175 EFI Knock...what's causing it???!!

[mhines]

1999 Mercury 175 EFI
Serial# OG870939

Motor has developed a knock, but I don't believe it's in the block...it only knocks while in gear, and doesn't seem to be RPM dependent...its a knock every second or so. I tried to take a video, but with the wind noise it's hard to hear, but I can hear it. In person it's very loud! Boats running great.

It will NOT knock in neutral...it almost sounds like it's in the mid section...

What could it be??

[mhines]

Video

[leonreno]

My first guest would be a loose exhaust tuner but that’s from your description not the video, hard to tell what it could be from video.

[mhines]

Would that only happen in gear?

[leonreno]

Not sure, if loose it would always be loose, but it might vibrate more when it’s in gear. BUT, that sound is just sounds so rhythmic it seems like it’s something else, but not sure what would rotate/vibrate on the motor at a constant speed independent of motor rpm. A loose tuner usually will show up with performance issues also.

[mhines]

Yeah I can't for the life of me figure it out...it's fine in neutral

[mhines]

Well I may have narrowed it down to a slipping prop hub...I went out tonight and put the boat in gear and tried to turn the prop...there's definitely some play in the out drive, but I was able to spin the prop about a 1/2in or so, and the prop shaft doesn't spin...it's definitely slipping

[JOE54]

never seen one do that, report back with results , replace with quicksilver part please,on your tempest

[mhines]

Well a new prop hub didn't help...it's not slipping anymore, but, it's still knocking...also today, my holeshot sucked, and top end was definitely slower...how can I check/fix the exhaust tuner to rule that out??

I'm leaning towards a lower unit issue now, since its only happening while in gear..

[leonreno]

Pull lower unit off and use a flashlight to look in midsection to see if tuner is loose, also a long screwdriver can help to check to see if it can move.

[EuropeanAM]

Pull lower unit off and use a flashlight to look in midsection to see if tuner is loose, also a long screwdriver can help to check to see if it can move.

+1

Have you verified there is no metal in the gear lube, also?

[mhines]

I pulled the lower, and it's toast! When turning it by hand, I can feel the gears grinding!!

[JOE54]

yup