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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.
 
We have a 2001 BFP8D "tiller model" with a very similar problem. It always starts right up (using its electric starter), then it idles normally for a few to several minutes, then it quits. I thought it must be a fuel issue, but I replaced the fuel pump and filter and fuel hoses and fuel tank, all without effect.

Now I suspect that the cooling jacket is clogged with salt and calcium deposits, resulting in overheating even at idle. I tried changing the thermostat, but I could not easily remove the old one. It has no gauge, buzzer or beeper for a overtemperature indicator.

The last time I worked on it, I noticed that sometimes after starting there is a healthy cooling water indicator flow, but other times the flow seems to be blocked, and I could clear it by jamming a wire up the output port. So it seems like debris is in the discharge that sometimes clogs the port and sometimes doesn't.

I'm planning to do an extended-duration fresh water flush, and to replace the thermostat. The cooling pump was rebuilt only a year or two ago.

My question is this:

Is there a flushing scheme that would clear the cooling jacket of salt deposits? Would a dealer service department know how to do it? Would it be more effective, on a one-time basis, than the recommended 5-10 minute freshwater flush after each use in saltwater? The boat is kept on a mooring, without any nearby source of fresh water (such as a garden hose), so flushing water must be hauled out to the boat in small tanks.

It is possible that the motor in the original post has the same problem.

Thanks!
 
A fresh water flush won’t help with salt/mineral buildup. You will need to submerge the lower unit into a barrel with a water and vinegar mix or a commercial product mixed with water. I’m a fresh water guy, I can’t offer any ratios or recommend a commercial chemical.
 
Okay thanks. Now I need to find out if there exists a desalting chemical that is superior in this application to simple vinegar and water, if so how much better, and what is the optimal vinegar and water ratio. 10% to 50% vinegar in water?

But probably a more important question is can a non-recirculating flush, where the desalting solution goes through the motor only once, practically accomplish the desired degree of desalting? With a recirculating flush the motor is suspended in a barrel of desalting solution and then started and run for as long as it takes to dissolve the deposits in the cooling jacket, right? That's the benchmark. A non-recirculating flush might require an impractical amount of desalting solution.

The boat has a simple flushing system consisting of a water tank and a washdown pump with a hose fitting. But it doesn't have a way of doing a recirculating flush. However the motor can be brought ashore where it can be done. I guess that answers my question. The motor needs to be brought ashore and transported to my yard where I can do a recirculating flush. But can I? The motor only gets flushed that way if the motor is running. But it quits after a few minutes due to the presumptive overheating. I guess worst case would be 1. start motor; 2. flush; 3. if system quits, wait 10 minutes; 4. repeat. Or something like that. If it overheats, wait for it to cool off, and resume flushing.
 
To Joe...
Your problem could be caused by the "electric bystarter" (Service Enrichment valve) aka: auto choke.

The auto choke is defaulted to richen the mixture when the engine is cold. If the auto choke gets stuck or doesn't receive voltage after the engine starts, the mixture will not lean out and the engine will usually sputter and die between 2 to 3 minutes after start up.

Checking the the bystarter wires for expected voltage (approx. 12 volts) after start up is the first step in testibg.

If the voltage is good while the engine is running, the bystarter (item 15 in link below) should be removed from the carb and tested for operation while applying 12 volts directly from a battery.


If your outboard has the manual choke,, ignore the above.

Good luck.
 
Do what Jgmo says, he knows these small Hondas very well. I’d say based on the speed of the outboard shutdown, it may not be an overheat.

On a vinegar flush, yes you need the engine running in a barrel. I used about 4 jugs of food grade vinegar in a barrel, ran the outboard at 2000 rpm for a while, then at 2900 for a while until the barrel water got warm. Ran it 20-30 minutes in total with the thermostat removed and the telltale plastic tip removed from the outboard to open up the water flow passage.

Did your thermostat look crappy?
 
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