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I have an engine noise of unknown origin on '95 Pontiac Bonneville with 3.8 liter V6 Engine. I am hoping someone can help identify the source of the noise for me. It sounds like belt noise to me, but the sound is unchanged with the serpentine belt removed, so it's clearly internal to the engine.

Here's a link to my YouTube video:
https://www.youtube.com/watch?v=4eGZhdm ... e=youtu.be

Engine has 115,000 miles with following:
- Intake Vacuum: 22 in Hg at idle, 23 Hg in at 2000 RPM
- Compression: 180 psi +/- 10 psi on all cylinders
- Catalytic Converter Temp at 1500 RPM: 400degF in, 500degF out
- Harmonic Balancer shows no visible signs of deterioration
- Loss of power and poor fuel economy began around same time as engine noise
- Oil changes every 3000 miles for life of vehicle

I would double check the balancer by getting a firm grip on the drive belt with some gloves, and try rotating it back and forth with the engine off. There should be no movement.

I would then pull the valve covers to see if anything is loose or broken.

Thanks, Randall, for your reply to my post.

As you suggested, I double checked the balancer -- I can't detect any movement between inner and outer parts of the balancer, and noise is unchanged with the drive belt removed.

This doesn't sound like any valve train noise that I've heard before -- it seems like more of a chirp than a clicking or clacking noise.

Since my post, I borrowed a scanner and observed some unusual spark advance readings. At idle, I have about 15 deg advance, and up to 30 deg when driving at steady speed. But when accelerating, the advance disappears and I have as little as -5 deg advance. When the "Knock Ret" is monitored, it's zero at steady speed and idle, but increases to a maximum of 15 deg under acceleration. I think the knock sensors are detecting the engine noise and are retarding the spark accordingly, which I guess explains the poor power and fuel economy.

Mark

Pull the oil filler cap off while it's running; see if the noise changes.
Inspect your PCV valve. My 3800 made horrible screeching noises when the PCV was *missing* from neglect by previous owner or previous owner's mechanic. Or, since the PCV runs about $3.27, just replace it and eliminate it as a possible source of the problem.

Then if you still have the problem, you might need to pull the oil pan and inspect the lower bearings.

As suggested, today I pulled my oil filler cap with the engine running and checked my PCV valve. No difference in the noise.

I then removed the serpentine belt again and carefully listened to the engine in another effort to determine where the noise is coming from. I came away thinking that it might be a valve train noise after all. So I pulled the rear valve cover, looking for loose or broken parts -- I didn't find anything obviously wrong, and I could detect no lash in any the valve trains on any of the cylinders.

I then probed around with a stethoscope with the engine at idle. I found that when listening at the head of the rocker arm bolt for one of the valves on cylinder #4, I heard a louder sound than on the rest of the cylinders, and it's matched perfectly in frequency with the engine noise I've been looking for. So I think I may have found my noise. Now the question is: what's the source of the noise within that valve train? I pulled the rocker arm and push rod for that valve and its neighbor. They both look pristine except for some wear between the push rod and rocker arm, but the wear looks identical for the the two valve trains. This leads me to conclude that I must have a faulty lifter causing the noise. Does anyone find fault with that logic?

As I described earlier, my knock sensors starting retarding the timing in this engine around the same time that I started hearing the noise. Is lifter noise enough to trigger the knock sensors, or do I have another problem here?

Incidentally, I also hear a lot of noise when listening at the timing cover (with the serpentine belt removed), though I don't think it's the source of the noise I've been looking for. Is that normal -- or should the timing chain run quietly?

I'm now planning to pull the intake manifold to replace the lifter for cylinder #4. What's the conventional wisdom on lifter replacement -- should I replace all 12 lifters while I'm there, or just the one causing the noise?

Any guidance you can offer will be much appreciated.
Thanks, Mark

Easier way to check it without taking everything apart is to try spinning the alternator, if the belt moves, you need a new balancer.

Update on my engine noise problem:

Before tearing into the engine to install new lifters, I decided to pour some Rislone Engine Treatment Concentrate into my crankcase to see if it would reduce my valve train noise.

Within about 20 miles of driving, the noise was slightly quieter than before, and the engine had much more power and much better fuel economy than before. Apparently, the knock sensors are no longer detecting enough noise to retard the engine timing as was the case earlier (I haven't double-checked this with a scanner yet).

I currently have driven about 200 miles since adding the Rislone. My plan is to continue driving the car and hope that the Rislone will eventually make the valve train noise goes away completely. If that fails, I think I'll be installing new lifters.

I'd be sure to follow Rislone's instructions or at least the part about changing the oil after 100 miles.
"For severely dirty engines, use two quarts of Rislone Engine Treatment and change the oil after 100 miles of driving."
https://rislone.com/wp-content/uploads/ ... Sheet1.pdf

Hope it quiets the noise for you.
Let us know how things turn out....