Knock in bottom end

[JFratzke]

Hello! I've got a rod knocking noise in the bottom end of my 1987 4y. We were wondering if it is possible to remove the oil pan and diag/repair a connecting rod or bearing issue from under the motor. We do have a lift.

Looking at diagrams, it seems like we could repair quite a bit from underneath. The engine runs great and has lots of power and only 117,000 miles on it. Obviously we'd rather not yank the motor if we don't have to.

Thanks

[timsrv]

In my experience, when rod bearings get bad enough to hear a knock, there will almost certainly be damage to the crank as well. In order to drop the crank the engine or the transmission (your choice) will need to be removed.

You can however take the pan off and inspect bearings/crank journals without removing the engine or transmission. If somehow you were lucky enough to only need a rod bearing, you could even change that from underneath. Tim

[JFratzke]

As soon as I get the time we're going to take a look. A friend has volunteer to help and has a large shop. Bracing for the worst but hey, I've got another 4y already (of completely unknown quality!) so at least I've got that going for me. Sad I won't get to drive my LE for awhile but hopefully this will be a learning experience.

[Burntboot]

I would want to do a little more diagnosis before making the assumption it is a bearing issue, just to be sure, before ripping into the engine.
Knowing what is most likely causing the noise, before doing exploratory surgery, can be worth its weight in gold.

A bad lifter(s) has been known to sound a lot like a "bearing knock".
It would suck to spend a couple of days replacing stuff, only to find your noise is still present.
BB

[Burntboot]

Is this still the same noise you eluded to in your blog entry when you first got the van?

I would also want to make doubly sure that there are no issues with the exhaust manifold, I have seen exhaust leaks that can make a heck of a racket, not at all dissimilar to a "bearing noise".

[timsrv]

I agree 100% with Burntboot (I was assuming this was already diagnosed correctly). Whenever it comes to major engine work, don't assume anything. If it's a rod knock, it will lessen or go away completely with that cylinder disabled. Pull a spark plug wire and rev the engine. Do this with all 4 (one at a time) and listen for a change. If the engine gets noticeably quieter with a particular plug pulled, then it most likely is a rod knock and that's the cylinder with the problem. Armed with this information, you can save time once the pan is off by only ripping into that one. Tim

PS: You need to know the difference between an exhaust noise and a knock..........because depending on the location of a possible exhaust leak, that could also get quieter when a particular cylinder is not firing.

[JFratzke]

Thanks for the replies. The engine definitely had lifter noise before that was loud (tapping). Replacing all of the lifters and rods quieted all racket coming from the top end. While my performance improved significantly, there was now noise in the bottom end that you could hear more clearly that I wasn't happy about. Shortly after this I ran into issues with the cooling system and front end so it sat for awhile until I got those things squared away. I have been driving it more often recently and the other day a friend of mine commented on the noise. We put it up on a lift running(I work at a dealership) and when you put you hand on the oil pan, you can feel that something isn't right up there vibration-wise. We had ruled out exhaust noise but maybe. Neither of us thought of pulling plugs to isolate the problem cylinder but that's next (thanks Tim)

Disappointed I haven't dealt with it yet. I've put too much into this van already to not keep going. I'll update as I go.

[JFratzke]

I pulled each wire one at a time and revved the motor, listening for changes to the knock. It neither quiets or gets worse with any of the wires pulled. Bummer. That sounded like a promising avenue to explore. I think the next step is to pull the pan and inspect bearings.

[Burntboot]

Hopefully Tim is around to shed better light on this.
It is my belief that pulling a wire removes the load from that particular piston/rod assembly so IF it really is a rod bearing, the noise SHOULD change.
The fact it has no effect would make me want to look at other possibilities before tearing it all down.

If you do end up pulling the pan, make sure you remove the oil level sender BEFORE dropping the pan, bad things happen if you don't.
Pulling the pan isn't a lot of fun so I always look at ways to avoid that, having already been down that road.

[timsrv]

If the noise wasn't reduced, eliminated, or at least changed by pulling plug wires then it's probably not a rod bearing. When your engine is running the piston and rod are basically being drug around by the crank, but when there's fire things change from being pulled by the crank to pushing it. Kind of like a bad universal joint (on a drive shaft) only banging and making noise when you change from acceleration to deceleration or visa versa. By disabling the power stroke on that cylinder you are changing the way force or pressure is being applied. Of course there's still changes when the piston changes direction, so a really bad bearing will still be audible, but the hammering sound becomes more uniform. When the cylinder is firing it's louder on the compression/fire stroke, so the frequency of the knock (at least the loud part) would change. Does that make sense?

Before pulling the pan, I'd at least pull the belts off, start the engine and rev. Don't run the engine for more than a minute or so without belts, but this is plenty of time to determine if the noise is gone. Sometimes accessories can make noises that sound exactly like rod knocks. Probably a long shot, but it's easy to check. Sometimes there can be noises due to other rotating parts like bolts backing off and allowing things to wiggle (like flywheels or flex plates touching other surfaces, loose pressure plates or torque converters, etc). If you have a stethoscope this can be helpful. Sometimes a wood dowel held up to different possible places will help too. Just place against different parts and press the other end up to your ear. Tim