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

[ryanagler]

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. I've tried a completely different fuel tank, bulb, and bypassed the fuel/water separator, same result. It just started randomly the other day, any ideas?

 

[alldodge]

My first thought is weak fuel pump
I would suggest putting a pressure gauge on it to see if it's falling off

 

[ryanagler]

My first thought is weak fuel pump
I would suggest putting a pressure gauge on it to see if it's falling off

Thanks for the suggestion, I just ordered a gauge, should be here Friday. My understanding is there is a Schrader valve to attach it to, I'll take the cowling off and watch it out on the water

 

[muc]

42-44 psi at all speeds

 

[dog22]

Sounds like a fuel or air problem. It's not getting enough fuel, which can indicate a bad fuel pump or a clogged fuel filter, or it's sucking air from somewhere else rather than the intake- a bad seal or a vacuum leak somewhere in the engine block

 

[Lou C]

Another possible cause might be a clogged screen (if it has one) on the fuel tank pick up tube.
Also look in the stickies, there is a nice section on troubleshooting your boat's fuel system that goes through all the pressure tests, vacuum tests, clear hose test, etc. All good to check and rule things out.

 

[ryanagler]

Another possible cause might be a clogged screen (if it has one) on the fuel tank pick up tube.
Also look in the stickies, there is a nice section on troubleshooting your boat's fuel system that goes through all the pressure tests, vacuum tests, clear hose test, etc. All good to check and rule things out.

I replaced everything fuel related up to where it enters the engine. Tomorrow a fuel pressure gauge comes it will be interesting to see what it shows!