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2018 Mercury 40 EFI – rhythmic RPM surging at WOT after 5 minutes – video

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ryanagler

New member
Serial 1C563963, runs perfectly for ~5 minutes at full throttle, then RPM oscillates very rhythmically about 2 cycles/sec, no warning horn, immediately clears if throttle is pulled back and reapplied, but returns. Ive tried a completely different fuel tank, bulb, and bypassed the fuel/water separator, same result.

 
Interesting issue. Since you've already tried a different fuel tank and primer bulb and bypassed the fuel/water separator, it may be worth looking beyond the basic fuel supply setup.

Have you had the engine scanned for stored or pending fault codes? Even without a warning horn, checking the ECU fault history could provide some useful clues.

If possible, I'd also check fuel pressure under load and monitor live data, particularly throttle position and sensor readings, when the RPM starts fluctuating. That may help narrow down whether it's a fuel delivery issue, an electronic control issue or something else.

Does the RPM fluctuation happen at lower throttle settings too, or only after running at full throttle for a few minutes?
 
Yeah, cleaning the VST might be worth a try. Since it runs fine for a few minutes before the RPMs start jumping around, maybe check the VST filter and fuel pressure while it's running at full throttle
 
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