Knocking 4.3 merc

[timharper89]

Got a knock in my 1987 4.3 mercruiser at around 2000 rpms. I noticed it the first time while out on the lake one day after towing tubes, skis, and kneeboards. The noise is absent at idle and presents only around 2000 rpms and then seems to disappear again after 2500 or so. It is quite loud /serious sounding, so I'm not sure what is safe to continue with troubleshooting. I don't want to be poking around at 2000 rpms if I don't have to. The engine does have blowby and lacks power (4600 wot rpms =32-33 mph gps) and hole shot is pretty bad, especially with a full boat (sometimes a minute or more to plane if fully loaded).
Here is a video of it idling. The ticking is the solid state fuel pump.

New video by Tim Harper

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Here is it knocking toward the end of the video

New video by Tim Harper

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Thanks for any direction.

 

[Rick Stephens]

Doesn't sound like a rod knock to me.... might pull all the belts, see if that changes anything. Then pull the drive and see if that changes anything. I wonder if it might be a broken piston. A compression check would be a good indicator of how things are inside.

 

[timharper89]

Doesn't sound like a rod knock to me.... might pull all the belts, see if that changes anything. Then pull the drive and see if that changes anything. I wonder if it might be a broken piston. A compression check would be a good indicator of how things are inside.

I can get a compression test maybe tomorrow. To clarify, I didn't notice a LOSS of power. It's been sluggish since we bought it a few years ago. I do know there is blowby from one piston because smoke puffs out of the dipstick tube.

I hope it's not a broken piston . I can do most things on my own, but a rebuild probably isn't one of them.

 

[timharper89]

Did a compression test.
1. 75
2-5. 165
6. 175
I figured as much since I have smoke puffing from my dipstick tube.
I checked the oil Level and the dipstick was dry. Added almost 2 quarts to get it to the full line.
Removed the belts, and the sound is still present, definitely coming from the port side. Could be cylinder 1 (low compression). Did running out on low oil screw something up?

 

[Scott06]

Did a compression test.
1. 75
2-5. 165
6. 175
I figured as much since I have smoke puffing from my dipstick tube.
I checked the oil Level and the dipstick was dry. Added almost 2 quarts to get it to the full line.
Removed the belts, and the sound is still present, definitely coming from the port side. Could be cylinder 1 (low compression). Did running out on low oil screw something up?

Probably a hole in the piston or broken ring grooves

 

[Rick Stephens]

As a side note, your gauge is reading high. A new motor will put up 150 PSI numbers. So cylinder 1 is probably even lower than 75.

 

[timharper89]

Probably a hole in the piston or broken ring grooves

Wouldn't a hole in the piston read no compression?

 

[timharper89]

As a side note, your gauge is reading high. A new motor will put up 150 PSI numbers. So cylinder 1 is probably even lower than 75.

Hmm. So what are my options at this point?

 

[Lou C]

before thowing in the towel, I'd check to see if you have an intake or exhaust valve on that cyl not seating, that can cause low compression as well. The low compression result doesn't tell you where you are losing the compression pressure.

 

[timharper89]

before thowing in the towel, I'd check to see if you have an intake or exhaust valve on that cyl not seating, that can cause low compression as well. The low compression result doesn't tell you where you are losing the compression pressure.

I haven't done a leak down test. I tried to use compressed air today but couldn't get a good seal. What else would cause puffs of smoke out the dipstick tube aside from blowby. Would a valve cause that? Also what would cause the knocking/rattling noise?

 

[RaceCarRich]

Can try a borescope in cylinder 1.

 

[nola mike]

Wouldn't a hole in the piston read no compression?

Not necessarily. The gauge reads max pressure in the cylinder.

I haven't done a leak down test. I tried to use compressed air today but couldn't get a good seal. What else would cause puffs of smoke out the dipstick tube aside from blowby. Would a valve cause that? Also what would cause the knocking/rattling noise?

Stuck valve, worn valve guides, worn cylinder/rings, broken rings. Sticking valve, broken rings, piston slap could cause a racket

 

[timharper89]

Not necessarily. The gauge reads max pressure in the cylinder.

Stuck valve, worn valve guides, worn cylinder/rings, broken rings. Sticking valve, broken rings, piston slap could cause a racket

The nose is also inconsistent in tempo. It doesn't sound cyclical to me. Would a valve not be cyclical and always present, not just at certain engine speeds? Could any of these be linked to running with low oil or could that have just been a coincidence?

 

[Lou C]

I’d do a leak down test but the questions asked may only be answered by dis assembly

 

[timharper89]

I ordered a borescope and will check that out when it comes in.

 

[timharper89]

I checked it out with the scope. At least there isn't a hole in the piston. Nobody loans out leak down testers that I know of. I'll link the borescope videos in case anyone wants to look at them. Harder to maneuver those things than I thought it would be. There are a few scrapes on the cylinder walls and some small dings in rattling piston, but it seems to look ok to me. It seems like the lack of compression would be blowby due to the cyclical puffs of smoke out of the dipstick tube. I'm most interested in the knocking /rattling sound and how serious it is since it is only at certain engine speeds.
How much would I have to invest in rebuilding the engine? I don't have a hoist or engine stand or the measurement tools needed. I'm sure I'm capable, but I need to decide if the boat is worth the time, money and energy.

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[timharper89]

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[havoc_squad]

I'm going to repeat what everyone else is saying, you need a leak down tester.

Or, if you have a standard air compressor adapter on your compression tester, you can remove the valve out of it and install it with about 90 PSI into the cylinder spark plug hole with the piston held at TDC. Then listen where most of the air leakage is coming from.

Remove the exhaust manifold for that cylinder or at least disconnect the riser, unplug the raw water inlet hose to the engine, remove the oil dipstick, unplug the lines going to the exhaust manifold still connected, and open the throttle on the carb.

I'm guessing you are going to be hearing most/all of your air coming out of the dipstick tube which means a bottom end issue (rings/piston/cylinder wall). That means an engine pull job and highly likely machine shop work needed.