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i need expert help with MFS40 motor coming out of low speed idle (not staying where its set at)

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keakar

Contributing Member
like the title says, i need expert help with motor coming out of low speed idle (not staying where its set at)

this is the first time this happened but i havent used the slow idle in a few months so im not sure when this started.

what is happening is i use the key to set the idle to the lowest speed idle for trolling for a type of fishing i do, so this is a very BIG deal to me for my fishing style so this isnt a feature i can afford to not have working.

what is happening is i get ready to fish and push the key in twice to change to slowest idle speed and it stays set that way as it should, but after something between 20-30 minutes of idling in gear this way, the engine decides, all on its own, to raise itself back up to the regular factory set idle speed, and it does this without receiving any form of driver input at all.

there are times i fish by slow trolling so it just idles as i steer it so i am not even touching the controls. when i tried to see what was going on with it, it doesnt seem to want to use any of the alternate idle speeds and it didnt "seem" to want to do anything but idle at the normal default idle speed unless i ran high rpms or turn the key off and back on in order to reset it, then, and only then, would it let me reset it back to low rpm idle again, which only lasted for a short time until it returns to normal idle and i have to repeat this.

this is a 2014 nissan 40hp 4 stroke engine that runs fine with no warning lights or codes flashing and it starts and runs perfect and smooth as silk.

i am guessing this must be a dirty sensor thing somewhere that needs cleaning or replacing, but i dont know how to to sort out or even where to look to solve this problem

thanks for any help you can give me.
 
darn, that doesnt sound good then.

i was hoping it was just a dirty sensor or something

i will have to bring it in then, do you think it is the ECM acting up?
 
Could be the ECM, ISC, TPS, or poor wiring... could even be a bad key switch, but you need the service manual and software on a laptop, plus the cable kit to properly diagnose and repair the issue.
 
wow, i didnt realize it gets that involved trying to diagnose something with these new engines.

do you think it would be safe to still use it for another week or two? other then the slow idle, it works fine but i dont want to get stuck on the water

fishing is really great right now and the shop is backed up anyway so they cant get to it right now.
 
The EFI systems are complex; one can plug it into a computer and read out what the ECU has to say for clues, but it's not a shade tree mechanic kind of motor. I see no reason why you can't continue to use it for a couple of weeks; if someone didn't activate the slow idle, they wouldn't even know there was an issue.
 
someone on another forum suggested it could just be a water pump issue, does this sound possible? their premis was a weak impeller will trip the tempo sensor when water flow slows at slow idle and that is why its doing it then with no other signs of a problem. i respect your opinion much more then i do some random internet comments

there are no outward signs of a bad impeller and it "appears" it still puts out a good water stream, but i know it is due for a new one and im thinking it makes sense "maybe" during slow idle when water flow slows down, it may be causing a temperature sensor to warm up just enough to trigger a reset in the idle. i realize it could be many things but this sounds very plausible as a cause since i know im overdue to replace the impeller at 3 years old and it has never been replaced.

im going to go ahead and order the new impeller anyway since its easy to replace and i dont need the shop to do that
 
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Possible, but I doubt it. If you run in dirty or silty or salt water, the water pump will wear faster, but in cold clear Great Lakes waters where I am, we do water pumps about every 3 years or so just to be safe. I recently did the water pump on a 10 year old MFS4, and it was definitely time (the blades had some cracking), but the Tohatsu impellers last a long time and really hold up well.

If the MFS40/50A sees an overheat, it triggers an alarm (lamp B flashes), so you should know if that happens.

Even so, it's probably time to do a wp kit (NOT just the impeller). The side clearances of the impeller are critical, and the cup and wear plate are easily changed when doing wp work. Always do a complete kit, especially if having a shop do the work, since the cost is a little more, but the labor is the same. If doing it yourself, of course, the cost is about double for the kit: Part #: 3T5873223M, $63.88 vs 29.75 for the OEM impeller. Any dealer (even me), can send you the kit.

It's more likely that the ECU is flaky (never had that happen though), or a poor ground or other bad wiring connection is the culprit. That said, if the battery is low, the idle speed will default to 850. A poor battery cable or connection could cause that to happen.
 
hmmm, that is not the best thing to hear

im in south louisiana marsh so yes its very salty water and often silty

thanks for the kit number, it wasnt shown on the parts diagram, if it was i would have ordered it because as you said, its the best way to go

the impeller was already ordered so it too late, i will get the kit next time, i have to believe at just 3 years old the kit shouldnt be "needed" this time

well im going to do a vinegar flush to descale the engine and put in the new impeller and see what happens, from your comments though imm not very hopeful it will solve this

the battery is strong and the connections show no sign of weakness so i dont think its a voltage issue however i will check the electrical plugs and ground for corrosion

to be honest i dont even remember how long the warranty is on this motor, i "thought" it was something like just 3 years so i wasnt expecting it to still be in warranty
 
thanks for all your help, i will check back and let you know what the dealer finds out about it

im sure you must be curious as well
 
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