Diagnosing Yamaha Outboards on Twin and Multi-Engine Boats

Two F300s on the transom, and one of them turns noticeably fewer rpm at the same lever position. Or on a triple, only the center engine throws an occasional warning. On a multi-engine boat, comparison is the best tool you have, because the engine next door runs under the same conditions. With M-JET DIAGNOSTIC you read each engine on its own and compare the numbers.

Every engine has its own ECU

On a twin or triple, every outboard has its own ECU and its own diagnostic connector. You plug the dongle into each engine in turn and read it separately. With Command Link digital controls the engines are also tied into a network at the helm, but the app uses the connector right on the engine. Many ECUs have a dedicated live value showing the status of the multi-engine system.

Which values to compare

• Engine hours by rpm range. Big differences show whether an engine was replaced or used alone for slow running
• Engine temperature and water pressure at the same rpm
• Oil pressure at idle and at cruise
• Fuel flow in gal/h. If one engine burns noticeably more at the same rpm, look at injection, ignition and the prop
• Ignition timing, injection duration and intake pressure
• Throttle valve angle and lever sensors, when the engines respond differently at the same lever position

Always read the fault memory on every engine

A fault is stored in the ECU of the engine that had it. When a warning pops up on the gauge, read all engines, not just the one you suspect. For entries about communication between the engine and the remote control, check the connectors and wiring between the engine and the helm first. Water in the fuel filter on just one engine points to that engine's filter, on all of them more likely to the tank. Read the live data of the engines back to back, while conditions are the same, meaning the same water temperature, load and rpm.

Picking the right license

Licenses come for 1, 3, 5 or unlimited engines. Pick it based on how many engines you want to read in total. If you look after several boats, say for a club or a rental operation, go with unlimited.

What comparing won't replace

Comparing shows you differences, not causes. If one engine runs hotter than the other, it could be the impeller, a clogged water intake or the thermostat, and those are mechanical checks. A difference in top rpm can come from the prop or the trim, not just the engine itself. So when you compare, make sure both engines are trimmed the same and running the same prop.

Related topics

• Yamaha Outboard Live Engine Data on Your Phone
• Yamaha Outboard Won't Shift: Remote Control, Shift Sensors and Neutral Detection
• How to Inspect a Used Yamaha Outboard Before You Buy
• All Yamaha outboard diagnostics topics

Everything runs via app over Bluetooth, for Yamaha outboards with control unit generations from 2003. Learn more about the M-JET DIAGNOSTIC for Yamaha WaveRunner & Outboard.