You pulled the throttle body on an F70 to clean off a season of varnish, put it back together, and now the engine idles noticeably too high, or hunts up and down for a minute before it settles. Nothing is broken. The ECU spent a long time adapting its idle control to a dirty throttle body, and that adaptation doesn't fit a clean one.
Why the ECU has to relearn
On engines with an ISC valve, the ECU holds idle speed by opening and closing that small bypass valve. As deposits build up in the throttle body, less air gets past the plate, and the ECU slowly learns to open the ISC valve further to make up for it. Clean the throttle body and suddenly more air gets through at the same valve position, so the idle jumps. On engines with an electronic throttle the principle is similar, except the ECU positions the throttle plate itself. Either way it carries a learned correction that no longer applies.
Idle learning throws that correction out and lets the ECU find a new baseline. Many Yamaha outboards support it, from small F-series engines up to the XTO and the HPDI Z series, and the app offers it where your engine's ECU supports it.
When to run it
- After cleaning or replacing the throttle body
- After cleaning or replacing the ISC valve
- After replacing a TPS, or after work on the electronic throttle body on ETV engines
- After fixing an intake leak the ECU had been compensating for, sometimes for a whole season
Getting the conditions right
The engine should be fully warm, in neutral, with the boat in the water or on a proper flushing setup. Switch off anything that puts a load on the electrical system, like livewell pumps, a spotlight or a stereo turned up loud. Make sure the fault memory doesn't hold anything that affects idle. A stored intake air leak message or a TPS out of specification will spoil the result, because the ECU can't learn properly from a signal it doesn't trust. Then start the procedure in the M-JET Diagnostic and keep your hands off the throttle until it finishes.
Reading the result in live data
Afterward, watch engine speed and ISC valve opening angle at a warm idle. A good result is a steady idle with the ISC valve sitting at a moderate opening rather than near either end of its range. Write those two numbers down. If the idle starts drifting up again later in the season, comparing against today's values tells you whether deposits are building or whether something else changed. You can also trigger the ISC valve as an actuator test to make sure it moves at all before you blame the learning. The idle learning page lists which engines offer the function.
When learning doesn't fix it
Learning only adapts to what's there. An air leak at the intake manifold gasket, a cracked hose or a sticking ISC valve will defeat it, and the idle will wander again within a few runs. If the ECU logs an intake air passage leak, find the leak first. A trick that still works: with the engine idling, spray a little soapy water around the gaskets and hose ends and watch engine speed in the live data. A leak shows itself as a brief change in rpm when the water seals it. More causes of a bad idle are on the rough idle page, but on an engine that idled fine until the throttle body came off, the gasket you just reused is the first thing I'd look at.
