200 Fourstroke RPM’s fluctuating / surging

[Pathfinder2200v]

1 year old 200 Fourstroke has less than 200 hrs and I’m noticing the RPM’s aren’t holding steady, regardless of whether idling in neutral or at any range running. There is a 80-100 RPM range (surge) that tach shows where it fluctuates up & down.

I suspect this isn’t normal and it might be time for my mechanic to take a peek…

Any suggestions?

[thommo59]

At idle my smart craft tach does exactly the same thing but I haven’t really looked while running, it’s really hard to hold perfectly steady and ensure boat drag or chop doesn’t affect it. I had a service, then new plugs and leads and it made zero difference

[EuropeanAM]

Mike: What maintenance has been done on your engine so far? While ALL items are important, I'd be specifically interested in the Thermostat and the Fuel Filter.

[LTZ25]

Just a wild guess but could this be tied to the alternator bumping up rpm's to charge the battery ?

[Pathfinder2200v]

Mike: What maintenance has been done on your engine so far? While ALL items are important, I'd be specifically interested in the Thermostat and the Fuel Filter.

Don - super keen observation on your part, as the other issue that occurred just before this, was water leaking from the 2 tiny holes in the cap of fresh water flush hose…so could be the “T” valve is clogged. I put a solid rubber washer in it to prevent water leaking until mechanic can peek at it.

Had 100 hour service about 5 months ago by my buddy who is a certified Mercury mechanic, at just over 100 hours, including full water pump replacement and thermostat.

Thoughts?

[Pathfinder2200v]

Just a wild guess but could this be tied to the alternator bumping up rpm's to charge the battery ?

Nope. Batteries are new and kept on Battery Tender maintenance charger.

[Pathfinder2200v]

At idle my smart craft tach does exactly the same thing but I haven’t really looked while running, it’s really hard to hold perfectly steady and ensure boat drag or chop doesn’t affect it. I had a service, then new plugs and leads and it made zero difference

Nope. Didn’t do this before and I’ve always been able to hold steady RPM’s with my boats, especially in calmer conditions. Something ain’t right with the motor.

[thommo59]

Looking forward to hearing that it’s fixed and what the cause was

[EuropeanAM]

Don - super keen observation on your part, as the other issue that occurred just before this, was water leaking from the 2 tiny holes in the cap of fresh water flush hose…so could be the “T” valve is clogged. I put a solid rubber washer in it to prevent water leaking until mechanic can peek at it.

Had 100 hour service about 5 months ago by my buddy who is a certified Mercury mechanic, at just over 100 hours, including full water pump replacement and thermostat.

Thoughts?

Make sure that the FUEL FILTER was replaced at that time (probably not a bad idea to replace it now anyway, just to rule it out as the potential cause). Very important that the filter be replaced ANNUALLY or every 100 hours, whichever occurs FIRST.

Just a wild guess but could this be tied to the alternator bumping up rpm's to charge the battery ?

Good idea, but this should not occur when the boat is in gear (only when idling in neutral). The Advanced Charge Strategy can raise the engine idle RPM's (in NEUTRAL) up to approximately 900 RPM's to help replenish battery when needed. This does not present as a "surge"- simply as a high idle that stops as soon as shift input (and likely when throttle input) are made.

[LTZ25]

Make sure that the FUEL FILTER was replaced at that time (probably not a bad idea to replace it now anyway, just to rule it out as the potential cause). Very important that the filter be replaced ANNUALLY or every 100 hours, whichever occurs FIRST.



Good idea, but this should not occur when the boat is in gear (only when idling in neutral). The Advanced Charge Strategy can raise the engine idle RPM's (in NEUTRAL) up to approximately 900 RPM's to help replenish battery when needed. This does not present as a "surge"- simply as a high idle that stops as soon as shift input (and likely when throttle input) are made.

That makes sense .