Mercruiser 3.0LX noisy - Can the cylinder compression be too high?

[tremblaylaroue]

Hi,
My boat is a Maxum 1996 with a Mercruiser 3.0 LX engine. I bought it abot a year ago.
Since I got it, engine i starting easily and is running smooth but it is noisy (some knocking sound) and is also burning oil for few seconds after a cold start.
I did regular maintenance and tried to locate and fix this annoying noise (spark plugs, oil, ignition timing adjustment, valve rockers adjustment) but didn't find anything wrong and engie is still noisy (knocking noise like a diesel engine).

Today I checked the cylinders compression: After 4 compression strokes, engine warm, with carb throttle open, I got 175 psi on #1, 170 psi on #2, 195 psi on #3 and 180 psi on #4. I checked my compression gage and it is good. From what I've found the normal compression should be around 145 psi, I'm wondering why I got these higher compression readings and if it could be related to this diesel engine sound it has?

I also checked again valve rockers adjustment (engine running at idle, unscrewed adj nuts until rockers knocks, screwed in 3/4 turn. Engine noise is still the same and it's not related to valve train.

I also unplugged sparkplug wires one by one while engine was running to see if the noise was related to one piston but it looks like it isn't.

I also checked engine oil pressure using a pressure gage directly on the engine:34 psi at 750 rpm, 40 psi at 2000 rpm
I'm wandering what to do or to check next? Should I take the engine out to check cylinders, pistons, connecting rods and crankshaft?

Thanks so much for any help you can provide.
Dany

 

[Walt T]

Are you sure you didn't buy a diesel? No seriously, You may have a damaged exhaust flapper or water shutter. Its in the exhaust pipe past the riser down by the exit to the transom. See if you can hear it coming from there. You can buy a mechanics stethoscope at Harbor Freight and poke it down on the pipe and if it is the shutter you will hear it. High compression at cranking is a symptom along with the noise. Or maybe stick a long screwdriver down there and put your ear on it. I cant hear it that way but others can. Even just reaching down there and putting fingers on the pipe can tell you. Be careful of moving parts. If thats okay then take your handy stethoscope and see if you can isolate the noise. Bearing knocks are unmistakable and don't sound like a diesel. Timing belt, any rotating parts on front of the motor all can make noises. I recommend everyone own one of those stethoscopes as they are great for isolating noises. Next time you're at a friends house and they keep blaming the dog, you'll be able to use that mudda to pinpoint where that noise is coming from.

 

[tremblaylaroue]

Thank you Walt T. Actually I already checked with a stethoscope to try to pinpoint noise source without success. It definitly comes from the engine block, a bit louder around the first cylinder but not much. Took it out for a ride today and I felt "knocking" was increasing or maybe I'm more concerned about it... Should have asked the neighbor's dog about it.

I didn't check noise on the exhaust pipe but I checked the exhaust manifold and it wasn't coming from there
I'was thinking to borrow a small "snake like" camera to check within the cylinders thru sparkplug holes next weekend. Do you think it can be useful?

BTW, I was wandering what are the most common reasons to have those Mercruiser 4 cyl. engines rebuilt ?

 

[81 Checkmate]

Has anyone else heard the noise?
I know my 4 cylinder makes alot more noise than the V-8.....It shakes...rattles..and rolls...but starts and runs like a champ.

 

[tremblaylaroue]

I asked a friend who's been working as a master mechanics at a local ATV, snowmobile and outboards dealer to listen to it. He wasn't shure either where the noise was coming from, and couldn't tell if I should have the engine overhauled. He just told me to check if noise start to increase.Does your engine also produce a blue smoke exhaust cloud at starting? I don't think this is normal.

 

[alldodge]

Trying to diagnosis a sound without hearing is a biggy. If it is an actual knock then IMO you need to pull the engine apart. Most probable cause is there is a connecting rod bearing being damaged. If it's a grind then maybe something else, even in the outdrive. If it's more a tap or loud tick then maybe just a collapsed lifter.

As for the smoke at startup this needs to be clarified. Black smoke, to much fuel, Blue is oil burning.

 

[tremblaylaroue]

It can't be a lifter problem because I adjusted the valve rockers while the engine was warm and running at idle. Buy doing this I could easily ear the tipical clapping valve noise while I was unscrewing rockers adjusting nuits.

Concerning the smoke, it's blue and smell burnt oil. I figure that it probably coomes from worned or dried valve stem seals, but it doesn't seem to be a common problem on these engines?

 

[81 Checkmate]

Blue smoke on start up.........Worn valve Guides/ valve stem seals……worn oil rings causing oil seepage into the combustion chamber. Its common on a old and tired 4 stroke engine and needs freshened up.

Have you taken the drive off and started it to be sure its not in the drive?

Just me and only me........I would think.... Unless you have someone verify that its in the bottom end, front end, valve train...id run it till it pukes

 

[alldodge]

It can't be a lifter problem because I adjusted the valve rockers while the engine was warm and running at idle. Buy doing this I could easily ear the tipical clapping valve noise while I was unscrewing rockers adjusting nuits.

Concerning the smoke, it's blue and smell burnt oil. I figure that it probably coomes from worned or dried valve stem seals, but it doesn't seem to be a common problem on these engines?

