True Mystery: 2016 Mercury 250 Pro XS - Loss of Power but still running at 5700rpms

I have a 2016 Mercury 250 Pro XS, <Serial# 2B125759> mounted to a 2016 Ranger Z521.

Here is what the boats doing - Cranks up and sounds fine, however, she is slow to plane. Full throttle it reaches 52mph - Fully trimmed. The speed will increase a few mph on a long stretch.

The Complete mind bender is, the RPM's still read 5700. I have been questioning my eyes, as in "did i really see 5700? did i misread?" but i don't believe i misread the rpm.

The prop is a Fury, 24p. I have not checked the coils yet, but that wouldn't account for the RPM's. The only thing that makes sense, would be a spun prop, but i did not hear anything, and 52mph seems high on a spun prop.

If you have any ideas as to why this is happening, please let me know. Any help would be appreciated, i imagine this one will take some serious thought.

In my experience, a spun prop will not even get a boat on plane. Are you sure your tachometer is working correctly?

Is the RPM reading from Smartcraft instrument?

You do not have a "spun prop".

Have you changed the height of the jackplate?

I am going to tach the prop this weekend to check. And yes, the gauges are smartcraft. As for the jackplate, i have tried everything trim related while running; the best i can get is a few additional mph. Short of checking the RPM's this weekend, i am going to check the coils out.

Is there anything else you can think of, that would cause this?

Thanks for responding, i am trying my best to avoid taking her to the shop - which around here, means a guaranteed month with no boat and a roll of the dice if the problem will be resolved. Thanks again.

KEV I spend a lot of time in z521 boats and yo speed is way off should run mid to high 60s easy with 24 fury/ one runs 70 light with poles and 2 people its a 2012

I had a similar issue with a Nitro, that on a good day, didn't lift very well with the prop I had. It seemed weak, like I could trim it high enough to get close to normal WOT, but lost some speed. Tried another prop, with no change. A friend told me to replace the plugs, even though they only had around 50 hours on them. I did, and everything returned to normal. That said, I never checked the gaps if the existing plugs, but they didn't look terrible when I pulled them.

you may have a fouled spark plug...had a buddy with an Optimax and his boat ran "great" just slow holeshot and slow top end....turned out he had a fouled plug or two. Changed plugs and issue went away. Make sure you use the appropriate plugs for your Optimax.

http://www.bbcboards.net/showthread.php?t=20401

If the RPM is right, this isn't a plug issue. Sounds more like a prop issue. IE running across a sand bar which will lower the pitch and propeller efficiency significantly. Your numbers indicate a prop slip of 33% which is way beyond reasonable. I'd check the prop first, otherwise make sure you are not dragging a sea anchor behind you.. :)

Prop issue...5700 Rpms is way too high for that speed...even with motor too high on jack plate

water trapped in the hull?

Update: After checking the motor at the house, and becoming more confused as to why everything was checking out fine, i gave in, and took the boat to the shop saturday. Surprisingly, this shop was much quicker than i expected, and called me Monday to say they figured out the problem. The thermostat on 2 cyclinders was bad. I spoke with the mechanic, and he said 1 of the thermostats was popping at around 160+, and the other was popping around 180. Because of this, the motor was going into guardian mode. I said, so this would make the engine only run around 50mph, and the mechanic said "yes". Since this is something they can't check at the shop, i plan to take it out this weekend, and figure out if this is has solved the issue. Does this make sense to anyone? does this sound plausible?

no warning horns for overheat ????? make sense NOPE 5700 is not in gaurdian

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Originally Posted by JOE54

no warning horns for overheat ????? make sense NOPE 5700 is not in gaurdian

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This. You will not see that kind of RPM in guardian mode.

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Originally Posted by Locknload59

This. You will not see that kind of RPM in guardian mode.

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Can't really say that- as Guardian can limit you to anything from 3% to 98% available power, depending on the SEVERITY of the situation.

That being said- I sincerely doubt that the thermostats were the issue in the above scenario. If they were indeed sticking until 160-180 degrees, you would have had multiple warnings to advise you of the OVERHEATING condition.

Much more likely something like a spark plug, or even minor leading edge damage or scuffing to the 24P Fury (they can lose SIGNIFICANT lift and speed with only very minor edge damage).

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Originally Posted by EuropeanAM

Can't really say that- as Guardian can limit you to anything from 3% to 98% available power, depending on the SEVERITY of the situation.

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Thank you for correcting my error Don. I had no idea that it would allow up to 98% power for some conditions.

I agree on the warning horn, as the horn has not been present while running - at least, i believe i would be able to hear it while running. I bought the plugs, and will change them as a backup before heading to the lake. I was reading earlier that if the battery is below 10v and above 16v, then the motor will go into guardian mode at 75% - which, 75% of 70mph, is 53 (rounded 17.5), so 53mph. I am going to test them tonight when i get home. Why would this even be a guardian mode item?

Engine cannot run properly with inadequate voltage.

