Engine knocking at 2000 RPM

N

NWhippie

New Member
Aug 29, 2009
61
Puget Sound, WA
Boat Info
1994 250 Express Cruiser
Engines
5.7L Mercruiser

Bravo II
We went out yesterday to drop the crab pot and after leaving the no wake zone, I throttled up and right at 2000 RPM's, the engine started knocking pretty bad. I pulled over to a safe spot and checked under the 'hood' and found nothing abnormal. My first thought was low oil so checked the stick and found the line between 'fill' and 'full' so I added about a 1/3 of a quart. Started up and tried again but still had the knocking right at 2000. We decided to float instead (a nice day can't be wasted in Seattle) and then went back to our slip. No noise at all below 2000 and no other symptoms. Pressure and temp were both normal throughout the episode.

The knocking seemed to be coming from the port side of the 5.7 and sounds like valves to me. From reading a few other 'engine knocking' posts, it could be a rod about to go, a valve dropped on top of a piston, bad fuel, a broken tooth (timing) or blown head gasket. Is my summer season over? We just paid for another month at the slip so I hate to waste it by trailering it to the shop in August. Any thoughts?

Eddie
 
A 'knock' could be a rod noise. Valves generally are a 'click' or 'clack'... lol.
Do like a doctor does... use a stethoscope a.k.a. a long screwdriver. Put the handle against your ear and put the tip on vaious parts of the motor to try and pinpoint the location of the noise. If you have a mechanical fuel pump... start by listening there. A bad fuel pump sounds like a 'cli-clunk'... lol.
Seriously, the screwdriver method works really well.
 
magster65 said:
A 'knock' could be a rod noise. Valves generally are a 'click' or 'clack'... lol.
Do like a doctor does... use a stethoscope a.k.a. a long screwdriver. Put the handle against your ear and put the tip on vaious parts of the motor to try and pinpoint the location of the noise. If you have a mechanical fuel pump... start by listening there. A bad fuel pump sounds like a 'cli-clunk'... lol.
Seriously, the screwdriver method works really well.
Click to expand...

Ok, I guess it's more of a clack-clack than a knock then. Since it only starts making the noise at 2000, is it ok, to rev that high in neutral? Otherwise, I might end up with a screwdriver handle dent in the side of my head when the admiral hits a wave and we bounce. :grin:

Eddie
 
A guy who used to own a garage (who also happens to be my current boss) said it might be a bearing in one of the cylinders. He's a small boat guy (Boston Whaler) and thought if I just idle around, wouldn't hurt it. I'm skeptical and don't want to make matters worse. I 'spose we could just sit on it at the slip and watch the other boats go out.:smt089

I haven't been to the boat in the last couple days so haven't been able to try the screwdriver trick yet. Will try tonight.

Eddie
 
NWhippie said:
A guy who used to own a garage (who also happens to be my current boss) said it might be a bearing in one of the cylinders. He's a small boat guy (Boston Whaler) and thought if I just idle around, wouldn't hurt it. I'm skeptical and don't want to make matters worse. I 'spose we could just sit on it at the slip and watch the other boats go out.:smt089

I haven't been to the boat in the last couple days so haven't been able to try the screwdriver trick yet. Will try tonight.

Eddie
Click to expand...

Which marina are you at?
 
Dave S said:
Have you tried it with the engine in neutral?

Dave
Click to expand...

No, will do that tonight in order to listen with the screwdriver. No one responded to my question about revving it to 2 grand in neutral so I assume it's ok.

Eddie
 
Trying it in neutral will allow you to determine if it is a knock that shows up just under load or not.
 
Ok, new facts:

Knocking (actually more of a clatter) does NOT occur when in neutral. Ran it up to 3000 RPM and no noise. The drive oil reservoir was below the line by about an inch and a half so I added to the line. I had filled the reservoir a couple of weeks ago so it's either leaking or burning it. No difference after filling tonight. The screwdriver stethoscope sounded fine at all points in neutral. Smooth and no abnormal noise.

The sound seems to be coming from either the port side valve cover or the exhaust manifold or exhaust elbow. My Clymer manual shows the raw water pump on the starboard side so I don't think it's that. Question: if the clattering is coming from the engine and not the transom area, ie: valves or exhaust shutters perhaps, wouldn't it occur in neutral as well as in gear? Or would the load of being in gear running at 2000 RPM make a difference in the exhaust? Maybe somewhere between the motor and drive?

Still stumped,

Eddie
 
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get the engine warm 120 160 range Put in N have some one rev the engine. as they bring it up to about 2 or 3 K easy and slowly pore a bottle of Disani water into the intake. Should go away then.......

You ask why?
if it's a chunk of carbon it will explode and clear up.
If it's a rod bearing or wrist pin it will explode any way with the water...
Ya got to find out now rather than nurse a bad condition till you are too red face and unsure and take your friends and kids 5 miles off shore....
 
Last edited:
ok... the screwdriver has shown that it has to have a load on it to make the noise.
I guess it's possible that its a carbon build-up... there's a ton of fuel additives for carbon removal... try one. Keep your fingers crossed.
Here's a story you may not like...
I bought a vehicle off a 'friend' once. He said it was running fine and then it started ticking... he thought it had a collapsed lifter... and that's what it sounded like (sort of). After going through the valvetrain with no luck I pulled the engine and dropped the oil pan. He had obviously run it low on oil and the front 2 rod bearings were beat to death. It made a clacking sound when it was under load.
:huh:
 
I took my mechanic (auto) friend out for a ride to listen to the noise this morning and here's his conclusion: Ignition. He said it sounds like the port side rear cylinder is not getting enough spark when it reaches into the 2k range. He explained how the parts inside rely on that firing of the fuel (explosion) to keep everything tight. When that spark doesn't occur or doesn't have the correct amount of 'fire', it loosens everything up and that's the clattering I'm hearing. Suggests replacing wires, plugs and cap and rotor and thinks that will solve the problem. Does this sound like a reasonable answer to my problem? I appreciate any responses to his diagnosis.

Eddie
 
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