2015 Honda 50hp sensor problem

[Snapdragon III]

I am hoping for some good advice on getting my 2015 50HP Honda running properly. I bought the motor as part of the 13' tender on my trawler I purchased it last year. It had been sitting for at least a couple years unused/broken down, in the tropics before I got it. It had a seized steering bar, and the high pressure fuel pump was corroded and seized, so it was not getting any fuel. I replaced those two parts and got it running pretty good, but I think I have a sensor related heat soak issue that I can't figure out. Here is a detailed description of the current problem.

After the work mentioned above, and new fuel, the motor starts and runs perfectly when it is cold. It will continue to run perfectly as it warms up, and everything is great. If you run it hard, then shut it down and park it at a dock, then come back about 30-40 minutes later, it will start right up, but if you give it more than about 1/4 throttle it will die immediately. If you let it run at idle, after 5-10 minutes, it will allow you to get up to 1/2-3/4 throttle before it dies, then another 5-10 minutes later it is back to running perfectly again. This is repeatable. I don't seem to be getting any check engine lights when this is happening. The conditions I was testing it in are winter/spring in Seattle area with cold water and air temperatures.

My theory is that one of the sensors is malfunctioning when it gets heated up past a certain temperature and gives an erroneous reading that causes the engine to die. It is not heating up from the engine running, but from the hot air inside the case when the hot engine is turned off. As the engine runs it draws cool air into the cover, and eventually cools the sensor off and everything works great again. The part I can't figure out is how to figure out which sensor might be causing this? I have not tried jumping the diagnostic port pins to flash error codes yet, but I assume I won't get anything that way since there is not check engine light? I did some googling for scan tools, but they all looked pretty expensive, of questionable quality, and require a windows computer which I don't have, so I would rather not go that route if I don't have to.

Any help would be greatly appreciated.

 

[CharlieO.]

I wonder if it is some time of vacuum leak that occurs when the engine starts cooling down due to two dissimilar metals the cool off at different rates. Then when you start running again it gradually brings both pieces to the same temp.

 

[boathealer]

Had a Suzuki OB a while back that would develop an air bubble (vapor lock) in the fuel hose (which ran past the exhaust manifold) when shut down after running hard/long. It took a while for the air bubble to "bleed off" when restarting.

Solution was to re-route and insulate fuel hose from engine heat.

 

[AlaskaProf]

I had a Honda 20 on my Bullfrog tender which displayed similar symptoms...but I have no idea if the two motors function the same.


Tom and Ray Magliozzi, the NPR "Car Guys" used to say that half of the calls they got could be diagnosed as "failed choke pull-off". My Honda didn't have a choke, and it is likely yours doesn't either. Instead, on the 20, there is a sensor which functions as a "pseudo-choke" and calls for extra fuel during a cold start to enrich the mixture, and then shuts itself off as the engine warms up. If your engine does not have this function, this is where you can tune out.


I was in the wilds of BC, actually Pruth Bay, when the Honda started misbehaving. Cold starts were good, warm starts impossible. Once I got it started, it wouldn't run above an in-gear idle. Any application of power would kill it, requiring a half-hour cool down.



After a day of frustration, I called the dealer in Tacoma who had done an annual some months before. After a couple calls followed by experiments, he explained the above to me. There is a single wire, on top of the carburetor IIRC, which enables the "not-a-choke". With coaching, I found it and yanked it out of its socket. Cured the issue. Started fine, warm or cold, ran normally, making me wonder why the device even existed.


I sold the boat/motor some years later without reconnecting it. Subsequent owner may still be wondering what this loose wire is for.