F115 EFI, strange idle after running

[Lejestrand]

Hi,
Couple of weeks ago I bought a 2006 F115 with 584 hours on the meter. Today was the first time I took it for a ride on the boat. I have started it attached to a garden hose before just to test it and it was running fine.
Started up OK and after warming up we went for a short ride. Ran like a charm, but when we were back at the dock the engine kept changing the idle between 700 rpm to 1200 rpm. Sort of up-down-up-down. Was doing like that for several minutes until I turned it off. Back home I hooked it up to the garded hose again and the idle was the normal 700 rpm, without going up or down.
Any ideas what might cause a behavior like that?
Should mention that no fault codes are set and I emptied the engine fuel filter and when starting the engine the filter was filled up nice and easy. New gasoline in a new tank, with new hoses.

Regards // Niklas Lejestrand

[rejesterd]

Although you didn't get an error code, I would adjust the TPS per the service manual. A bad signal from the TPS (or another sensor) would confuse the ECM and cause it to go up and down like that (as it tries to stabilize the idle speed). A bad MAP sensor will usually throw a code 29 or 37, so I'd be more suspicious of the TPS. The ECM also gets signals from the pulser coil(s) but if there was a problem with timing, you'd have other symptoms than just the rocking idle speed.

If the sensors are fine, then it could be a sticky ISC valve. To clean that safely, I believe you have to remove it. But there's isn't any official guidance from Yamaha on how to clean them.

[Lejestrand]

rejesterd Thanks for the tip. I have the service manual, so I have to find a way to measure the TPS value to ckeck that it is within specification.
This is a bit annoying, but is it any risk to keep driving the engine like this until I can fix it?

[rejesterd]

The risk would be overheating the ECM. I would say it's a lower risk, but still a risk. If you don't run it for hours at a time, it might be ok.