Update: I checked the batteries last night, they were all 12.63. So that easy fix is out the door. I'll update again after i get the new plugs in, and test her out this weekend on the water. Thanks for everyones comments.

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Originally Posted by kevinbryant1

Update: I checked the batteries last night, they were all 12.63. So that easy fix is out the door. I'll update again after i get the new plugs in, and test her out this weekend on the water. Thanks for everyones comments.

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You need to have your cranking battery load tested.

^^^True^^^ 12.6 volts doesn't mean good battery.

Any idea on how to do this? The only thing that comes to mind, that is do-able in the garage would be pulling the spark plug caps so she won't fire, then hooking up the meter to the battery and pressing the start button. Would this be a capable test, or is there a better way to load test the starting battery?

Just take the battery to an auto parts store or a local mechanic shop and have it tested.

You must fully charge the battery BEFORE load testing......Go to a battery supply house, or auto parts store for testing. Don't try testing on your own.

Update: I just load tested the batteries and all of the checked out, but i did notice that one of the batteries, is a car battery - AC DELCO 31900CT . I believe this to be the cranking battery, but i can't say for certain. With that said, if this is the cranking battery, which is a car battery, could this be the issue?

Kevin,

There is no way you could have lost speed if your RPMs are the same before the event as they are now unless you changed the prop or damaged the prop or the hub is spinning. It is a direct connection. The transmission power is direct. There is no loss of shaft revolutions so you should be going at the speed you have always gone. Did you check the hub yet? Change the hub first and if that doesn't work, borrow a prop from someone and try it out. It is either prop damage or the hub.

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Originally Posted by tomloans

Kevin,

There is no way you could have lost speed if your RPMs are the same before the event as they are now unless you changed the prop or damaged the prop or the hub is spinning. It is a direct connection. The transmission power is direct. There is no loss of shaft revolutions so you should be going at the speed you have always gone. Did you check the hub yet? Change the hub first and if that doesn't work, borrow a prop from someone and try it out. It is either prop damage or the hub.

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Actually there is a way Tom, I have personally had this happen to me, Lost a coil and dropped over 10 mph and only lost about 300 rpm, I believe this is a result of a tremendous loss of Torque where yo can not get the boat all the way on the pad and losses almost all of the bow lift. In turn prop slip goes way up resulting in far less speed and minimal rpm loss, just take a lot longer to get there.
Instead of say 10% prop slip you could now have 30% slip
Not as dramatic as the op but similar, I suspect there may be more than one problem going on here
Need to start over on this one and provide all the info required in forum announcements and accurate data as well as previous performance data
Check all the basics first
Battery currently in use and actual test results
Compression test results
Spark test
Fuel pressure test on the water etc...….

I'm sticking with a bad battery! I experienced something similar to this with a 2003 225 opti!

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Originally Posted by kevinbryant1

Update: I just load tested the batteries and all of the checked out, but i did notice that one of the batteries, is a car battery - AC DELCO 31900CT . I believe this to be the cranking battery, but i can't say for certain. With that said, if this is the cranking battery, which is a car battery, could this be the issue?

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What are the specific results of the load test?
How was the load test performed?

too much DIY without specific results/ true numbers on this thread...
there is physically NO WAY possible to RUN identical RPMS of 5700 with same prop with a 10-15mph drop in speed...
if so, that prop is complete trashed or the motor is now raised an INCH or 2" higher than on previous runs...

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Originally Posted by low slung cruiser

too much DIY without specific results/ true numbers on this thread...
there is physically NO WAY possible to RUN identical RPMS of 5700 with same prop with a 10-15mph drop in speed...
if so, that prop is complete trashed or the motor is now raised an INCH or 2" higher than on previous runs...

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He did not say the RPM was identical.
Read the first post carefully and my post #10

Hey, i understand, i am going to put her in the water this weekend, and video. Like i said in the original post, it is possible that i misread the RPM's - When i looked at the gauge, it wasn't a study look, it was just a glance to see where she was running at wide open, it is possible i misread - i just didn't feel like that was the case. As for the numbers, i will see what i can do to get some better information on here. As i want this mystery solved, badly.

Your smartcraft should be able to recall the max rpm so you can check after your speed run.

Don't put it on the water until all the basic testing is done, could cost you motor depending on what is wrong

Listen to Leon. I am an apprentice compared to him. If you have a drop in RPM then my remarks will likely not apply.

Guys, please see update from taking the boat out - Pictures attached. As suspected, the thermostat change on the 2 cylinders did not resolve the issue. RPM's were around 5400-5500, top speed 53mph, water Pressure 19-20psi, Voltage reading 12.3v-14.1v. I did not get to put the new plugs in, as the delivery was pushed to Tuesday. I do not see any issues with the prop, as far as being bent.

Oh, and the trim gauge does not work, no clue why.

Your 3 or 4 hours from Don get an appointment and drop it off.

i'm actually in Midland TX with work.

Dash gauges are not reliable "Testing" and "Diagnostic" instruments, when it comes to electrical system related problems.

Texas is a bit of a drive to SC did you make any headway on issues My money is on an ignition problem plugs,coil