Engine knock

[east302]

This is on a 1998 K1500, 350, 230k miles.

Earlier this year (7,000 miles ago) the #5 hydrolocked due to a cracked head. The other head was also cracked and I replaced both.

It’s my son’s truck so I rarely drive it, but I noticed a knocking sound a week ago. Changed the oil (5W30 Mobil 1) with no change.

Attached are videos of it. It only does this when warmed up. When cold, it’s quiet and gauge shows oil pressure of 40-psi. As it warms up, the knocking is louder and oil pressure is about 20-psi. I’ve not verified with a mechanical gauge.

The noise increases with engine speed, becoming so fast as to be hard to hear past 2000-rpm. It does the same when in gear.

So, can y’all suggest a next step on diagnosing this? I’d thought of pulling the intake and swapping the lifters, but it’s not really a ticking and the noise seems louder when heard from below - particularly on the oil filter side.

I readjusted the valves with no change.

 

[iamdub]

It's difficult to tell from these types of videos. Between the sounds in the videos and your symptoms of it happening only when warmed up, I'm leaning towards a lifter collapsing. I'm thinking the oil thins out when warmed up and the weak lifter collapses. You could try 20W-50 and see if there's any improvement. Maybe even try some Motor Honey just as a diagnostic measure. Since you just changed the oil with Mobil1, you can drain it into a clean container to put back in later. If it were me, and if I were pretty sure of the side, I'd pop off that driver side valve cover and look for a loose rocker arm.

 

[east302]

Thanks dub.

From above, it sounds more prominent on the right bank but using a stethoscope on the valve cover it didn’t sound louder in any particular area. I pulled the cover earlier today and none of the rockers were loose (engine not running and shut off for maybe five minutes after running). I’d love for it to be lifters. Would it be visually obvious with it running?

In the back of my mind is a concern that bearings are damaged after the cracked head and hydrolock. There was definitely coolant in the oil after that jamboree.

 

[OR VietVet]

If you pull the plug wires, one at a time while running, the noise change should narrow down the cylinder effected. I know that typically is to narrow down a rod knock, but it could change for a lifter, in a small way. It is worth a shot. It does sound like a lifter to me though, but I am old. Good luck

 

[iamdub]

Thanks dub.

From above, it sounds more prominent on the right bank but using a stethoscope on the valve cover it didn’t sound louder in any particular area. I pulled the cover earlier today and none of the rockers were loose (engine not running and shut off for maybe five minutes after running). I’d love for it to be lifters. Would it be visually obvious with it running?

In the back of my mind is a concern that bearings are damaged after the cracked head and hydrolock. There was definitely coolant in the oil after that jamboree.

Right bank? I was thinking the oil filter was on the left side. That's why I said driver's valve cover. My mistake. Regardless, you already pulled the cover.

It might be obvious when running, but it'd be really hard to see. They flick too fast. Also, you'd have a dead or really weak cylinder if it was collapsed enough to be visually obvious.

I like @OR VietVet's suggestion of killing cylinders until the sound changes. This would at least point you to the cylinder.

On that note, have you verified all spark wires were secure and all plugs were tight? A spark jumping a gap can "tap". A loose plug can make a similar sound that can change with temperature due to the metal expanding. This doesn't explain the drop in oil pressure unless it's affecting the idle enough to lower the RPM that much.

 

[OR VietVet]

When you pin point the cylinder, do a compression test.

 

[east302]

Right bank? I was thinking the oil filter was on the left side. That's why I said driver's valve cover. My mistake. Regardless, you already pulled the cover.

It might be obvious when running, but it'd be really hard to see. They flick too fast. Also, you'd have a dead or really weak cylinder if it was collapsed enough to be visually obvious.

I like @OR VietVet's suggestion of killing cylinders until the sound changes. This would at least point you to the cylinder.

On that note, have you verified all spark wires were secure and all plugs were tight? A spark jumping a gap can "tap". A loose plug can make a similar sound that can change with temperature due to the metal expanding. This doesn't explain the drop in oil pressure unless it's affecting the idle enough to lower the RPM that much.

Sorry about that, from above it sounds like it’s coming from the right bank but from underneath it sounds slightly louder-ish on the left. It’s just not as obvious as I wish it were. Going to go listen again and check the plugs.

It shows slight (2 to 6) current misfire counts on #2 when running which may have predisposed me to think right bank.

The oil pressure has been low for some time now even before all of this.

@OR VietVet I’ll pull some plugs when running. Also have a compression tester.

 

[OR VietVet]

I am sure you meant to say "pull plug wires" and not "pull plugs". Pull plug wires first, one at a time, to try and locate/pin-point the effected cylinder. Then pull all plugs to do the compression test. When you pull the plug wires, you can do it at the spark plug a or at the cap, whichever is easier.

