1989 mercruiser 4.3 engine knock

[BomoBoater]

So, after putting $2500 into a new deck this spring my '89 mercruiser 4.3 developed this knock. It's not really rhythmic, but it's definitely there.

Pulled all plugs and they looked decent. No fouling, burning etc. compression test gave up fairly good #'s. from 159-163. No water visible in cylinders. Oil from dipstick shows no clouding or metal flakes. No idea of hours on motor, there's no indicator. Fires right up, but does shake a lot at idle. Good oil pressure too...

Started yesterday. after 15 min of WOT I dropped down to 1500rpm to cruise in. Sudden knocking from motor. Shut her down and came back today to investigate.

Ideas?
http://m.youtube.com/watch?feature=...top_uri=/watch?v=pyUzcFCygKg&feature=youtu.be

 

[thumpar]

Re: 1989 mercruiser 4.3 engine knock

Can you tell where it is coming from? In the video is sounds like something on the outside of the engine rattling.

 

[achris]

Re: 1989 mercruiser 4.3 engine knock

How's the oil pressure? And spark on all cylinders. Don't just check spark plugs, put new ones in... Also check (= remove wires, clean and reattach) things like grounding on the distributor and ignition amplifier box...

Chris........

 

[alldodge]

Re: 1989 mercruiser 4.3 engine knock

new deck this spring

Is your alternator a bit close to the deck

 

[BomoBoater]

Re: 1989 mercruiser 4.3 engine knock

I didn't like running it with that knock, but it seemed to be low down. I agree with the rattling sound, but what could cause that?

Replaced and gapped new plugs (.035), before vid. Old plugs looked like they were firing fine. All wires rechecked.

I'm thinking rod knock, but the non rhythmic thing is stumping me.

Alternator is close, but not touching.

 

[BomoBoater]

Re: 1989 mercruiser 4.3 engine knock

Oil pressure has always been good. Drops with rpms, but never even close to yellow.

 

[alldodge]

Re: 1989 mercruiser 4.3 engine knock

I don't think it is an ignition knock, rod of lifter. If you have a velvet drive I would say it is the transmission but you don't have one. Have you pulled the drive and checked u-joints, flywheel and maybe engine coulper

 

[achris]

Re: 1989 mercruiser 4.3 engine knock

This is an old outboard trick, but might just work in this case...

Remove all the plugs. Turn the engine until you have one piston at the top. Now keep turning another 90 degrees (puts the piston about half way down the cylinder on a down stroke). Now get a screwdriver or the like and push carefully on the top of the piston, and see if it moves down the bore any... You shouldn't get any movement, if you do, you have a shot bearing (big end or small end).... Check all 6 pistons the same way...

My first impression of that noise was massively early spark firing, on an irregular basis. That would account for the noise and the roughness.... Just a thought. Maybe throw a timing light on each cylinder and verify that the spark timing is not wandering.... ?

Chris.......

 

[ericga]

Re: 1989 mercruiser 4.3 engine knock

Could it be an engine mount or any kind of mounting bolt rattling? Engine cover?

 

[alldodge]

Re: 1989 mercruiser 4.3 engine knock

This is a WAG but maybe, Remove the belts on your water pump, grab hold of it and see if you get any noise from it but rocking it back and forth quickly.

 

[britisher]

Re: 1989 mercruiser 4.3 engine knock

Have you tried using a stethoscope?

 

[BomoBoater]

Re: 1989 mercruiser 4.3 engine knock

'89 mercruiser 4.3 motor knock - YouTube

More

 

[BomoBoater]

Re: 1989 mercruiser 4.3 engine knock

The sound appears to be coming from the bottom end. I tried pushing on the pistons, and got no movement. I tried removing the plug wires one by one, and also noticed no difference. I like the water pump idea, but since the knocking appears to be low down, and even started making a horrible metal screeching noise ( just as I ended the 2nd video), I'm probably not going to test it.

I looked and looked for something loose. Found nothing. The motor does hop around a bit on the mounts... But I'd find it hard to believe that's the culprit. I did recall that the dipstick had a tendency to pop up and inch or so under power. Since it never spewed oil I just chalked up as a worn out dipstick.

On a side note. While messing around I managed to mess up the ignition module and have a no start issue to top off my day. So now I have new plugs and a new module in a more than likely toasted motor.

I guess it's time to seek profe$$ional help.

 

[BomoBoater]

Re: 1989 mercruiser 4.3 engine knock

In conclusion....

My mechanic also believed it to be rod knock. So he removed the engine, pulled off the oil pan and inspected everything. He was shocked to find nothing. Actually, better than nothing. He said the bottom end looked immaculate. So he then flipped it over, pulled everything off ( including the heads) and did another inspection. Once again revealing a perfect motor, but no cause for the noise. He replaced all gaskets, reassembled it and ran it on the stand. No noise. I think he kicked himself at this point.

Turns out the u joints in the upper unit were bad. So one rebuilt upper unit, and $1k later, everything is running. 100%

Since he winterized, shrink wrapped, stored the boat. R&Rd the motor, disassembled, reassembled, and put in a rebuilt upper unit, I feel like $1k was highway robbery, and I was the robber.

Thanks for all your input.

 

[alldodge]

Re: 1989 mercruiser 4.3 engine knock

Thank You very much for coming back and filling the outcome. Will be great info for all those who come afterwards.

Have a great boating season