You could run it without the drive on like checkmate mentioned. Connect a hose to the thermostat housing where the drive water comes in. If the knock is still there, and it is a knock, I would tear into the engine. If it is a rod bearing the engine is salvageable now, but when it locks up it may not be

 

[tremblaylaroue]

OK, it sounds like a good idea. But could a drive produce a knocking sound timed with engine revolution even on neutral? I was also thinking to cut the oil filter to look for brass flakes that could confirm rod bearing wear. In case of no metal particules I will probably remove manifolds and cylinder head to inspect valves guides and remove carbon deposits and replace valve stem oil seals. Nobody seem to be concerned by 195 psi compression reading, is it usual? Is it possible that some carbon deposits in cylinder head and on top of piston are raising the compression ratio and compression readings? Anyone as tried to pour some water into carb while engine running to clean carbon deposits? Sounds risky buisness to me...

 

[Bondo]

OK, it sounds like a good idea. But could a drive produce a knocking sound timed with engine revolution even on neutral? I was also thinking to cut the oil filter to look for brass flakes that could confirm rod bearing wear. In case of no metal particules I will probably remove manifolds and cylinder head to inspect valves guides and remove carbon deposits and replace valve stem oil seals. Nobody seem to be concerned by 195 psi compression reading, is it usual? Is it possible that some carbon deposits in cylinder head and on top of piston are raising the compression ratio and compression readings? Anyone as tried to pour some water into carb while engine running to clean carbon deposits? Sounds risky buisness to me...

Ayuh,..... Everything, but the prop shaft turns With the crank shaft, no matter the gear selected,...

I'd guess water or oil elevated the compression on that hole, donno,....

 

[Silvertip]

Several things to check that cost nothing:
1) Fuel pump lever return springs can break or get weak. The knocking can come from the clearance between the pump lever and cam. It may even be a fuel pump going bad.
2) There is no timing belt or chain on the 3.0. The cam gear is fiber while the crankshaft gear is metal. A worn cam gear can cause this noise and it would be at the front of the engine.
3) These engines can develop piston slap. since the oil pressure is apparently ok, the likelihood of a rod or main bearing going bad is nil (that's what causes low oil pressure). To diagnose piston slap or rod bearing noise, pull one plug wire at a time. If the noise suddenly stops, you found the culprit cylinder.
4) Excessive carbon build-up due to oil consumption can cause a knock. Buy a few cans of SeaFoam and give it the "decarb process" (carbon removal). With the engine warmed up and at a fast idle (do this in the water - not on muffs) pour some SeaFoam down the carb. Dribble it in until the engine almost dies. Continue this for one full can. Then add SeaFoam to kill the engine. Let the engine sit overnight. Next morning fire it up, warm it up, and then run the be-jeepers out of it. Yes -- it will look like the boat is on fire so explain what you are doing to the neighbors. Repeat the process if necessary.

If none of these checks/processes works, then remove the outdrive and run the engine. Noise gone? Yes = outdrive problem. No = Engine problem

 

[tremblaylaroue]

Thanks Silvertip,
I already tried to pull one sparkplug wire at a time at idle but it didn't change the knocking noise, every time I pulled one wire the engine was badly shaking. Does the no noise change result means that there's no bad rod bearing and/or piston slap?

I've read somewhere that one way to test for piston slap is to pour some engine oil into each cylinder and start the engine. If for few seconds after starts there is less noise then it should confirm that there is piston slap. It's an easy test to do, what do you think about it?

I will also check again for noise source with my stethoscope closer to the fuel pump to make shure it's not comming from it. I'll also have a look inside the combustion chambers with a camera to check for carbon buildup. I think the puzzle is about to get solved.
This would all makes sense to me:
1- Leaking valve seals let some oil drop on top of pistons that produce blue smoke at startup.
2- Burning oil is turning into carbon buildup on top of pistons and around valves
3- Oill leaking and carbon buildup might be worst on cyl.#3 which could explain higher compression readings
4- If there's enough carbon buildup, the top of the pistons might hits the cylinder head and produce the knocking sound...

I'll do some testing during next weekend and will post results. If I'm right, some new valve seals and few shots of Seafoam should fix it (cheap fix hopefully)

 

[tremblaylaroue]

Testings results:
1) I was wrong about the carbon buildup theory. Looking thrue plug holes I found just a little trace of normal carbon deposit on top of piston with no trace of hiting with cyl head.
2) Cylinders wall where a bright as a mirror (no more trace of honing) wich means that this engine as probably many running hours on it...
3) Did the Seafoam treatment thru carb - No noise reduction.
4) Replaced engine oil and filter (there was about a quart of oil missing)
5) Cut old oil filter casing to look for some metal flakes - Nothing almost as clean from the outside as from the inside.

So I guess that oil smoking and diesel like knocking is coming from normal wear of pistons, rings, bearings ... and I have the choice to keep it running until it fails (I use it about 50 hours per summer) or get the engine out and have cylinders rebored, and replace wearing parts pistons and bearings.

 

[Stingray190LS]

Treblaylaroue... What did you end up resulting to after dealing with the engine noise? Were you able to isolate where and what it was? I'm dealing with same dilemma.

 

[Bondo]

Ayuh,.... Welcome Aboard,.... This is a Year Old thread, 'n Treblaylaroue ain't been here since,....