An F90 on a pontoon shakes at idle and smooths out a bit above 2000 rpm. That's a textbook single-cylinder miss. The usual guessing game starts with a new set of plugs, then a coil borrowed from a parts bin, then an injector cleaning, and each round costs money and a weekend. The actuator tests built into the ECU let you work through the cylinders one by one instead.
What an actuator test actually does
An actuator test tells the ECU to trigger one component while the engine is stopped. An injector answers with a sharp, regular click you can hear or feel. A coil gets fired by the ECU, and with an inline spark tester you can see whether it delivers. How many tests you get depends on the engine: 4 to 8 on the F20 to F100, 6 to 11 on the F115 to F150, 9 to 16 on the F175 to F250 and 16 to 25 on the big V6, V8 and XTO engines. The electric fuel pump and, where fitted, the ISC valve usually come along too.
Working through a four-cylinder
- Read the fault memory first. A pulser coil or knock sensor message changes the whole picture, and on the bigger engines an injector message can name the cylinder directly.
- Find the weak cylinder. Pull the plugs and compare them, or note which cylinder runs coolest on the exhaust side with an infrared thermometer after a short idle.
- With the M-JET Diagnostic connected, key on, engine off, trigger injector #1 and listen with a mechanic's stethoscope or a long screwdriver against the injector body. Repeat for #2 to #4. A silent or dull one stands out right away.
- Trigger each coil. A coil that doesn't respond at all is either dead or isn't getting its signal, so check the connector and the wiring before you buy anything.
- If everything responds, swap the coil from the weak cylinder with a neighbor and run the boat. If the miss moves, it's the coil. If it stays, look at the plug, the injector spray and the compression.
The coil or injector page has more on reading the symptoms before you start testing.
Paired tests and how to split them
Some ECUs don't fire each cylinder on its own. They work in pairs instead: #1 and 4 with #2 and 3 on a four-cylinder, or #1 and 4, #2 and 5, #3 and 6 on a V6. The pairs follow cylinders that share a firing position in the cycle, and it means a weak result points at two cylinders, not one. Swap one coil of the pair to a healthy cylinder and see whether the problem moves with it, or listen to each injector of the pair separately while the test runs. It takes ten minutes longer and saves buying parts in twos.
Where the test stops
The test proves that the ECU can drive the circuit and the part responds. It doesn't prove that the injector sprays a clean pattern under fuel pressure, and it doesn't prove that the coil holds up under compression and heat. A coil that breaks down only when hot, or an injector that clicks happily but dribbles, needs a running test, a swap, or a bench check at a shop with a flow rig. Valve and ring problems sit completely outside anything the ECU can see.
The full list of tests per engine family is on the actuator test page. If every coil and injector checks out and the miss stays in the same cylinder, put a compression gauge on it before spending another dollar on ignition parts.
