 550 ohms same as was measured at the cdi plug, tried moving wires and wiring harness while measuring and reading remained the same.
Tried to measure cdi unit resistance pin to pin but readings were all over the map and did not match the chart in the service manual.
I pulled the thermostat while taking temperature measurement and temperatures accros the board are much lower with a warmed up engine and the thermostat pulled. The temperatures with thermostat in are 63 c at the sensor, and 60 at the cylinder heads when the motor shuts down . With thermostat out we have 36 deg at the sensor and 45 degrees at the cylinder heads and the engine will not shut down in this manner.
So with either sensor unplugged or thermostat removed the engine will not shut down.
Since it shuts down with thermostat and sensor installed while still in normal operating temperatures I think the cdi is not interpreting signal from the sensor properly. (2 sensors give same result)
Thermostat was tested in boiling water and opens at 140 deg f.
New cdi is $450 cnd from dealer so I'm looking used.
Better to run for the time being with the sensor wire disconnected or the thermostat pulled?
 
Just my opinion but I would choose to run without the Tstat rather than disconnect the sensor.

While not ideal, at least that way there's still the possibility of the engine protection system still working if the pump suddenly failed and truly severe overheating were to take place.

$3,000 + replacement cost.
 
FWIW, I've tried doing those "cross check" resistance measurements a few times too and the readings never seemed to match that chart or make any sense to me either.

hondadude, a once VERY KNOWLEDGEABLE contributor here agreed. He was an accomplished dealer tech that helped MANY people on this site repair their Hondas and save THOUSANDS.
I miss his presence.
 
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