I have a yamaha f250 as stated above. I have owned the motor for two years now and it has been great. Recently while offshore the motor just above idle in gear started to sputter and miss. after about one minute and a little more throttle it cleared up for the the rest of the day. After this i changed the spark plugs and the fuel filter on the front of the motor and the water fuel separating filter under the console. I thought maybe the fuel let some water get through and that was the problem. The next two trips offshore the motor ran great as usual with out any signs of the strange idle until the last trip out. During trolling at about 7mph it ran great but two times the motor was brought down to the slowest speed to boat fish and it started to act like it wanted to stall. While it was doing it i placed it into nuetral and it idled fine again until it was put back in gear at the low speed. After this it again ran strong all day I need help.
If your engine is NOT coughing/sneezing and stalling because of a low idle, your problem could be related to bad thermostats, ISC valve or incorrect TPS voltage.
Here was Rodbolts advice on a similar thread...
here's what creates the 700 RPM or so idle.
mostly its by the temp sensor,TPS and ISC.
its possible the t-stat is stuck its possible the ISC is sticking or dirty and its possible the TPS voltage is incorrect.
the ECU will adjust air,via the ISC and ign timing based on signals from the ECT and TPS to try to maintain the ECU mapped idle speed for the throttle angle and engine temp inputs.
now ya just gotta find out whats not quite right.
remember the ECU is rather stupid.
wait wait wait.
hold the music.
the F250 is a totally different animal than the F225 or F200.
you need to take this to a shop that employs at least one master technician and actually look at his certs on the wall.
the Yamaha dealer locator web site can help you find a dealership with a master tech.
I dont think so.
the F250 uses a throttle position potentiometer that monitors cable motion and transmits the data to the throttle stepper motor.
from there the stepper motor electronically opens and close the throttle valve.
the F250 compared to the F225 is like comparing an Estes rocket with the space shuttle.
we dont have many non fuel related issues with that motor and most techs are absolutly clueless when dealing with it.
I am still very careful and I see a lot of them.
just not many broke ones.
make sure your tech has a masters cert and pray.
Did you get it fixed? I have a 2005 f150 with the same problem. I cleaned the injectors and the problem stopped for a trip or two but it didn't totally solve the problem. It is worse when the boat is in reverse at under 800 rpm and is noticable when the boat is in forward at under 800 rpm. Some people I talked to attributed it to running a stainless prop but I can't see that being the problem. I was going to try running some ring clear and changing the plugs. I will post if there is any improvment.