 

[tom3]

If not discovered to be a loose plug or exhaust manifold leak I'd go with a thicker oil to start with. That old school small block might not like that 5wt, syn. oil when cold. 15w40 Rotella or the 20w50 mentioned above might do the trick. That's pretty high miles on that machine.

 

[iamdub]

Sorry about that, from above it sounds like it’s coming from the right bank but from underneath it sounds slightly louder-ish on the left. It’s just not as obvious as I wish it were. Going to go listen again and check the plugs.

It shows slight (2 to 6) current misfire counts on #2 when running which may have predisposed me to think right bank.

The oil pressure has been low for some time now even before all of this.

@OR VietVet I’ll pull some plugs when running. Also have a compression tester.

This is actually an upside, diagnostically speaking. Check that #2 spark system- the wire (both ends), the plug, etc.

 

[east302]

Thanks for the suggestions, guys.

Pulled all plug wires with no change in the noise. All plugs are torqued to spec and boots are on good at both ends. I meant to swap the plug and wires from #2 to another but got sidetracked. Will keep that misfire on the list as well as the heavier oil.

Checking again, it’s definitely loudest coming from the bellhousing with the inspection cover held back. I’ve read of cracked flex plates at the bolts…maybe that’s the culprit? I couldn’t see anything from my limited view, but believe those cracks may not be visible without sliding the transmission back?

 

[OR VietVet]

I almost said that earlier but it just was too rhythmic and did not sound like that but hell, anything is possible. You can see without pulling transmission back. Remove the inspection shield. Get a flashlight that is bright. The crack could be anywhere in the circle. You may have to inch around that plate with the teeth. Look for a discolored/dark/black looking crack radiating out from the center and typically from an attaching bolt area. May be able to see clearly by using a small mirror, like dental mirror size or even the bigger ones if can get in there.

 

[Fless]

Don't know if you can fit an inspection cam in there, but some come with 90° mirror heads for very tight spaces.

 

[east302]

Roger that, will take a better look tomorrow and get the whole cover off - it looked like the starter may need to come out so I didn’t get very far with it.

I do have a camera similar to that…never would have thought to use it, that’s a good idea.

 

[east302]

Looking at the flex plate was inconclusive, I just couldn’t see the area inside the bolt circle. Since the noise is rhythmic and only when warm, I’m thinking it may be a red herring but it sure seems louder right there around the end of the oil pan at the bellhousing. Hell, I don’t know, it’s just kind of “there” and everywhere, lol.

Using a timing light, I get one knock per flash which (per GM) should point to the valve train.

Pulled the right valve cover and watched the rockers. I’m not exactly sure what a collapsed lifter would look like, but they all seemed to move identically. The noise didn’t seem to be focused on any of them in particular. Is there a way to tighten/loosen the rocket nuts to verify if it is actually lifter issue?

Using a different stethoscope probe (a rod instead of the plastic cone), the #6 and #8 areas of the valve cover have more of a bell-like ringing or higher pitched metallic ping than the others. I need to compare it with my other 98s and see how they sound but called it a night instead.

Different oil is next, I suppose.

 

[OR VietVet]

Compression test results? Did I miss it?

 

[east302]

Compression test results? Did I miss it?

Nope, didn’t get that far with it. On the list, though.

 

[OR VietVet]

If you have the valve covers off at the same time, that would be a great time to watch the rockers and push rods at cranking speed instead of running speed. You can also lay your finger tips on the various parts to "feel" the noise.

 

[exp500]

From sound in video and descriptions, I think all you can do is feed it heavy oil till it blows. Think you have bent rod and it will get worse fast. Will be 5-8 cyl 9 of 10 times.
There are still a few under /around 100k L31 near me in the yards for 500 or so last time I looked. I aleays look for bad trans which usually means good engine. I don't like crash cars unless very fresh. Good Luck.

 

[east302]

Small update. Changed the oil to 15W40 and noticed a bit of “fuzz” on the oil drain plug magnet. Ran it for probably an hour over the last few days with no change in the noise.

Pulled the filter (Mobil) which had 230 miles on it and poured some in a bowl.

Cut the filter open and do have metallic dust in the pleats (upstream face). None are magnetic and you have to hit the light just right to see it. The largest piece is this one on Abe’s chin, the majority are the size of the two pieces to the right above 2022.

Would a compression test be academic at this point or what do y’all think?

To recap- the tapping doesn’t change when the plug wires are pulled one at a time. Using a stethoscope, tapping is loudest at the back of the engine in the oil pan near the bellhousing. The #7 and 8 lifters really ping with the rest just being “less” loud as you go forward. My other two 350s are sewing machines in comparison.

Just in case anyone is curious, the GM dealer here quoted me a smidge over $8,000 for a new 350. Local shop that I trust said around $7500 and no longer installs remans like Jasper and ATK - too much trouble to fool with, they said.

Local junkyard 350s range from $1000 with 200k miles up to $2000 with 126k on the engine.