Random knock over 4k RPMs...

XJ99

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Feb 18, 2013
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  • Engine make/model - Mercruiser 4.3 V6 (EFI)
  • Is it a new problem and previously engine was running fine - Just bought the boat
  • Engine hours(if you don't know, make a good guess) 211.6 hours
  • To the best of your ability best describe the problem:
I just bought this Sea Ray 190, and lake tested it before purchase. It runs great,
starts right up no problem. When running around 4k RPMs it sounds just fine,
no power problems, gets right up on plane. When I push it past 4K I get a very
random knock sound coming from the stern area. It only does it under a load.
In neutral, I can rev it up with no issues. It does it on a calm lake no waves, doesn't
matter if I turn or just keep it straight. The knock isn't constant, or predictable at all.

A friend thinks it's old ethanol gas in the boat. I added some marine stable that didn't help.
Any other info I need to provide, just ask... Thanks for any help.... Gino
 

tank1949

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Apr 4, 2013
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Not 100% proficient on EFIs in boats, but I suspect knock sensor is not doing its job. Clean and reconnect. Mechanic may have "bumped" loose. Or,, perhaps spark plug wire is leaking to it before going to ground???
 

XJ99

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Not 100% proficient on EFIs in boats, but I suspect knock sensor is not doing its job. Clean and reconnect. Mechanic may have "bumped" loose. Or,, perhaps spark plug wire is leaking to it before going to ground???
Thanks, will check both! Also just heard back from the owner, and he said it's
been a while since he fueled up with ethanol. (last season?)
 

alldodge

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The knock sensor picks up on a per-ignition knock, but is actually heard as a ping. If this is an actual knock (some louder) then might be something else. As has been mentioned would be good to check in any case.

Could use a serial number, but if its a MPI
Might be a distributor cap going bad and the spark is cross tracking. Those flat caps are know to go bad and look great
 

Bondo

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Ayuh,..... 1st thing I'd do is dump the fuel filter, 'n look for water/ ethanol sludge,.......

If it's a lean detonation, every knock ya hear, is abit of yer pistons goin' out the tailpipe,.....
 

tank1949

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Thanks, will check both! Also just heard back from the owner, and he said it's
been a while since he fueled up with ethanol. (last season?)
OUCH!!!!!!!!!!!!!!!!!!!!!!!!! Discard safest way possible!
 

alldodge

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IMO (others do vary widely) one season isn't that big of a deal. As Bondo said, just check the filter

Edit: see if its clear and that it smells like gas
 

XJ99

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Alright, will start with all the suggestions! First things first, get rid of the
ethanol. I'm sure the Jeeps will handle it better, so it won't go to waste.
Going to see if I can get some clear line to siphon out the old stuff first.
 

Scott06

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Alright, will start with all the suggestions! First things first, get rid of the
ethanol. I'm sure the Jeeps will handle it better, so it won't go to waste.
Going to see if I can get some clear line to siphon out the old stuff first.
Check the filter and dist cap as advised first. I use ethanol pump gas in my seadoo for years , no issue in the spring. So many issue blamed on ethanol without due diligence troubleshooting,
 

tank1949

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Alright, will start with all the suggestions! First things first, get rid of the
ethanol. I'm sure the Jeeps will handle it better, so it won't go to waste.
Going to see if I can get some clear line to siphon out the old stuff first.
How old is motor??? If my memory serves me correctly, the ethanol sometimes breaks down some of the older fuel lines and the crap ends up in places that it is not supposed to. I had a proline that gave me engine problems until I cycled clean fuel through the tanks/filters and replaced rubber fuel lines. Good luck!
 

XJ99

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I checked the fuel filter/water separator and it looks very clean.
Strong gas odor, but I'm going to replace it anyways. Not knowing
when it last done. The cap and rotor look a little tight, so I'll youtube
how to get to it...
 

tank1949

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I checked the fuel filter/water separator and it looks very clean.
Strong gas odor, but I'm going to replace it anyways. Not knowing
when it last done. The cap and rotor look a little tight, so I'll youtube
how to get to it...
Look inside dist at rotor for any crap, fractures or looseness. U might also have a plug wire arcing or breaking down.. Good luck!
 

XJ99

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So I drained about 35 gallons of ethanol out of the boat. The tank
is nice and clean. Also replaced the sending unit with a KUS while
I had the floor off. Side note, I added stabil to the gas when I got
the boat, and it turns out the bottle treats 120 gallons. Sounds like
I over did it by not reading the directions.

Today I plan to pull the cap, rotor, plugs, and plug wires for inspection before
I take it back to the lake on Friday. Also it will have fresh non-ethanol
from here on out....
 

Lou C

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I don’t think a “random knock” is really reflective of detonation that would be more repeated knocking at certain loads. I think you’ve got something else going on there. I’ve been using E10 gas stabilized for at least 15 years and have not had knocking it pinging problems using either 89 or 93 octane fuel.
 

XJ99

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I don’t think a “random knock” is really reflective of detonation that would be more repeated knocking at certain loads. I think you’ve got something else going on there. I’ve been using E10 gas stabilized for at least 15 years and have not had knocking it pinging problems using either 89 or 93 octane fuel.
I like to use non-ethanol in all my engines that can/will sit for a while.
They always fire right up when needed. Now Something new..
I changed the sending unit and the fuel filter/separator. I ran it on
muffs yesterday, and it was running rough. The only thing I can think
of is I didn't fill the new filter up all the way with gas and got air in the
system. The more it ran the better it sounded. Trying to see if I can
bleed air out of the system.
 

XJ99

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I wish I could edit my post.
I put the muffs back on today, and before start up
turned the key on and off a few times and heard the
fuel pump kick in each time. I started it up and it ran
much smoother than yesterday. It sounded perfect.
 

Scott06

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I wish I could edit my post.
I put the muffs back on today, and before start up
turned the key on and off a few times and heard the
fuel pump kick in each time. I started it up and it ran
much smoother than yesterday. It sounded perfect.
idling on muffs doesn't tell you much, running under load does.

A shot in the dark - check your PS level. Had a neighbors boat (4.3 VP) making a rattling, the only thing I could visually see was low level and foam in PS reservoir. I never would have made the connection between the noise and ps level.
 

Lou C

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idling on muffs doesn't tell you much, running under load does.

A shot in the dark - check your PS level. Had a neighbors boat (4.3 VP) making a rattling, the only thing I could visually see was low level and foam in PS reservoir. I never would have made the connection between the noise and ps level.
Interesting thought, I had a bad leak from my old P/S actuator this summer and one time the fluid level got low enough for the pump to rattle, but I think it would keep doing it, as the pump is ingesting air...
you're talking about a random knock that doesn't seem to follow a pattern, correct?
 

Scott06

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Interesting thought, I had a bad leak from my old P/S actuator this summer and one time the fluid level got low enough for the pump to rattle, but I think it would keep doing it, as the pump is ingesting air...
you're talking about a random knock that doesn't seem to follow a pattern, correct?
rattling noise was what i observed was related to engine speed. Basically started getting on a plane. Return line had loose clamp at ps cooler. Apparently clamps VP used tend to loosen up.
 

XJ99

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Feb 18, 2013
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Think I found the problem!!! Was going to pull the plugs, and saw this!



 
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