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Does your Yamaha F90 run clean at cruise, then die the second you shift into gear? That points at the idle speed control valve, which Yamaha calls the ISCV.

The ISCV is a small gate for air. When the throttle plate closes, this gate feeds the idle air that holds a steady 700-800 rpm. When it sticks, the engine has nothing left to breathe and stalls.

The fix is a bolt-in part. The ISCV idle speed control valve for the Yamaha F90 and F75 is $33.34, and it needs no tuning after install.

What a stuck ISCV does to your day on the water

The throttle plate sits nearly closed at idle. Every bit of air the engine gets must pass through the valve. That is why idle trouble shows up at the dock and not at cruise.

Carbon, fuel varnish and salt crust build up on the pintle — the little plunger that meters the air. The gate ends up hanging open or hanging shut.

• Stuck shut → idle drops low, the engine shakes, and it dies worst on a cold start.
• Stuck open → idle races past 1,000 rpm and the engine will not come back to neutral cleanly.
• Stuck mid-travel → idle hunts up and down while you sit at the dock.

The five-minute dock check

Run this before you order anything. It separates a stuck valve from a fuel problem, which looks identical from the helm.

1. Cold engine, no throttle. If you must hold the throttle open to keep it alive, the valve is not passing idle air.
2. Warm idle in neutral. It should settle at 700-800 rpm and stay there.
3. Shift into gear at the dock. A stall here means the valve cannot add air under load.
4. Pull the valve and look at the pintle. Black gum is a cleaning job; white crust or pitting means replace.

Still hunting after a clean valve and fresh plugs? Then the fuel side is the likely suspect, not the air side.

Which valve your engine takes

Yamaha's number for the F75/F90 part is 6D8-13105-01-00, and catalogs carry it under F75, F80, F90 and F100. Pick by engine family, not by the number on the box.

All four valves are in stock and ship as standard parts. Fuel and air metering parts for other engines sit in the air and fuel delivery collection.

F115 owners: your part carries a different number

The F115 (2001-2011) never used the 6D8 valve. Its valve carries 68V-1312A-00-00, and the two do not interchange.

That is why dealers charge about $513-538 for the OEM unit. The ISCV for the F115 115hp (2001-2011) covers the same engines at $44.12. If your service log needs the exact number, the 68V-1312A-00-00 version is $52.40.

Salt water is why marine ISCVs die early

Yamaha tells owners to flush with fresh water after every run in salt or dirty water. Salt left inside the intake tracks crusts over the pintle and its seat. Once the metal pits, cleaning will not save the valve.

• Flush after every salt or dirty-water run, engine first, then water on.
• Check the sacrificial anodes at each service. They corrode instead of the valve.
• Keep fuel fresh. Old fuel leaves the varnish that glues the pintle in place.

How to replace the valve: about 45 minutes

1. Engine cold, battery switch off. The valve sits on the intake with a two-wire connector.
2. Remove the top cowl and the airbox cover so the valve and plug are clear.
3. Unbolt the valve and lift it out. Note which way the gasket sits.
4. Compare old and new side by side. The new pintle should move and return on its own.
5. Fit a new gasket, torque the bolts evenly, and reconnect the two-wire plug.
6. Start the engine and let it idle warm in neutral. The idle should settle at 700-800 rpm by itself.

There is no relearn routine on this system. If the idle still sags on a cold start, clean the throttle plate area while the airbox is open.

The same bypass-air valve runs in cars, and it fails the same way. The IAC valve guide for the Mitsubishi Mirage (1997-2000) walks those identical symptoms on a car engine.

Frequently asked questions