RPM's Drop to about 2000

Chuck 21SLX

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I have a 2018 VF115 VMax SHO. I am experiencing a drop in RPM's. I can cruise along for a bit at 6300 and suddenly, it drops to around 2000. Turn it off for a minute or two and it will run again. If I keep it to around 4000, it doesn't happen. No warning alarms are sounding. This began the first trip out after completing the 100 hour maintenance (oil and filter change, fuel filter, and lower unit lube change) which I performed myself. What might my problem be?
 
You may have better luck posting in multiple forums (such as The Hull Truth) but my guess is a fuel starvation issue. When you drop RPMs check the priming bulb to see if it is sucked flat which could indicate there is a restriction upstream. Water separator, fuel tank pickup crud etc.

Edit - Downstream of the bulb it could be VST filter, injectors, fuel pumps.

Since it happened right after your maintenance, I'm no rocket surgeon but I would retrace steps on anything fuel related and make sure things are screwed on tight and gaskets are seated so you're not sucking air into your fuel system.

Good luck!
 
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Hey Chuck, any updates? Another thing to check is any "limp mode" issues. For example low oil pressure will drop RPM to 2K. I would think there would be an audible alarm but something to consider.

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That's what it is...your engine is programmed to save itself. Low oil pressure, overheated, etc will drop the rpms to 2000 as Potamac described.
You need to find the problem before damage is done.
TT
This seems correct except no alarm sounds. I tested the alarm by pulling the kill switch and trying to start the engine. The alarm works.
 
I have a 2018 VF115 VMax SHO. I am experiencing a drop in RPM's. I can cruise along for a bit at 6300 and suddenly, it drops to around 2000. Turn it off for a minute or two and it will run again. If I keep it to around 4000, it doesn't happen. No warning alarms are sounding. This began the first trip out after completing the 100 hour maintenance (oil and filter change, fuel filter, and lower unit lube change) which I performed myself. What might my problem be?
I would avoid "cruising along at 6300 rpm" for extended periods. Back off a little bit and see if the problem goes away. I limit myself to 6000 on my Vmax just to be safe.
TT
 
I talked to a boat mechanic, who seemed certain that my problem was the water pump. I replaced the water pump and the problem remains the same. Just a minute or two at full speed and it drops back to limp mode 2000 rpms. It runs fine at 4000 rpms.
 
Any gauges so you can monitor water and oil temp? If not maybe pay the dealer to pull any ECM codes that might be stored?

Otherwise you're going to be throwing a lot of darts blindfolded, but this seems to be a common issue which hopefully means the right mechanic will be able to solve for you.
 
Fuel flow is way higher at 6300 than 4000, so the problem may still be in the fuel system. Did you check the fuel filter for water? I have seen a problem where the filter would not flow enough for high demand, but was fine for normal ops. Also, just to check another box, what oil did you use? At 6300 RPM, some oils are going to have a foaming issue. The quantity can be exactly right, but foaming can cause circulation/pressure problems. Good luck.

BTW: You have carefully checked the oil quantity? Over-service matters too
 
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Since the issue began immediately following the service work, I suspect the two are connected. I'm not a mechanic, but I do think today's engines are very sophisticated and will not tolerate oil levels that are either above or below the correct amount. Case in point: A friend's Yamaha would not run properly because the oil level was just slightly above the correct amount, like PartyBarge wrote above. I would check the oil level following the manufacturer's procedures (i.e. cold vs. warmed up, dip stick screwed in vs. resting on top of threads like my 4-wheeler, etc. etc.) and adjust the level if needed. If you have already done that, it may require a trip to the shop, but I wouldn't continue to run it like it is. Good luck.
 
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