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Mercruiser 5.7L Engine Knock - when accelerating - incident likely caused this

minn_dc

New member
Hello, I have a 2002 Chapparal deckboat with a Mercruiser 5.7L engine. I have had it for 4 years and keep it very well maintained. It has been completely fine (and actually great!) up until now. The ticking happens once I get the RPMs up over 2k. There is NO ticking when it's idle or if I rev it in neutral. Also, the knocking happens often but it's not a consistent knock. Like I said, there was an INCIDENT. I was getting ready to pull a water skier and the depth finder said 7ft. Stupidly I ignored the low depth and tried pulling him anyway. He ended up falling right away but as I went back to get him I saw a trail that I created from the bottom of the lake. Also, when I was trying to get him up i could tell the boat was struggling, but I didn't realize I was likely hitting bottom until after. I didn't hear the noise so I tried getting him up again and then I heard it once the RPMs got up there. I stopped immediately and kept it at low speeds and brought it back to the dock. I took it out again today to see if it magically cleared and it's the same issue. I am not that mechanical but I checked the oil dipstick to see if there was any bubbling or cream color and it looks just fine. I don't want to get completely screwed by a mechanic so if you have ideas of what happened, that would be helpful so I can direct a mechanic, without having them completely screw me. Any ideas and suggestions on what happened? Thank you!

Let me know if you can't see the video. The knocking starts when I rev it higher towards the end of the video:
https://photos.google.com/share/AF1QipPFhdXxAj4U-TbOIg5KEKIiY5qoCIwFEPI4NKU1L8P7lU8MU-ihccl1aAo10E5l2g/photo/AF1QipOCz_iyef_zGiO3Xf01OWXCIRa9zsbbgSnmIALa?key=dVRTa04xdjdNMGxTX3BGbWl5Y0hLNF81cWF5RExB
 
Are you sure it's knocking because the ignition is too far advanced? There's a trick to timing these things and perhaps someone screwed up.

Jeff
 
Jeff, given the boat was running fine before the incident, I doubt timing was an issue but who knows....

Bt Doctur, do you think the incident could have caused a detonation issue? Could it have messed up my knock sensor somehow? Also, does this engine likely have a computer connection that can detect engine issues, like a car code reader? Thanks!!
 
If you damaged a gear by dragging bottom the gear oil would be silver or making a lot more noise even in neutral . .If the oil is normal and clean then I say detonation
code readers are for EFI and MPI , not used on carb motors
 
Easy on the loud pedal there! Your knocking sounds like pre-ignition (detonation) and some of that may be due to you trying to push the boat too much. Thats a lot of fiberglass that you're asking that Chevy to make go. Get a few guests on board and it's maxed out real soon.
Try raising the rpm over a longer period of time and not just whack it to WFO in one go. You might find your issue will go away on its own. Maybe not but it costs nothing to try. Well, almost nothing..
 
o2batsea, if I raise the RPM's gradually, and let it run for a long time (while it's clicking/knocking) couldn't I be damaging the engine if it's a detonation issue? Could it be the knock sensor? Any other thoughts?
 
I will tell you what that is..... That is the head gasket, head casting or block casting missing between two cylinders. Time to pull the spark plugs and do a leak down.

If you want to pull some plug wires while it is making the noise you can find the cylinders directly, the noise will stop when the miss starts..
 
Hi Chris from Dockside...wouldn't I hear the knocking in neutral and revving it up? I don't hear it in neutral....Also, wouldn't this knock simply happen the entire time rather than at higher RPMs....if in fact it's a blown head gasket? Thanks.
 
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Hi Chris from Dockside...wouldn't I hear the knocking in neutral and revving it up? I don't hear it in neutral....Also, wouldn't this knock simply happen the entire time rather than at higher RPMs....if in fact it's a blown head gasket? Thanks.

It is not knocking.... It is detonating. It only happens under load.

Just pull the plugs and do a leak down test.
 
Hi Chris (dockside), wouldn't the knock sound be happening in neutral too, when I rev it up? Pistons are moving in neutral. why do you think it's detonation upon load? Thanks. I am only dangerously knowledgeable about engines
 
Hi Chris (dockside), wouldn't the knock sound be happening in neutral too, when I rev it up? Pistons are moving in neutral. why do you think it's detonation upon load? Thanks. I am only dangerously knowledgeable about engines

Pre-ignition and detonation are two entirely different phenomenons.

Detonation occurs primarily while the engine is under load.
Causes may be from:

... poor combustion chamber design, such as with GM's SBC Full Dished pistons.

... undesirable fuel octane rating.

... excessive ignition advance while under load.

... excessive cylinder temperatures.


Detonation will not be created while in neutral with no load.


 
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