Two boats in the same marina both wear an F200 on the transom. One owner is chasing a rough idle, the other an alarm at cruise. Pull the cowlings and you may find a compact inline four on one and a V6 on the other, because Yamaha has sold the F200 both ways. The F175 to F250 range as a whole covers more than 40 model entries, from the F175A to the F(L)250 versions, and once you plug in, the engine layout decides what you get to see.
One cam value or two
On an inline four there's one cylinder bank and one intake camshaft, so the ECU reports a single intake cam timing value and drives a single OCV (the oil control valve that moves the cam). On the V6 each bank has its own variable cam timing, labeled S for starboard and P for port. You get intake cam timing (S) and intake cam timing (P) as separate live values, two OCVs you can trigger as actuator tests, and separate messages for each side: VCT irregular cam timing, cam position sensor irregular signal and OCV irregular load current, each tagged with its bank.
That split is handy. If only the port bank logs a VCT message, the problem lives on that side: the OCV, its small filter screen, or the wiring to it. The check the ECU itself uses is easy to repeat. With the shift in neutral and the engine idling, the intake cam timing should sit at 10 degrees or less. A bank that reads well above that at idle while the other sits low is your lead. The cam timing page goes deeper into it.
Firing coils and injectors one at a time
Actuator tests in this family range from 9 to 16 depending on the ECU. V6 versions offer coils and injectors #1 to #6, either individually or, on some ECUs, in pairs: #1 and 4, #2 and 5, #3 and 6. The inline four gets #1 to #4. When an engine misses at idle, triggering each injector with the engine stopped and listening with a mechanic's stethoscope tells you quickly whether one of them stays quiet.
The pair tests are a small trap. If the #2 and 5 test sounds off, you still don't know which of the two is the culprit. Swap the coil from one of them to a healthy cylinder and see whether the miss follows it.
What both layouts share
Most of the remaining live data is common ground. Depending on the ECU you get 22 to 57 values, typically including:
- engine temperature and oil pressure in kPa or psi
- cooling water pressure on versions that carry a water pressure sensor
- intake air pressure next to atmospheric pressure, which should match closely with the key on and the engine stopped
- battery voltage, and on some ECUs a separate accessory battery voltage for the house side
- fuel flow in l/h or gal/h, a nice way to compare cruise settings when you're trying to stretch range
Messages for the thermo switch, oil pressure sensor, knock sensor and water in fuel filter read the same on both layouts. All 18 ECU variants in this family support idle learning.
A case that shows the difference
Take a V6 F250 that pulls fine out of the hole and then feels flat through the mid-range. On an inline four you'd look at one cam timing value and maybe the knock sensor. On the V6 you compare both banks while the boat runs at a steady rpm, which takes a second person, one to drive and one to watch the phone. If both cam values track each other as rpm changes, the cam system is probably fine and you move on to fuel. If one bank lags behind, you've narrowed things down to one side of the engine before a single bolt comes out.
The M-JET Diagnostic shows exactly what each ECU offers, so the list on an inline four will simply be shorter. That's the engine, not the app. Covered versions are on the F175 to F250 overview. What none of the readings will tell you is the state of the timing chain, the thermostat or the water pump, and on a V6 that's been run hard in salt water, the thermostat housings are the first thing I'd open.
