Mercruiser 4.3 engine knock

rcdent · Apr 24, 2005

I purchased a 1995 Rinker with a Mercruiser 4.3 at a great price, knowing that the engine is toast. There is evidence of water in the oil inside the portside valve cover only, with no water evidence on the dipstick or elsewhere. The engine starts easily and runs smoothly, but with a loud knock. Compression is low (140) in the forward port cylinder compared with the other 5 cylinders (average about 165). Any ideas before I start turning wrenches? I was hoping it might be just a head gasket, but the knock doesn't help. I'm familiar with this engine in automotive aplications, but not the marine version.

Don S · Apr 24, 2005

Re: Mercruiser 4.3 engine knock

A head gasket will not cause a loud knock. Low compression yes, a loud knock NO. So back to your wrenches, guessing won't find which rod or bearing is making the noise. Assuming the knock is in the lower end and not a valve lifter or bent push rod, can you tell where the sound is coming from?

Bondo · Apr 24, 2005

Re: Mercruiser 4.3 engine knock

Pull the Outdrive,....... Then Pull the Motor,........ Bolt it to a stand, Open it up,+ See What Ya Got..........

I'm familiar with this engine in automotive aplications, but not the marine version.

There Ain't much difference......... Neither will Heal themselfs...........And,... Headgaskets Don't Knock.......

bigbrownbuku · Apr 24, 2005

Re: Mercruiser 4.3 engine knock

yeah i got a visit from a headgasket the other day, bugger just let himself in. the knock is most likely bearing or piston failure. find out why.

rcdent · Apr 27, 2005

Re: Mercruiser 4.3 engine knock

ok, ok, I know head gaskets don't knock...just unfortunate wording. I'm pulling the head off this week and will take a look. I'm hoping for a stuck lifter or bent pushrod, but the sound is pretty solid, so I probably won't get off that easy. I can't tell where th noise is coming from, but if it's anything like an automotive engine, I should be able to isolate the bad cylinder by pulling off plug wires one at a time and listening for change. Any thoughts on a likely cause for water under the port valve cover but no evidence of water on the dipstick? Thanks for your input.

TilliamWe · Apr 27, 2005

Re: Mercruiser 4.3 engine knock

rcdent, now you didn't say it was "missing", you just said it had a knock. You may not isolate the cylinder by pulling wires, cause they all may be firing (even the one with the lower compression- which isn't that bad by the way). What did the spark plugs look like? Regardless, if you want to be absolutely positively sure, Bondo's way is the only way.

TilliamWe · Apr 27, 2005

Re: Mercruiser 4.3 engine knock

Have you thought about running it until breaks, then changing the engine? Just a thought

Bondo · Apr 27, 2005

Re: Mercruiser 4.3 engine knock

I can't tell where th noise is coming from, but if it's anything like an automotive engine,

Ayuh,.......... It's the Same Motor........

Any thoughts on a likely cause for water under the port valve cover but no evidence of water on the dipstick?

Condensation.............

bluewater19 · Apr 27, 2005

Re: Mercruiser 4.3 engine knock

RCDENT, Put the motor on a stand and plastigauge/mic the rod bearings and mains. If there is no damage to the crank put in a new set of bearings and while your at it throw a new oil pump in it. Button it up and go boating! Good luck!

loriaw1 · May 18, 2005

Re: Mercruiser 4.3 engine knock

rcdent, Any luck w/ your knock. Just curious...two days ago my merc 4.3xl just developed a knock. scoped it and thought it was a rod, before tearing into it, decided to take it by a local shop. The owner said that he also just had a 4.3 w/ 2 barrel carb in w/ the same issue. They had it out and on a stand, changed rod bearings, head gasket, and valve job, just to put it back together again w/ same knock. Tehy then got in touch w/ Merc and come to find out they know about this problem. I guess the piston skirt shrinks and rattles aroung....said that it doesnt hurt anything just is anoying....not sure what to do with mine yet.

puppydan · Sep 6, 2008

Re: Mercruiser 4.3 engine knock

Hi we have a larson 5.7 merc and it has a knock at 3000 rpm the dealer has peplaced alot of electrical parts even the main computor and the same problem then pulled the motor could not find anything wrong and think merc sent them a bad computor and are sending a new one, anyone help

Don S · Sep 6, 2008

Re: Mercruiser 4.3 engine knock

Best to start a new thread, not open some 2005 post and add your question to the bottom of it.
In no time your question will be lost, as few people read every reply.

BTW, Welcome to iboats.

Mercruiser 4.3 engine knock

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