Is this rod knock? (1 Viewer)

[CruiserTrash]

Spoke with some of you today via text and on IG, but thought I’d throw it out to a wider audience. This noise is pretty concerning. It does vary with rpm, but engine temp, clutch, and steering have no effect. This original engine has 137k miles and compression was around 150 across the board when I checked it in November.

The engine feels and sounds smooth under acceleration. All gauges are as normal as they’ve always been, and there aren’t any new leaks or significant increases to old leaks.

This started very intermittently about 6 weeks ago after some “spirited driving” in the city - maybe 40mph and acceleration pulls to 3300rpm or so. It stayed intermittent until two days ago, when it became pretty consistent at idle.

I have not done any thorough checking but will be doing so tonight. Thanks for any help!

 

[CruiserTrash]

First steps I’ll be doing:
Check compression
Check clutch fork for rattle
Stethoscope
Run it briefly without belts
Check heat shields for rattle

 

[CruiserTrash]

I did suspect a plugged cat, so I straight piped it last weekend by welding a pipe onto the cat flanges and bolting it back in. Huge improvement in power so I think the cat was clogged. I have some new exhaust leaks now from my booger welds, but this noise seems different. Wondering if excessive exhaust back pressure caused internal issues.

I also changed the oil. Zero metal flakes. Not a single microscopic shiny bit. There were some black specs at the end though. They disintegrate when you touch them, so I’m guessing some carbon deposits or crud broke loose. The truck sat 2018-2022, so there could be residual crap left in there.

 

[Zjohnsonua]

Stethoscope is a good one. In a previous work life we'd do a low budget audio frequency analysis with Handbrake (free A/V converter) to get the knock frequency, which can pin things down in a hurry. Only so many things in an engine that make a racket at a given speed.

 

[CruiserTrash]

Stethoscope is a good one. In a previous work life we'd do a low budget audio frequency analysis with Handbrake (free A/V converter) to get the knock frequency, which can pin things down in a hurry. Only so many things in an engine that make a racket at a given speed.

So any software capable of frequency analysis would do that huh? Do you have a table or chart of frequencies and what common causes they correlate to?

 

[whitey45]

Old school method, pull one plug wire at a time while running, if the noise goes away, you know what cylinder its on( removing spark takes the load off the rod bearing)

 

[CruiserTrash]

Pressure plate adjustment? I recall a similar noise when I have adjusted a slave cylinder incorrectly…

@ryaneddy16 is this what you were suggesting?

 

[whitey45]

It would go away when pushing clutch in?

 

[60series777]

A few weeks after I had my engine rebuilt one of the rod nuts loosened and came off. Also happened after some spirited freeway driving. Probably not what happened with yours (I certainly could not have driven my rig for 6 more weeks before addressing things), but just throwing out another possibility. Hope it is something minor and you get things squared away soon.

 

[CruiserTrash]

A few weeks after I had my engine rebuilt one of the rod nuts loosened and came off. Also happened after some spirited freeway driving. Probably not what happened with yours (I certainly could not have driven my rig for 6 more weeks before addressing things), but just throwing out another possibility. Hope it is something minor and you get things squared away soon.

The tone of the noise is really similar to that, but it’s more localized. Can’t really hear it up too at all. Maybe I have a slightly loose cap. This is a good reference though, thanks.

 

[CruiserTrash]

It would go away when pushing clutch in?

No change with clutch

 

[whitey45]

Then not the t/o bearing, probably a rod bearing, if the noise increases with rpm its a rod bearing( pretty common on 2f’s) pull plug wires!!!

 

[Spike Strip]

^^
^^

Pull plug wires. Report back.

 

[CruiserTrash]

@whitey45 @Spike Strip

Pulled plug wires one at a time, the noise remains every time. It seemed to get worse pulling plug wires for cylinders #1, 2, & 3. That could be engine vibrations from the misfire exacerbating whatever the issue is.

Onward to more testing now that I’m finally at the shop. Worst case scenario I lose a season of camping & wheeling while refurbing this:

 

[Spike Strip]

If you don't get a noticeable diminishing of the knock when pulling plug wires (when the wire on the offending cylinder is pulled), it's not a bottom end issue.

You're gonna need an automotive stethoscope or something of than nature to isolate the approximate location of the sound.

 

[67azcruizr]

auto tranny? if it's a manual, put it in 3rd gear and load it up. if noise changes it could be wrist pin(s).

doesn't sound like a bearing knock to me. maybe a cracked for loose flex plate. someone else mentioned belts, could be a loose/bad harmonic or better yet a bad smog (especially with a clogged cat).

 

[CruiserTrash]

Stethoscope is saying flying wheel inspection cover. Diving in to that now.

 

[CruiserTrash]

the sound is definitely originating from the clutch/flywheel/pressure plate area, or possibly the rear crankshaft bearing. The flywheel spins true and nothing in the vicinity is rattling. Checked the bell housing bolts I could reach and they’re good, the hard to reach ones pass visual inspection with the motor running. Rear main cap bolts I gently tested - so not to change the torque - and they’re not loose.

I’m thinking rear main bearing at this point.

 

[CruiserTrash]

Looks like the FSM calls for 76-94 ft-lb of torque on that rear main cap. If I’m reading things correctly. Yes?

Having never done this before, would I be good to remove that, inspect, then replace and torque it down? 76-94 is a wide margin, what am I looking for on that? Is it a feel thing?

 

[Spike Strip]

That's an very odd sound for a end bearing to make, but I suppose it's possible. They call it a 'Rod Knock' for a reason 'cuz there's a rod involved with the ignition thrust.

Can you carefully put the stethoscope rod on one or the other of the rod cap bolts while the engine is running?


Isn't # 3 the end cap ?