1989 4.3L Mercruiser - Bad rod bearings?

[Fiveo Invader]

Hi guys,

I'm working on a 1989 4.3L Mercruiser (GM 262 V6). Got it cheap, knowing there are some engine issues. Intake gasket turned out to be fried, allowing water into the oil... which can only mean bad things, I know. I've fixed the intake, misfire, and other issues, and she's running on all 6 cylinders and idles smooth now. No water in the new oil.

However, there is this constant knock. I thought it might be in the valvetrain, as some rockers were so loose that the cylinder wasnt firing. Tightened the rockers, now it fires on all 6, but the knock is still there. Sounds like a solid knock, not a tap, and happens at idle through all RPM, both on acceleration and deceleration, speeds up with engine speed. I'm thinking the water in the oil may have fried the rod bearings on the crankshaft, and when it takes up the slack it knocks.

I've done rod bearings before, so its no big deal. Just curious if you guys agree with my assessment, or if you have some other suggestions?

Thanks

 

[Haut Medoc]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

Sounds like a rod bearing to me....​

 

[bomar76]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

With engine running, pull and replace plug wires one at a time.
If knock ceases, it's a rod bearing.
BTW..you say you "tightened the rockers"...I hope what you really did was adjust each of them correctly....if not, god knows what may be going on, hell, you could have a piston hitting a valve....

 

[Coors]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

With engine running, pull and replace plug wires one at a time.
If knock ceases, it's a rod bearing.
BTW..you say you "tightened the rockers"...I hope what you really did was adjust each of them correctly....if not, god knows what may be going on, hell, you could have a piston hitting a vale....

Yep, and the rod knocks more when throttle is pulled back. The mains make a lower knock, under load.

 

[Fiveo Invader]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

Hi Guys,

Yes, by 'tighten' i mean that I put each cylinder at TDC and adjusted properly... Just didnt feel like writing that whole bit out at the time .

Whats your suggestions on how to short the wires? I made the mistake not too long ago of pulling the plugs off the cap one by one bare-handed, thats what led me to finding the misfiring cylinder (loose rockers). Only took about 3 plugs until I accidentally bumped into another wire at the cap and zapped the little remaining good sense out of myself. Would rather not do that again.

I imagine that when I have the motor out, I might as well do the main bearings at the same time as the rod bearings. I just want to be relatively sure it's the bottom end before I go through the fun of pulling the motor.

Thanks

 

[Fishermark]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

The only way to do that is to pull the wires off one at a time. I too have gotten jolted by a spark plug cable and it's not fun! It makes me wonder if you have bad cables however. (Not the cause of your knocking, just leaking too much voltage).

 

[ziggy]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

they make sparkplug pliers that are insulated. so ya can pull a wire and not get zapped..... most auto parts stores have'em....

 

[bomar76]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

Helpful hints to avoid having the BeeGeesus shocked out of you when doing the pull-the-plug-wire-one-at-a-time thing:

1. Talk one of your dumber friends into pulling the wires while you "work the throttle and watch the tach"....

2. Fire the boat up at night and look for the Nothern Lights eminating from the plug wires....if so, replace them before you go any further cause you'll get your azz lit up with bad wires no matter what you do.

3. Before you pull any wire, do this - pre pull them one by one to make sure the come off easily (and you want to pull them @ the plug to avoid the electric chair phenomenom).

4. Put some dielectric grease in each plug wire (good practice anyway)

5. Go to Sears and buy a set of the insulated wire pullers (in the aisle with the Lisle tools)

6. Tell you dumb friend you have a case of beer to drink when we are done with this little project.

 

[brunolund]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

just use a jumper wire from needle nose pliers to a groud souce. no worries about a shock after that. pre-pulling them is also a good idea, as it makes it very simlpe and ther is no chance of damaging your wires with the pliers then.

 

[Mischief Managed]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

Run it without the accessory belt momentarily, and see if the sound goes away. A failed water pump bearing can sound remarkably like a rod bearing.

 

[Fiveo Invader]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

Good suggestions... I'll do the plug checks and try to determine which cylinder is causing the knock, so I can take a close look at that one when I do the bearings, make sure nothing else is wrong with it.

Great suggestion about the water pump. I'll give it a shot before disassembling the motor.

 

[Chris1956]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

That motor may have a mechanical fuel pump. They can tick-tick-tick, but don't usually knock.

 

[Fiveo Invader]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

I guess my biggest question as to figuring out if this is a rod bearing issue, or a valve train issue, is the consistency of the noise with increasing RPM / Idle.

I've listened to several rod-knock videos on you-tube, and the sound seems less consistent, and sometimes comes/goes with rpm... even tends to sound more 'hollow' than I'm hearing.

The noise from this engine is more of a solid knocking than a ticking, but it is dead constant, and speeds up with the engine on both acceleration and deceleration. There is no pause in the sound when the load lets off, etc. Noise exists at IDLE and just gets louder/faster with RPM. Guess I'll pull a couple of lifters before pulling the motor, just to check for the chances of a few being collapsed...

Is there any rule-of-thumb as to whether a noise comes/goes with RPM, and how that connects to it being bottom/top-end related?

Thanks everybody. Hopefully I can tear into the motor tomorrow.

 

[Bondo]

Re: 1989 4.3L Mercruiser - Bad rod bearings?

Guess I'll pull a couple of lifters before pulling the motor, just to check for the chances of a few being collapsed...

Ayuh,........

Take it right down to the Block, right in the boat if you want,......
It's Alot lighter that way......

Just keep Diggin' til you Find the Issue...........
From your discription,..... I'm thinking you'll find it after it's on the Stand,+ upside-down.......

Good Luck.....