Rod Knock on Park '96 Avenue.

[mbg]

Be kind - my first post.

My daughter has a '96 Buick Park Ave. - 3.8L. At idle it started to have an intermittent oil press. light come on. I told her to pull over immediately and I met her to look at the car. I first checked the oil (I just changed it a couple of months ago) and the level was fine. When I started it up I could hear a knock in the engine. Sounds louder from underneath the car especially at increased rpm's.

Tonight I thought to remove the injector wires one at a time to see which cylinder might be the coulpirite. The result was unexpected. When I removed one of the wires the noise increased by tenfold (maybe a little exazerating).

Is this just a rod bearing? Can I get by with just changing the rod bearings? Looks like on this big Buick I can drop the pan and get at the bottom end w/o removing the engine.

Thanks,

Mike

[rksk]

Mike,

Even though the intermittent oil pressure light and the increased noise level when removing the injector wire from one cylinder does sound like a rod bearing problem, before you drop the pan, you may want to inspect the harmonic balancer first. I have had personal experience with a loose or cracked harmonic balancer, that sounded exactly like a rod knock.

Regards,
rksk

[mbg]

Originally Posted by rksk

Mike,

Even though the intermittent oil pressure light and the increased noise level when removing the injector wire from one cylinder does sound like a rod bearing problem, before you drop the pan, you may want to inspect the harmonic balancer first. I have had personal experience with a loose or cracked harmonic balancer, that sounded exactly like a rod knock.

Regards,
rksk

How do I check the HB?

Mike

[HotZ28]

Originally Posted by mbg

Be kind - my first post.
When I started it up I could hear a knock in the engine. Sounds louder from underneath the car especially at increased rpm's.

Is this just a rod bearing? Can I get by with just changing the rod bearings? Looks like on this big Buick I can drop the pan and get at the bottom end w/o removing the engine.
Thanks,
Mike

To answer your last question, you can put your hands on the outside ring of the harmonic balancer and see if it can be rotated clockwise/counterclockwise, back & forth. If it can the balancer may be the culprit, however; usually the harmonic balancer makes a hammering noise at idle and will have a very distinct “metallic hammering” sound, then the noise will fade away “at increased rpm’s”.
Now, several things you need to check, before you proceed. First, look inside the oil fill cap and valve cover (inside), to see if you have any foamy chocolate-milk looking substance clinging to anything inside. It does not take much coolant in the oil to generate this foamy gook! If all this looks clean, the oil pressure needs to be checked with a manual gage to eliminate the possibility of a bad IC gage, or sending unit. If the manual gage indicates a fluctuation of 10-20 psig, now you could suspect a bearing, or oil pump problem. The knocking sound would indicate a rod bearing that has worn enough to allow the piston to tap the head. When the bearings wear that bad, the crank has been scored bad enough that it will need to be turned or replaced with a new one. If you put new bearings on a grooved & worn crank, the bearings will fail sooner, not later! The only long term solution to this dilemma, would be a replacement crank kit! i.e. crank, rod & main bearings.

[mbg]

Good news is it was easy to remove the oil pan and rod caps from below the engine.....as you probably guessed bad news is the rod bearing spun and wore the crankpin down approximately 0.060". I did remove the adjacent rod cap and the crankpin looked great. The bearing showed some wear but nothing I'd call excessive. Although this is the first time I examined bearings this small - at work I have designed the bottom ends of a few locomotive diesels one having an 8" crankpin.

Just wondering w/o signs of damage to the adjacent cylinder what caused this one to fail???

Thanks for the help,

Mike