'90s Chevy truck elusive engine knock. Rod/Main bearings, or what?

[Just Dropped In]

Hi folks. I have a '92 Chevy C1500, 5.7 with 162k miles (owned since new). Been running Shell T5 15w-40 for a few years. A few months back, it developed an engine knock. Oil pressure in the past year has dropped somewhat, around 30ish at 50/60mph, and 10+lbs or so at hot idle. Not a ping or rattle type sound, a sort of deep knock. One shop said it might be the flex plate, but it checked out fine. They said it could be lower engine, but they don't do lower engine repairs.

Here's the things I can rule out causing the knock. All the front engine accessories are new (pow/steering original, but it's quiet). I replaced both exhaust manifolds and Y-pipe, no leaks there. In fact, virtually every sensor/component on the engine has been replaced over the last couple of years.

Here are the symptoms: the engine is quiet at startup, but within a few miles (reaching operating temp), the knock starts up. Changing serpentine belt can make the knock go away (to a degree). Looser belt, no knock, tighter belt knock shows up. Another mechanic said it was main bearings.

Pulled a few plug wires, and the knock all but disappeared at cylinder #2.

So, could this be main bearings, or rod bearings or what? Haven't pulled the pan, but can if I need to. Thanks.

 

[kschachn]

Please post a video of the sound. That will help.

 

[nwjones18]

My mom has a s10 with the 4.3 and low miles, but it was driven with a failed intake gasket which resulted in water in the oil and eventually truck being overheated from lack of antifreeze. That motor has oil pressures similar to yours (10w-30) and sometimes I feel like I hear a faint knock at warm idle.. Its been doing it for 5+ years and there isn't any glitter in the oil so I'm just not worrying about it..

You could do an oil analysis if you wanted and see if there's a bunch of bearing material present.

 

[Just Dropped In]

Please post a video of the sound. That will help.

I'll see if I can manage that, never posted a vid before.

My mom has a s10 with the 4.3 and low miles, but it was driven with a failed intake gasket which resulted in water in the oil and eventually truck being overheated from lack of antifreeze. That motor has oil pressures similar to yours (10w-30) and sometimes I feel like I hear a faint knock at warm idle.. Its been doing it for 5+ years and there isn't any glitter in the oil so I'm just not worrying about it..

You could do an oil analysis if you wanted and see if there's a bunch of bearing material present.

I removed a qt of oil the other day, and neither my wife nor I could see any metallic flakes.

 

[ARCOgraphite]

When hot if you rev the engine to 3500- 4000 rpm and then let off the gas and the reapply the gas does the sound get loud when you let off the gas a bit and then disappear when you step on the gas?

"Unloaded Rapping" is and indicator of worn rod bearings,

Another thing that fails over time is that the Harmonic Balancer soft rubber insert sandwiched between the hub and the balance weight deteriorates - can you wiggle the balancer front to back?

-Arco

 

[Lawnman23]

Pressure is ok but on the low side. My 98 5.7 runs ~20/40/60 hot idle/cruise/cold start with ~85k on it(rebuild). I'm running Castrol GTX HM 10w-40.

Low pressure means increased bearing clearance between bearing and journal.

Might be time to drop the pan and check rod #2 for play. Thankfully far easier to do on your 2WD vs a 4WD.

 

[wlk]

Did you try a new oil pump? When I had the op changed in my 93 5.7l my pressures came back to normal

 

[Just Dropped In]

When hot if you rev the engine to 3500- 4000 rpm and then let off the gas and the reapply the gas does the sound get loud when you let off the gas a bit and then disappear when you step on the gas?

"Unloaded Rapping" is and indicator of worn rod bearings,

Another thing that fails over time is that the Harmonic Balancer soft rubber insert sandwiched between the hub and the balance weight deteriorates - can you wiggle the balancer front to back?

-Arco

Thanks, I'll check those things later this afternoon.

Pressure is ok but on the low side. My 98 5.7 runs ~20/40/60 hot idle/cruise/cold start with ~85k on it(rebuild). I'm running Castrol GTX HM 10w-40.

Low pressure means increased bearing clearance between bearing and journal.

Might be time to drop the pan and check rod #2 for play. Thankfully far easier to do on your 2WD vs a 4WD.

Will do. Yeah, I'm glad it's 2wd right now.

Did you try a new oil pump? When I had the op changed in my 93 5.7l my pressures came back to normal

If I pull the pan and find the problem to be fixable (by me, lol), I will certainly replace the oil pump. Still debating whether to have the engine rebuilt or replaced if I can't fix it.

 

[BigCahuna]

Years back I had the same problem with the same engine. Took it apart and found I had a cracked /broken wrist pin. I also found it funny that the noise stopped, when the plug wire was removed.,,

 

[wlk]

Thanks, I'll check those things later this afternoon.



Will do. Yeah, I'm glad it's 2wd right now.



If I pull the pan and find the problem to be fixable (by me, lol), I will certainly replace the oil pump. Still debating whether to have the engine rebuilt or replaced if I can't fix it.

2wd, easy to replace the oil pump, just buy a Melling

 

[Just Dropped In]

Ok, checked the lower pulley/balancer with belt removed. No movement front to back or up/down (used scissor jack to test).

Then, with cold engine crank, about 4-5 light knocks at start-up only. Warming up, no knock. Around 15+ minutes of idle run time, knock begins to be audible.

Unable to get video to load. Will be back once I sort that issue out.

 

[cptbarkey]

The type of knock with a video will help somewhat, but ultimately trying to "hear" where the knock is coming from will be better for you for diagnosing next steps.

Get a pair of stethoscope's but be careful not to deafen yourself.

 

[Just Dropped In]

The type of knock with a video will help somewhat, but ultimately trying to "hear" where the knock is coming from will be better for you for diagnosing next steps.

Get a pair of stethoscope's but be careful not to deafen yourself.

I have a stethoscope, and it's been somewhat helpful. To my ears, without the steth, the sound appears to be loudest near the rear of the oil pan. Not getting anywhere with my video load to the forum. Still trying to sort that out.

 

[methusaleh]

My mom has a s10 with the 4.3 and low miles, but it was driven with a failed intake gasket which resulted in water in the oil and eventually truck being overheated from lack of antifreeze.

Not to derail the thread, but I haven’t owned an S10 series in over 20 years but a neighbor has one I’ve been considering.

How long did it run with the failed gasket and did the engine need to be replaced afterward?

 

[Slick3914]

Run it till she blows!

 

[21_6.7f450]

162k is WAAAY early for one that's been maintained.

 

[Just Dropped In]

Going to pull the pan in the upcoming days to see what's going on in there. Whatever is making the racket is going to get plucked out.

 

[Lawnman23]

Going to pull the pan in the upcoming days to see what's going on in there. Whatever is making the racket is going to get plucked out.

View attachment 230829

Very clean engine bay, very well maintained

 

[Just Dropped In]

Very clean engine bay, very well maintained

Thanks. Engine bottom and entire chassis is superclean as well.

 

[clinebarger]

162k is WAAAY early for one that's been maintained.

Got to account the inverse of that, A babied LO5 might last that long with a manufacturing defect vs one that was worked & failed during warranty.