View Full Version : Engine Knocking

06-26-2006, 06:14 PM
I know this is a super general question, but what causes engine knocking?

06-26-2006, 06:19 PM
It depends on whay you mean by engine knocking. If you mean preignition / pinging then there could be a couple of causes. What occurs is the fuel ignites before the spark was supposed to ignite it. Here a few causes that I can think of.

1. The wrong octane gas.
2. Hot spots in the combustion chamber. This could be due to many factors, such as burnt oil on the top of the piston or a defect in the pistson itself.
3. The engine is not timed properly
4. Air fuel ratio is too lean.
5. Engine overheating

Hoosier Bob
06-26-2006, 07:01 PM
I hope it is Pinging that you are hearing and not knocking ^^^^. :(

06-26-2006, 07:26 PM
yes Knockin is a really bad thing and pingin is just annoying and for the most part can be fixed.

06-27-2006, 09:18 AM
Alright, lets say for the sake of discussion that it's not pinging, but knocking. Knocking = real bad...so we're talking major engine work?

Hoosier Bob
06-27-2006, 09:36 AM
I would get it checked ASAP. Rod and main bearings are a good source for knock. I believe it is possible to catch it soon enough that all you may need is a crankshaft rebuild, bearings etc not cheap but better than total rebuild. You may also be due for a total rebuild hours? Find a reputable machine shop and let them find the problem. Hopefully it is something minor and if not post what the problem is, cost to fix and repost it here. :o

06-27-2006, 01:49 PM
Is the knockin happening all the time or only when the engine is warming up and then it goes away?

If it is all the time then you better do what HB says as that will certainly lead to engine failure in the future for sure. Bearings could definitely be the issue.

If the sound goes away after engine heats up then it could be the "skirts" on one or some of the pistons that are worn out. (not sure how many hours are on this engine). Won't really hurt the engine but I certainly wouldn't want to listen to it. Can add some oil additive to help that issue.

Can you give us more detail with the knocking.....when it happens, at what RPM, does it go away, subtle or loud knocking, engine burn oil or do u have to add oil frequently?

More details can certainly help to pin point what it might be.

06-28-2006, 01:46 PM
The good news is it isn't my boat. The boats I can afford seem to be the the ones come with some work to do. I have a buddy who's the engine guy that I'll see this weekend so I'll ask him is this is a project he'd like to help me tackle. I guess with my question, I was hoping for some tell-tail signs to see how in-depth the rebuild will be. I don't know much more about what's causing the knocking or what caused it so I can't give much more detail. I'll keep everyone updated if I decide to take it on.