Idle Run UP

[macleman]

When I start my 150HP outboard it starts to idle at 600 RPM. It will then run up to 1000 RPM and back down to 400 RPM back up to 1000 RPM. It will continue to do this until it dies. It is not a consistent problem sometimes it starts and will idle at 700 RPM fine.

It's a 2004 Yamaha F150TXRC

[jb123]

150 hp...........

[countrytn]

put the motor and model number in your post so they know if its carb,fuel injected,etc.

[TownsendsFJR1300]

YEAR TOO helps..

[rodbolt17]

my bet will be either a fuel rail issue or an ISC valve issue.
two basic ways to play with engine RPM, add air or increase ign timing.
either will make the RPM increase.
take away air or decrease ign timing and it slows down.

if rail pressure is incorrect or an injectors spray pattern is not right the ECU doesn't know that.
it does see a drop in RPM.
based on the assumption nothing else changed but engine RPM the ECU is going to REQUEST an opening in the ISC, maybe a bit of ign timing correction and a bit of injector on time trim.
now as RPM climbs and still no change in TPS or MAP feedback its going to send yet another REQUEST to the ISC and its going to trim timing and injector on time.

its why on a carbed stupid motor we can set an idle speed and if all is well it wont vary 10 RPM.
if you watch an EFI motor it constantly hunts in a 50-100 RPM band.

the ECU does this in response to external sensor feed backs compared to an internal map or "truth table".

means its looking at TPS,CPS,ECT,MAP.IAT to determine what should be the engine speed.

based on these 5 signals the ECU knows throttle angle, amount of air.
engine speed,some models use a CPS some use a pulser coil signal, for engine speed,this is the speed part of a speed density EFI setup..

it knows engine coolent temp, this is for correction of the A/F when cold or hot.

it knows barometric pressure,locked in for that key cycle at key on.
it knows engine load based on manifold absolute pressure.
and it knows intake air temp,this is the DENSITY part of the speed density EFI setup.

based on these basic signals the ECU knows about what RPM the engine should be turning and can trim fuell injector ON time to add or subtract fuel.

it can increase or decrease the amount of ign timing
at low speeds it can play with the amount of air it allows to bypass the throttle shutters,ISC REQUEST.

the ISC is NOT monitored for failures, that's why its a REQUEST.
the ECU has no idea if it actually moved.

its also why if the ISC mechanically sticks and the ECU keeps toggling the stepper motor ground path it can and will overcurrent the ECU causing ECU and other failures due to overtemp of the ECU internally.

I have seen some ISC failures on the f200/225 V6 that actually melted all 3 ign coils and the coil holder as well as fried the ECU.

its actually a fairly simple stupid EFI setup when compared to some others.